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1,000 terms

AP Biology

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active transport
Any transport of a substance across a cell's plasma membrane that requires ATP. * Moves substances against the concentration gradient (from low concentration to high concentration).Examples include cell membrane pumps and bulk flow (endocytosis and exocytosis).
amphipathic molecule
A molecule that has both hydrophobic and hydrophilic characters on separate ends of the molecule. * Characteristic of a phospholipid, which contains a hydrophilic head region and a hydrophobic tail region.
aquaporin
Channel protein embedded in many cell membranes that allows water molecules to diffuse freely across the membrane without having to interact with the hydrophobic interior of the phospholipid bilayer.
basal body
A structure found in eukaryotic cells consisting of microtubule triplets arranged in a 9 + 0 formation. * Organizes the assembly of microtubules in cilia and flagella.
cell membrane pump
A transmembrane protein that moves substances across the membrane against their concentration gradient through phosphorylation by ATP. * An example is the sodium-potassium pump, which moves three N⁺ ions into the cell followed by two K⁺ ions out of the cell and establishes an electrochemical gradient across nerve cells.Another example is the proton (H⁺) pump.
cell wall
The rigid surface exterior to the plasma membrane of some cells that functions as protection. * Composed of cellulose in plant cells, of chitin in fungal cells, and of peptidoglycan in many bacterial cells.
central vacuole
A large membrane-bound container found in plant cells used for storage and metabolism of various substances. * Separated from the cytoplasm by the regulatory tonoplast.
centrioles
A structure found in animal cells comprised of microtubule triplets arranged into a cylinder in a 9 + 0 formation. * A pair is involved in mitosis in establishing the centrosomes.
chloroplast
The pigment-containing organelle responsible for photosynthesis in plants and algae. * Comprised of an outer membrane, an inner membrane with internal folds called thylakoids, and an internal compartment called the stroma.
chromatin
The less organized, uncondensed form of DNA observed during interphase. * Allows for replication and transcription.
chromosome
The organized, condensed form of DNA observed during mitosis and containing a discrete set of genes from the organism's genome.
cilium
Short, thin, hairlike projections covering the outer surface of some eukaryotic cells used for locomotion. * Composed of microtubules in a 9 + 2 arrangement.
contractile vacuole
A flexible, membrane-bound organelle present in many freshwater protists that allows for regulation of internal water concentrations.
co-transport
A transport process that couples the passive movement of one substance with the active movement of another substance.
crista (pl. cristae)
A fold of the inner mitochondrial membrane. * Serves to increase the surface area along which the ATP-producing reactions of the electron transport chain occur.
cyclic AMP (cAMP)
A specific, ring-shaped second messenger in many eukaryotic cells constructed with ATP.
cytoplasm
The portion of a cell from the interior of the cell membrane to the nucleus (if present), including all the suspended organelles. * The aqueous portion is called the cytosol.
cytoskeleton
The framework of microtubules, microfilaments, and other intermediate filaments present in the cytoplasm of most eukaryotic cells. * Responsible for maintaining the structure of the cell and involved in various transport processes.
desmosomes
Structures composed of keratin proteins used for anchoring two cells together into a strong sheet.
diffusion
The passive transport of a substance down its concentration gradient requiring no ATP (cellular energy). * A movement from high concentration to low concentration.Examples include osmosis and facilitated diffusion.
electrochemical gradient
A type of potential energy established by the diffusion gradient of an ion. * Relates to the concentration gradient of that ion across a membrane and the tendency of that ion to cross the membrane.Established across the membrane of neurons by the sodium-potassium pump, allowing the nervous impulse to occur.
endocytosis
The active transport process of moving large molecules or small cells into a cell as the plasma membrane pinches in and forms a vesicle around the substance. * Called phagocytosis when the engulfed substance is a solid or cellular.Called pinocytosis when the engulfed substance is a liquid (including any dissolved solutes).
endomembrane system
The series of membranes within a eukaryotic cell extending from the nucleus and including the endoplasmic reticulum (ER), the Golgi, and the cell membrane. * Some are directly connected, as in the nucleus and ER; others are indirectly connected through vesicle action, as in the ER and Golgi.
endoplasmic reticulum (ER)
Part of the endomembrane system usually located just outside of the nucleus and consisting of a series of membranous channels. * Responsible for protein synthesis and packaging when studded with ribosomes (rough ER); otherwise functions in the synthesis of lipids and the breakdown of toxins (smooth ER).
eukaryotic cell
Any complex cell that possesses a nucleus and other membrane-bound organelles. * The cell type that evolved from prokaryotes through the process of endosymbiosis and is characteristic of the Protista, Plantae, Fungi, and Animalia kingdoms.
exocytosis
The active transport process of moving large molecules out of the cell as a vesicle fuses with the plasma membrane. * Helps to eliminate metabolic waste products from the cell and to distribute secretions to other cells.
extracellular matrix (ECM)
An assemblage of proteins and polysaccharides into which cells in an animal tissue are embedded. * Includes networks of collagen fibers and proteoglycan molecules in the extracellular fluid connected to integrins in the cell membrane through fibronectin.
facilitated diffusion
A type of passive transport that allows substances to move down the concentration gradient through an integral protein because the substance is too large or too charged to cross the phospholipid bilayer. * An example is an ion channel, a protein that allows charged particles to cross the membrane without having to interact with the phospholipid bilayer.
flaccid
A very limp, deflated cell that has experienced a recent loss of water due to a concentration gradient. * When a walled cell loses too much water, the cytoplasm shrivels and the plasma membrane pulls back from the cell wall (plasmolysis).
flagellum
A long, thin, hairlike projection from a cell used for locomotion. * In eukaryotes, composed of microtubules in a 9 + 2 arrangement.In prokaryotes, is formed from chains of the globular protein flagellum.
fluid mosaic model
The model used to describe the structure of the cell membrane. * Individual protein molecules embedded in the bilayer of phospholipids can move laterally and fluidly within the bilayer.
gap junction
A channel between adjacent animal cells in a tissue providing continuous cytoplasm and allowing communication and coordination between cells.
G-linked protein receptor
A plasma membrane signal receptor that is activated upon the binding of a signal molecule and initiates a cellular response by activating first a G protein and eventually an enzyme. * Used by hormones and neurotransmitters.
Golgi apparatus
Part of the endomembrane system consisting of flattened membranous sacs that modify, store, and release products of the endoplasmic reticulum (ER). * Also referred to as the Golgi or the Golgi body.
glycolipid
A lipid covalently bonded to a carbohydrate. * Often embedded within the phospholipid bilayer of the cell membrane, which functions in cell recognition and the maintenance of cell stability.
glycoprotein
A molecule consisting of a carbohydrate covalently bonded to a protein. * Functions in cell-cell communication and recognition.
hypertonic
A solution with a higher concentration of solutes than another solution. * A cell with this type of internal environment will gain water in its cytoplasm from the external environment through osmosis.
hypotonic
A solution with a lower concentration of solutes than another solution. * A cell with this type of internal environment will lose water from its cytosol to the external environment through osmosis.
integral protein
A transmembrane protein embedded in the phospholipid bilayer of a cell membrane. * Useful as channels across which ions and large, polar molecules can move into or out of the cell.
isotonic
Two solutions or sides of a membrane containing equal solute concentrations. * Movement will still occur after dynamic equilibrium is reached, but it will be balanced.
ligand
A molecule that binds to the specific receptor site of another molecule.
lysosome
A small, membrane-bound sac containing hydrolytic (digestive) enzymes. * Can fuse with vesicles and break down their contents.
microfilament
Part of the cytoskeleton of eukaryotic cells and an important component of the contractile unit in muscle tissue. * Consists of a solid rod of actin protein.
microtubule
Part of the cytoskeleton of eukaryotic cells and an important component of cilia and flagella. * Consists of tubulin protein filaments arranged into hollow rods.
mitochondrial matrix
The inner compartment of the mitochondrion where the citric acid cycle (Krebs cycle) of cellular respiration occurs.
mitochondrion
The organelle responsible for aerobic cellular respiration in eukaryotes. * Comprised of an outer membrane, an inner membrane with internal folds called cristae, and an internal compartment called the matrix.
nuclear envelope
A double membrane that surrounds and protects the DNA. * Contains perforations called nuclear pores that allow movement of select materials between the nucleus and the cytoplasm.The inner layer is connected to the nuclear lamina, a series of protein microfilaments that help maintain the shape of the nucleus.
nucleoid
The region in a prokaryotic cell where the DNA is concentrated.
nucleolus
The structure within the nucleus that produces rRNA, an integral component of the ribosome.
nucleus
The area in a eukaryotic cell surrounded by a double membrane and containing all of the cell's genetic code. * Comprised of the nuclear envelope, nuclear pores, DNA, and at least one nucleolus.
organelle
Any structure within a cell that is designed to carry out a specific function. * Most are membrane bound and exclusively found in eukaryotes; examples include the mitochondrion and the endoplasmic reticulum.Others lack membranes and are found in both prokaryotes and eukaryotes; an example is the ribosome.
osmoregulation
The ability of cells to regulate the levels of water and dissolved solutes in its cytoplasm. * The ability of organisms to regulate the levels of water and dissolved solutes in their tissues.
osmosis
The diffusion of water across the plasma membrane of a cell driven by the concentration gradient of solutes across the membrane. * Water will passively move from the hypotonic side to the hypertonic side in an attempt to dilute the more concentrated solution and reach equilibrium.
passive transport
The movement of a substance down its concentration gradient from an area of higher concentration to an area of lower concentration. * Requires no ATP (cellular energy).Diffusion is the most general type of passive transport occurring in all cells.
peripheral protein
A protein bound to one surface of a cell membrane but not embedded within the bilayer.
peroxisome
A small, membrane-bound organelle in plant cells containing metabolic enzymes that transfer hydrogen atoms to oxygen. * Involved in the creation of and then degradation of hydrogen peroxide (H₂O₂).
plasma membrane
The exterior boundary of a cell comprised of a selectively permeable phospholipid bilayer designed to regulate the internal chemical environment of a cell. * The outermost interface with the external environment and the innermost interface with the internal cytoplasm are hydrophilic; the internal thickness of the cell membrane is hydrophobic.The only membrane present in prokaryotic cells.
plasmodesma (pl. plasmodesmata)
A pore in the cell walls of neighboring plant cells providing continuity between the cytoplasm and allowing for communication and coordination between cells.
prokaryotic cell
Any relatively simple cell that lacks a nucleus and other membrane-bound organelles. * The earliest cell type to evolve, giving rise to the extant Archaebacteria and Eubacteria.
protein kinase
An enzyme that transfers a phosphate group from an ATP molecule to a protein. * Involved in many signal-transduction pathways of plants, animals, and fungi.
ribosome
The small, granular organelle responsible for protein synthesis (translation). * Comprised of rRNA and proteins and organized into a large and small subunit.
second messenger
A small, nonprotein polar molecule or ion that acts to relay information received from the external environment via a signal receptor protein to other structures within the cell's interior. * Examples include cAMP and C²+.
selective permeability
The characteristic of biological membranes that allows them to regulate the passage of molecules across the membrane based on the size and chemical composition of the molecules. * This characteristic is due to the amphipathic nature of the phospholipids that comprise the membrane.
signal-transduction pathway
A mechanism that allows a cell to initiate a given response to a mechanical or chemical stimulus.
stroma
The inner compartment of a chloroplast where the Calvin cycle (light-independent reactions) of photosynthesis occurs.
thylakoid
A disc-shaped fold of the inner membrane of the chloroplast arranged into stacks called grana. * Contains clusters of pigments called photosystems, which receive and transfer light energy to other molecules.
tight junction
Areas along adjacent cells where proteins are used to very closely bind the cells together in a tissue. * Useful for creating a seal between cells in epithelial tissue so that fluids cannot leak in between the cells.
transport protein
An integral protein useful in moving substances across the cell membrane that cannot directly pass through the phospholipid bilayer.
turgid
A very firm, plump cell that has experienced a recent influx of water due to a concentration gradient. * Plant and fungal cells can remain turgid due to their rigid cell wall; animal cells burst (cytolysis) if they take up too much water.
vesicle
A small, temporary, membrane-bound sac that is created when part of the endomembrane system pinches off and carries contents elsewhere. * Can fuse with a lysosome for digestion of its contents.
acid
Any substance that can act as a hydrogen ion (H⁺) donor and has a pH less than 7. * Any aqueous solution that has a greater concentration of hydrogen ions (H⁺) than hydroxide ions (OH⁻).
activation energy
The amount of energy required by the reactants for the initiation of a chemical reaction and formation of products. * Reduction is possible through enzyme catalysis, allowing the reaction to proceed more quickly.
active site
The specific location along an enzyme that interacts with the substrate via weak chemical interactions. * If bound to competitive inhibitors instead of the substrate, the enzyme's activity is restricted.
adhesion
The property of water that allows it to form chemical attractions with other substances and makes it a useful solvent. * Contributes to the presence of a meniscus in a graduated cylinder as water sticks to the surface of the container.Is an important component of capillarity in that it allows water to cling to the inner surface of the xylem tubes of plants.
allosteric regulation
The binding of a regulatory molecule to a protein at a site other than the site where protein functioning is actually affected. * Can result in activation and cooperativity (increased protein function) or inhibition (decreased protein function).
alpha helix
One of two major forms of secondary protein structure. * Associated with a spiral shape along a stretch of a polypeptide chain and maintained by hydrogen bonds.
amino acid
The monomer building block of a protein. * Comprised of a central carbon atom bonded to a hydrogen atom, an amino group, a carboxylic group, and an R group (unique side chain).Coded for during translation (protein synthesis) by each mRNA codon along a transcript.Twenty different forms exist among most organisms, each distinguished by its R group.
amino group
The (-NH₂) functional group that creates an amine when attached to a hydrocarbon. * Significant in amino acids, the building blocks of proteins.
anion
Any negatively charged ion resulting from a neutral atom that has gained one or more electrons. * Examples include Cl⁻, PO₄³-, and SO₄²-.
atom
The simplest naturally occurring particulate state of matter comprised of a central nucleus surrounded by one or more energy levels. * The nucleus contains protons and neutrons, while electrons move around the nucleus in different energy levels.
atomic nucleus
The small, dense, central region of an atom containing all of the atom's protons and neutrons.
atomic number
A unique value assigned to each chemical element based on the total number of protons contained within that element's atomic nucleus.
ATP (adenosine triphosphate)
The essential cellular energy transfer molecule produced through the cellular respiration and fermentation of sugars. * Comprised of a ribose sugar bonded to an adenine nucleotide and a chain of three phosphate groups.As the terminal phosphate bond is broken through hydrolysis, energy is released.
base
Any substance that can act as a hydrogen ion (H⁺) acceptor and has a pH greater than 7. * Any aqueous solution that has a greater concentration of hydroxide ions (OH⁻) than hydrogen ions (H⁺).
beta pleated sheet
One of two major forms of secondary protein structure. * Associated with repeating parallel folds of a polypeptide chain maintained by hydrogen bonds.
carbohydrate
One of four major classes of organic macromolecules responsible for short-term energy storage. * Used for the synthesis of ATP through cellular respiration and fermentation.
carboxyl group
The (-COOH) functional group that creates a carboxylic acid when bonded to a hydrocarbon. * Significant in amino acids, the building blocks of proteins.
catalyst
Any substance that speeds up a chemical reaction. * Biological catalysts are called enzymes.
cation
Any positively charged ion resulting from a neutral atom that has lost one or more electrons. * Examples include Na⁺, K⁺, Ca²+, and NH₄⁺.
chemical reaction
A change in matter involving the making and/or breaking of chemical bonds. * During the process, the reactants (inputs) are chemically changed into products (outputs).Significant to metabolism and regulatory feedback loops.
coenzyme
An organic substance that must bind to an enzyme to allow proper functioning. * Examples include most vitamins common to the animal diet.
cofactor
A nonprotein substance that must bind to an enzyme to allow proper functioning. * Examples include minerals in the animal diet and ionic components of soil important to plants.Some are permanently bound to an enzyme; others bind temporarily during catalysis.
cohesion
The property of water that allows it to hydrogen-bond with other water molecules and to be chemically "sticky." * Contributes to large droplets of water aggregating on a surface like a leaf.An important component of capillarity in that it allows for the creation of a continuous column of water throughout the xylem tubes of plants.
compound
Any combination of two or more chemical elements connected through a chemical bond. * Examples include methane (CH₄) and water (H₂O).
condensation reaction (dehydration synthesis)
The type of reaction responsible for the synthesis of organic polymers from their monomer subunits (anabolism). * Involves the removal of the components of water from adjacent monomers, allowing a new covalent bond to form.
covalent bond
A bond formed as two atoms' outermost shells overlap and valence electrons are shared between the two atoms. * If electrons are shared equally, the bond is nonpolar covalent, as in oxygen (O₂).If electrons are shared unequally, the bond is polar covalent, as in water (H₂O).
denaturation
The disruption of the structure of a protein or DNA molecule as hydrogen bonds are broken. * Achieved through increases in temperature or changes in pH.Utilized in biotechnology in the polymerase chain reaction to mimic the action of helicase and "unzip" the double helix into separate strands.
disaccharide
The dimer form of a carbohydrate consisting of two monosaccharides bonded through a glycosidic linkage. * Often called a simple sugar.Provides short-term energy.Examples are sucrose (table sugar) and lactose (milk sugar).
disulfide bridge
One type of interaction between amino-acid side chains responsible for the tertiary structure of a protein. * A strong covalent bond formed as two cysteine monomers bond with one another.Created as two sulfhydryl groups both lose their terminal hydrogen atom and form a bond between sulfur atoms.
DNA (deoxyribonucleic acid)
One of two major polynucleotide forms responsible for the protection, storage, and replication of the genetic code. * Contains nucleotides composed of a five-carbon sugar (deoxyribose), a phosphate group, and a nitrogenous base (either adenine, thymine, guanine, or cytosine).A double helix of complementary nucleotide strands.
electron
A negatively charged particle located within an orbital outside of the nucleus of an atom, the mass of which is too small to contribute to the atomic mass of an element. * Type of particle involved in bonding between elements to form compounds and molecules.
electronegativity
The affinity of an atom to attract electrons in a covalent bond.
element
Any of the pure chemical substances listed on the periodic table. * Carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) are the major elements relevant to organisms.
endergonic
A reaction the products of which are at a higher energy state than the reactants. * Non-spontaneous reactions requiring a free energy input from the environment.Can be coupled with an exergonic reaction to maximize cellular energy efficiency.
energy
The capacity to do work, an essential quality for the coordination and continuation of all life processes. * Stored forms (potential energy) useful to organisms are found in chemical bonds.The transfer in form from radiant energy (solar) to chemical energy (glucose) to cellular energy (ATP) drives all life on Earth.
enzyme
A protein that catalyzes a chemical reaction by lowering the activation energy needed to convert reactants to products. * Unique to each substrate (reactant) because of the unique structure of its active site.Its activity is explained by the lock-and-key and induced-fit models.
enzyme-substrate complex
The complex structure created while an enzyme is acting on its substrate to create the products of a chemical reaction. * Held together at the active site of the enzyme through weak chemical interactions.
exergonic
A reaction the products of which are at a lower energy state than the reactants. * Spontaneous reactions that release free energy into the environment.Can be coupled with an endergonic reaction to maximize cellular energy efficiency.
fat (triacylglycerol)
A lipid molecule consisting of three fatty-acid chains bonded to a glycerol backbone. * Provides the lipid basis for nutrients in the animal diet.
fatty acid
A carboxylic acid with a very long hydrocarbon chain. * Saturated forms possess all single bonds between carbons in the chain; unsaturated forms have at least one double bond between carbons.
functional group
Any group of atoms bonded to a hydrocarbon that confers a specific function to the molecule. * Examples include the hydroxyl group (-OH) and the sulfhydryl group (-SH).
glycosidic linkage
The specialized type of covalent bond created as a condensation reaction joins together adjacent monosaccharides. * The presence of one in a carbohydrate indicates a disaccharide; the presence of two or more indicates a polysaccharide.
heat
Thermal energy of a sample. * The total kinetic energy of matter due to molecular motion.
hydration shell
The spherical arrangement of water molecules that forms around each ion as an ionic compound is dissolved in water. * The cation becomes surrounded by the oxygen atoms in neighboring water molecules, while the anion becomes surrounded by the hydrogen atoms in neighboring water molecules.
hydrocarbon
Any organic molecule that is composed of a carbon backbone surrounded by hydrogen atoms. * The carbon backbone can be a straight chain, branched chain, or ring formation and can consist of single, double, or triple bonds between adjacent carbon atoms.Examples include methane (CH₄) and fatty acid chains.
hydrogen bond (H-bond)
The intermolecular interaction between a hydrogen atom on one molecule and an electronegative atom on another molecule. * Forms between the partially positive hydrogen atom in one water molecule and the partially negative oxygen atom in another water molecule.Also contributes to the secondary and tertiary folding in proteins.
hydrogen ion
A hydrogen atom that has lost its electron. * A single proton with a +1 charge.A substance that is a hydrogen-ion donor is considered an acid and has a pH measuring less than 7.
hydrolysis
The type of reaction responsible for the degradation of polymers into their monomer subunits (catabolism). * Involves the input of a water molecule to allow breakage of the bond between monomers.
hydrophilic
The chemical nature of a substance that dissolves readily in, and has a high affinity for, water. * In a cell membrane, the polar head region in each phospholipid is hydrophilic.
hydrophobic
The chemical nature of a substance that does not dissolve in water. * In a cell membrane, the nonpolar tails in each phospholipid are hydrophobic.
hydroxide ion
A polyatomic anion composed of a hydrogen and oxygen atom in the form (OH⁻). * A water molecule that has lost a proton.Often substances containing a hydroxide group are H⁺ acceptors and, thus, basic with a pH measuring greater than 7.
hydroxyl group
The (-OH) functional group that creates an alcohol when bonded to a hydrocarbon. * An example is ethanol (CH₃CH₂OH), the product of anaerobic alcohol fermentation in some fungi and bacteria.
induced fit
The slight alteration in the shape of an enzyme's active site to encourage binding to its substrate. * Occurs as the substrate enters the active site of the enzyme.
inhibitor
A substance that binds to an enzyme, preventing it from binding to its substrate and, thus, decreasing the activity of the enzyme. * Binding to the enzyme's active site results in competitive inhibition; binding to a site other than the active site results in noncompetitive inhibition.
ion
Any positively or negatively charged particle formed as electrons are lost or gained. * Can be simple (as with H⁺) or polyatomic (as with OH⁻).
ionic bond
A bond that forms between oppositely charged ions. * A bond that forms as a metal donates electrons to a nonmetal and creates an ionic compound (salt).An example is Na⁺ + Cl⁻ → NaCl.
isomer
Different organic compounds possessing the same molecular formula but different arrangements of their atoms and, thus, different functions. * Can be structural, geometric, or enantiomer in nature.Examples include alpha-glucose and beta-glucose.
isotope
Forms of the same atom that possess different numbers of neutrons and, thus, different atomic masses. * Some are radioactive and can be useful in radiometric dating of fossils.
lipid
One of four classes of organic macromolecules insoluble in water and responsible for long-term energy storage and protection. * Includes fats, oils, waxes, phospholipids, and steroids.
macromolecule
A very large molecule formed by the linking together of smaller subunit molecules. * Organic examples include carbohydrates, proteins, and nucleic acids.
mass number (atomic mass)
A unique value assigned to each chemical element based on the total number of protons and neutrons found in that element's atomic nucleus. * Because each element has isotopes (varying numbers of neutrons), the relative abundance of each isotope is used to calculate an average atomic mass.
matter
Anything that has mass and takes up space (that is, has a density). * All relevant biological materials are classified as matter.
metabolic pathway
A series of chemical reactions in which the products of one reaction become the reactants of the following reaction. * Each reaction is catalyzed and regulated by an enzyme, allowing for feedback.
metabolism
The sum of all the chemical reactions occurring within a cell or organism. * Reactions responsible for building up larger molecules are anabolic and include the synthesis of organic polymers from monomers.Reactions responsible for breaking down larger molecules are catabolic and include the digestion of organic polymers (food like carbohydrates and proteins) into their monomer subunits.
molecule
The structure formed as two or more atoms bond. * The simplest forms are diatomic, as in O₂ and N₂.
monomer
The simplest form of an organic molecule that acts as a building block for larger and more complex forms. * Examples include amino acids in proteins, monosaccharides in carbohydrates, and nucleotides in nucleic acids.
monosaccharide
The monomer building block of a carbohydrate. * Comprised of carbon, hydrogen, and oxygen atoms in the ratio (CH₂O)subscriptn.Forms a polysaccharide when linked together covalently through glycosidic linkages.
neutron
A neutrally charged particle contained within the nucleus of an atom. * Each neutron contributes 1 amu to the atomic mass of an element.
nonpolar covalent
A covalent bond between two atoms in which there is equal sharing of the electrons due to no or negligible electronegative differences between the atoms * Examples include the bonds in O₂ and CH₄.
nucleic acid
One of four major classes of organic macromolecules responsible for storage of the genetic code and expression of genetic traits. * Comprised of complex nucleotide monomers containing a five-carbon sugar, a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, and either thymine or uracil).Two major forms are DNA and RNA.
nucleotide
The monomer building block of a nucleic acid. * Consists of a five-carbon sugar (either ribose or deoxyribose), a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, thymine, or uracil).
peptide bond
The specialized covalent bond that connects adjacent amino acids within a protein. * The presence of one within a protein indicates a dipeptide; the presence of two or more indicates a polypeptide.
phosphate group
The (-PO₄³3-) functional group that creates an organic phosphate when bonded to a hydrocarbon. * Significant in cellular energy processes as a component of ATP and in nucleotides like DNA and RNA as a component of nucleotides.
phospholipid
The form of lipid that makes up the cell membrane and is amphipathic. * Comprised of a hydrophilic head region (including a glycerol and phosphate group) and two hydrophobic tails (fatty-acid chains).
phosphorylation
The transfer of a phosphate group from ATP to another molecule, increasing the energy of the recipient.
polar covalent
A covalent bond between two atoms in which there is unequal sharing of electrons due to considerable electronegative differences between the atoms. * Examples include H₂O and NH₃.
polar molecule
An assemblage of covalently bonded atoms in which valence electrons are shared unequally due to electronegativity differences between the atoms. * Associated with partial charges on its atoms even though the molecule is neutral overall.An example is water; the two hydrogen atoms have a partially positive charge (∂+) and the oxygen atom has a partially negative charge (∂-).
polymer
The more complex form of organic molecules created when monomer building blocks covalently bond. * Examples include polypeptides in proteins, polysaccharides in carbohydrates, and polynucleotides in nucleic acids.
polynucleotide
The polymer form of a nucleic acid created as nucleotides covalently bond. * The form typical of DNA and RNA.
polypeptide
The polymer form of a protein. * Examples include enzymes, hemoglobin, and cell membrane pumps.
polysaccharide
The polymer form of a carbohydrate created as glycosidic linkages are established between adjacent monosaccharides. * Examples include glycogen, cellulose, and starch.
primary structure
The first level of protein structure involving the sequence of specific amino acids present in a given protein. * Maintained by peptide bonds connecting adjacent amino acids.
protein
One of four major organic macromolecules essential for the structure and function of all organisms. * The monomer form is an amino acid; the polymer form is a polypeptide.
proton
A positively charged particle contained within the nucleus of an atom. * Each proton contributes one amu to the atomic mass of an element.The total number of protons found in the nucleus of an element is unique and defines the atomic number.
quaternary structure
The fourth level of protein folding associated with a complex interaction of two or more tertiary polypeptides. * An example is hemoglobin, which contains four polypeptide subunits.
RNA (ribonucleic acid)
One of two major polynucleotide forms responsible for the expression of the genetic code through protein synthesis (transcription and translation). * Contains nucleotides composed of a five-carbon sugar (ribose), a phosphate group, and a nitrogenous base (either adenine, uracil, guanine, or cytosine).Takes three major forms: globular rRNA, cloverleaf-shaped tRNA, and linear mRNA.
secondary structure
The second level of protein structure influenced by interactions between the different R groups (side chains). * Associated with repeated folding (beta pleated sheet) or coiling (alpha helix) along specific stretches of a polypeptide chain.
solution
A homogeneous mixture composed of a solute dissolved in a solvent. * The solute can be solid, liquid, or gas, while the solvent is a liquid.When aqueous, the solvent is water.
specific heat
The quantity of heat energy required to change the temperature of 1 gram of a substance by 1°C. * Water has a high specific heat due to the large number of hydrogen bonds present between neighboring molecules; the bonds must be broken before the temperature can increase.
steroid
The form of lipid that is characterized by four fused carbon rings with different functional groups. * Examples include cholesterol and the sex hormones (testosterone and estrogen).
substrate
The substance altered by the catalytic activity of an enzyme. * Possesses a structure recognized by a specific enzyme.
sulfhydryl group
The (-SH) functional group that creates a thiol when attached to a hydrocarbon. * Significant in tertiary polypeptides when disulfide bridges form to maintain the protein's structure.
surface tension
A measure of the level of difficulty involved in breaking the surface of a liquid. * A liquid with a high quantity of hydrogen bonds, such as water, has a high surface tension.
temperature
The average kinetic energy of the molecules in a sample of matter. * A measure of heat energy in degrees.
tertiary structure
The third level of protein folding associated with a specific three-dimensional globular shape. * Maintained by hydrogen bonds, disulfide bridges, ionic bonds, and hydrophobic interactions.
valence electron
An electron in an atom's outermost electron shell that is available for formation of a chemical bond between two atoms.
van der Waals interactions
Any weak intermolecular interaction due to fluctuating changes in charge distribution throughout the molecules.
absorption spectrum
The graph that expresses a pigment's absorption of various wavelengths of light.
action spectrum
The graph that expresses the relationship between photosynthetic rate and absorption of various wavelengths of light.
autotroph
An organism that utilizes either photosynthesis or chemosynthesis to produce its own organic food from inorganic CO₂. * Kingdoms Plantae, Protista, and Eubacteria include such organisms.
C₃ plant
A plant that directly incorporates CO₂ into the Calvin cycle. * CO₂ combines with five-carbon RuBP to produce a six-carbon intermediate, which immediately splits to the stable three-carbon G3P molecule used to build glucose and regenerate RuBP.
C₄ plant
A plant that incorporates CO₂ first into a four-carbon compound and, ultimately, into the Calvin cycle. * Allows for increased water efficiency by highly regulating opening and closing of the stomata during the day.
Calvin cycle
The series of biochemical reactions that occur in the stroma and utilize carbon dioxide (CO₂) and ATP and NADPH from the light-independent reactions to produce glucose. * Fixes inorganic CO₂ into organic glucose usable for energy by all organisms.
CAM plant
A plant that uses crassulacean acid metabolism, a specialized adaptation found in desert plants that allows for collection and storage of CO₂ at night when water loss is minimized. * During the day, when stomata are closed, the CO₂ is released from storage and used in the Calvin cycle.
carbon fixation
The process conducted by autotrophs during which inorganic CO₂ molecules are converted into organic glucose molecules. * Responsible for the production of the organic molecules that heterotrophs depend on for food.
carotenoid
An accessory pigment of photosynthesis in plants and green algae. * Maximizes absorption in the region of the spectrum missed by the chlorophylls and reflects back yellow-orange wavelengths to the observer.
chlorophyll a
The main pigment of photosynthesis in plants and algae. * Maximizes absorption in the red and blue wavelengths of the spectrum and reflects the green wavelengths back to the observer.
chlorophyll b
An accessory pigment of photosynthesis in plants and green algae. * Maximizes absorption in the red and blue wavelengths so as to complement the absorption of chlorophyll a.
cyclic electron flow
The movement of electrons during the light-dependent reactions that utilizes only photosystem I and produces additional ATP but no more NADPH or oxygen. * Helps reconcile the disproportionate amounts of ATP and NADPH used by the Calvin cycle.
electromagnetic spectrum
The spectrum of radiation based on the inverse relationship between wavelength and frequency. * Gamma rays possess the shortest wavelength, radio waves possess the longest wavelength, and visible light expresses medium-length waves.
glyceraldehyde-3-phosphate (G3P)
The three-carbon carbohydrate directly produced by the Calvin cycle and used to build glucose.
heterotroph
An organism that must acquire organic food substances by ingesting or absorbing nutrients from the environment. * Examples include animals, fungi, protozoans, and most bacteria.
light-dependent reactions
The biochemical pathway that converts radiant energy (sunlight) into chemical energy in the form of ATP and NADPH. * Relies on the work of pigments and electron transport chains for energy transfer.
NADPH
The energy shuttle generated by the light-dependent reactions and used by the Calvin cycle to help produce glucose.
noncyclic electron flow
The movement of electrons during the light-dependent reactions that utilizes both photosystems and produces ATP, NADPH, and oxygen. * Electrons move from water to NADP⁺.
photoautotroph
An organism that utilizes photosynthesis to produce its own organic food molecules by "fixing" inorganic substances in the environment. * Examples include plants, algae, and cyanobacteria.
photophosphorylation
The chemiosmotic process by which ATP is generated as an H⁺ ion diffuses across the thylakoid membrane via ATP synthase. * Relies on the energy generated by the proton gradient.
photorespiration
A metabolic pathway employed on dry, hot days when stomata close and leaf concentrations of O₂ exceed CO₂. * Uses O₂2 and generates CO₂ but no additional ATP, effectively decreasing photosynthetic productivity.
photosynthesis
The biochemical pathway utilized by photoautotrophs to produce carbohydrate monomers from carbon dioxide and water. * Represented by the sum equation 6CO₂ + 6H₂O + sunlight energy → C₆H₁₂O₆ + 6H₂O.
photosystem
A collection of pigments in the thylakoid membrane of the chloroplast that absorbs light. * Contains many light harvesting complexes that surround and direct light toward the reaction center.
photosystem I
One of two collections of pigments embedded in the thylakoid membrane of the chloroplast. * Contains P700 chlorophyll a at its reaction center.Directly connected via electron transport chains to [XXX] II and to the production of NADPH.
photosystem II
One of two collections of pigments embedded in the thylakoid membrane of the chloroplast. * Contains P680 chlorophyll a at its reaction center.Accepts electrons from the splitting of water and directs them down an electron transport chain toward [XXX] I.
primary electron acceptor
The first electron acceptor of the electron transport chain that starts a series of redox reactions.
reaction center
The area in the photosystem concentrated with chlorophyll a toward which all of the absorbed energy from light is directed from the accessory pigments.
rubisco (ribulose carboxylase)
The enzyme that catalyzes the reaction between CO₂ and RuBP to start the Calvin cycle. * Incorporates inorganic CO₂ into an organic molecule, "fixing" the carbon.
RuBP (ribulose biphosphate)
The five-carbon compound used to incorporate CO₂ into an organic compound in the first step of the Calvin cycle. * Regenerated in the last step of the Calvin cycle.
stoma (pl. stomata)
A small opening on the underside of plant leaves that allows for gas exchange through the otherwise impermeable waxy cuticle.
stroma
The space inside the inner membrane of the chloroplast surrounding the grana. * The site of the light-independent reactions (Calvin cycle) of photosynthesis.
thylakoid
The disc-shaped folds of the inner membrane of a chloroplast arranged in stacks called grana. * The site of the light-dependent reactions of photosynthesis.Serves to increase the surface area of the inner membrane of the chloroplast for increased efficiency of light absorption and, thus, photosynthesis.
visible light
The region on the electromagnetic spectrum where wavelengths range from 380 nm (violet light) to 750 nm (red light). * All wavelengths in this region are detected as discrete colors by the human eye.
wavelength
The distance from one peak of a wave to the next. * Inversely proportional to the frequency of the wave.
acetyl CoA
The product of the breakdown of pyruvate in the mitochondrial matrix that then enters the citric acid cycle for further degradation.
aerobic
Conditions where oxygen (O₂) is present. * The most efficient pathway for the catabolism of glucose, generating maximum ATP yield per glucose.
alcohol fermentation
The type of anaerobic fermentation utilized by bacteria and yeast that produces ethyl alcohol (ethanol) and CO₂ from pyruvate and regenerates NAD⁺.
anaerobic
Conditions where oxygen (O₂) is absent or in insufficient quantities. * A less efficient pathway involving a partial breakdown of glucose and minimal ATP production.
ATP synthase
The H⁺ ion channel that catalyzes the synthesis of ATP from ADP and Psubscripti as H⁺ diffuses across a membrane.
beta oxidation
A metabolic pathway that breaks a fatty acid down into two-carbon molecules that then enter the citric acid cycle as acetyl CoA.
cellular respiration
The aerobic, catabolic process involving the complete oxidation of a monosaccharide. * Generates 36 to 38 ATP molecules per glucose molecule invested.
chemiosmosis
The transfer of the energy from an H⁺ ion gradient to the production of ATP through the diffusion of H⁺ ions across a membrane using ATP synthase.
citric acid cycle (Krebs cycle)
The biochemical pathway that utilizes acetyl CoA to generate ATP, NADH, and FADH₂ as part of cellular respiration. * The second major phase of cellular respiration.
cytochromes
Proteins comprising the majority of the electron transport chain that facilitate redox reactions and the eventual transfer of electrons to oxygen.
electron transport chain
The series of molecules embedded in the inner mitochondrial membrane of aerobic eukaryotes that facilitates a series of redox reactions.
facultative anaerobes
Organisms including some bacteria and yeast that can utilize both anaerobic fermentation and aerobic cellular respiration to generate ATP depending on whether oxygen (O₂) is present.
FADH₂
An energy shuttle molecule generated during the citric acid cycle and later used in the electron transport chain to synthesize ATP. * FAD + 2H⁺ → FADH₂.
fermentation
The anaerobic, catabolic process of partial monosaccharide degradation. * Generates only two ATP molecules per glucose molecule invested.Utilizes glycolysis followed by either alcohol or lactic acid fermentation, depending on the type of organism.
glycolysis
The partial breakdown of a glucose molecule that occurs in the cytosol of all cells and generates two ATP molecules. * The first major phase of both cellular respiration and fermentation.
lactic acid fermentation
The type of anaerobic fermentation process utilized by some fungi, by some bacteria, and by mammalian muscle cells experiencing oxygen debt. * Produces lactate (lactic acid) from pyruvate and regenerates NAD⁺.
NAD⁺
A coenzyme and derivative of vitamin niacin that acts as an electron acceptor (oxidizing agent) during respiration to produce NADH.
NADH
An energy shuttle generated during glycolysis and the citric acid cycle and eventually used by the electron transport chain to generate ATP. * NAD⁺ + H⁺ → [XXX].
OAA (oxaloacetate)
The four-carbon molecule regenerated at the end of each turn of the citric acid cycle. * Combines with acetyl CoA to produce citrate during the first reaction of the cycle.
obligate anaerobes
Organisms that rely exclusively on anaerobic fermentation to generate ATP. * Oxygen is toxic to these organisms.
oxidation
The loss of electrons causing an increase in the oxidation state of the substance. * Process involved in the catabolism of glucose.Always coupled with a reduction.
oxidative phosphorylation
The process by which ATP is produced from the energy generated through the redox reactions of an electron transport chain. * The third major phase of cellular respiration.
proton-motive force
The energy generated from the gradient of H⁺ ions across the inner mitochondrial membrane that is used in the synthesis of ATP.
redox reaction (oxidation-reduction reaction)
A coupled chemical reaction involving the loss of electrons by one substance (oxidation) and the resultant gain of electrons by another substance (reduction). * Type of reaction utilized by an electron transport chain.
reduction
The gain of electrons causing a decrease in the oxidation state of the substance. * Process involved in the anabolism of organic molecules.Always coupled with an oxidation.
substrate-level phosphorylation
The process by which the energy used to hold a phosphate group to its substrate is transferred directly to ADP to produce ATP.
anaphase
The fourth phase of mitosis, during which the sister chromatids are pulled apart at the centromere and move toward the centrosomes at opposite poles. * The phase during which the cell begins to elongate.
anchorage dependence
The characteristic observed in cells that requires attachment to a substratum for division to occur. * Prevents clumping and overgrowth of cells.
aster
The short microtubules that extend from the centrosomes as the mitotic spindle forms.
benign tumor
A mass of abnormal cells that remains at the site of origin and usually does not affect the functioning of neighboring cells.
binary fission
The process of cell division and asexual reproduction in prokaryotes involving replication of the DNA followed by cytokinesis.
cell cycle
The repeating set of events that comprise the life cycle of a cell from one division to the next. * In eukaryotes, gap 1 (G₁) → synthesis (S) → gap 2 (G₂) → mitosis (M).
cell division
The process of a single parent cell copying its DNA and splitting its cytoplasm to form two daughter cells. * In prokaryotes, this process involves binary fission, while in eukaryotes the process usually includes mitosis.
cell plate
The structure formed along the center of a plant cell as vesicles deposit cellulose to create a new cell wall and separate one cell into two during cytokinesis.
centromere
The region in a replicated chromosome connecting the two identical sister chromatids. * This region is pulled apart during anaphase of mitosis to allow each chromatid to migrate toward opposite poles.
centrosome
The structure consisting of a pair of perpendicularly arranged centrioles that established the opposite poles of a cell during cell division.
checkpoint
A point of regulation in the cell cycle when the cell receives feedback to proceed with the cycle or to arrest the cycle. * The G₁ checkpoint allows a cell either to anticipate division and proceed with the S phase or to instead enter the G₀ phase and stall preparation for division.
chromatin
The uncondensed, thread-like form of DNA present during interphase that can be read for transcription and replication.
chromosome
The physical structure comprised of DNA and supportive proteins that becomes visible in the nucleus during mitosis. * Contains discrete units called genes that code for specific traits in an organism.
cleavage furrow
The structure formed as an elongated animal cell is pinched into two separate cells during cytokinesis.
cyclin
A regulatory protein the concentrations of which fluctuate in a cyclical manner during the cell cycle. * Creates active cyclin-dependent kinase (Cdk) when attached to a cell cycle kinase.
cyclin-dependent kinase (Cdk)
A protein kinase important in regulating the cell cycle. * Becomes active when attached to cyclin and, in sufficient concentrations, allows cells to move past a checkpoint.
cytokinesis
The process of the division of the cytoplasm allowing complete cell division to occur. * In animal cells, a cleavage furrow forms, while in plant cells a cell plate is created.
density-dependent inhibition
The effect observed when overcrowded cells stop dividing.
G₀ phase (gap 0 phase)
The nondividing state a cell will enter if it does not receive sufficient feedback to pass the G₁ restriction point. * Most cells in an adult human are in this phase.
G₁ phase (gap 1 phase)
The first stage of the cell cycle following division, during which the cell metabolizes and grows. * DNA present is in chromatin form to allow for transcription.
G₂ phase (gap 2 phase)
The phase of the cell cycle following the S phase, during which additional metabolism and growth of the cell occurs and the cell prepares to divide by forming two centrosomes. * The DNA present is in chromatin form to allow for transcription.
gamete
A haploid (n) sex cell specialized for reproduction in eukaryotes. * Usually found in the forms of sperm in males and eggs in females.
genome
The entire set of genes present in a typical cell. * Comprised of a characteristic number of chromosomes in each sexually reproducing species.
growth factor
A protein released by a cell that signals other cells to divide. * Contributes to density-dependent inhibition of cell division.
interphase
The portion of the cell cycle when active division is not occurring. * It includes G₁, S, and G₂.DNA present is in chromatin form.
kinetochore
The proteins that form around each centromere and provide attachment points for kinetochore microtubules of the spindle.
M phase (mitotic phase)
The phase of the cell cycle during which mitosis occurs. * DNA present is in chromosome form.
malignant tumor
A mass of abnormal cells that continue to divide uncontrollably, invade the functioning of neighboring tissues and organs, and signal the presence of cancer.
meiosis
The process of nuclear division in specialized diploid germ cells that creates haploid gametes for sexual reproduction.
metaphase
The third phase of mitosis, during which the centrosomes establish the opposite poles of the cell and the chromosomes are arranged along the metaphase plate (equator).
metaphase plate
The plane equidistant from each pole of the cell upon which the chromosomes line up and position each sister chromatid toward opposite poles.
metastasis
The spreading of cancerous cells from the site of origin to other tissues of the body. * Usually achieved through the circulatory system and lymphatic vessels.
mitosis
The process of nuclear division in eukaryotes that, when followed by cytokinesis, creates two genetically identical diploid cells from one diploid parent cell. * Prophase → metaphase → anaphase → telophase.
mitotic spindle
The apparatus consisting of microtubules and proteins that allows for chromosome movement during mitosis.
MPF (maturation-promoting factor)
A specific cyclin-Cdk complex that allows cells to move from the G₂ phase into the M phase for division. * Begins degradation during anaphase to signal the end of division and the start of the next G₁ phase.
origin of replication
The point at which DNA replication begins along the single bacterial chromosome.
prometaphase
The second phase of mitosis, during which the nuclear envelope breaks down, kinetochore proteins form around each centromere, and the microtubules of the spindle connect to the chromosomes.
prophase
The first phase of mitosis, during which the chromatin condenses into chromosomes, the nucleoli disappear, the mitotic spindle and asters form, and the centrosomes are pushed away from one another.
S phase (synthesis phase)
The phase of the cell cycle following the G₁ phase, during which the DNA is replicated and, thus, the information for the sister chromatid is created. * DNA present is in chromatin form to allow for replication.
sister chromatid
Identical pieces of a replicated chromosome attached to one another at the centromere. * The second copy is created during the S phase and precedes mitosis to ensure genetic information is present for the daughter cells.
somatic cell
Any diploid (2n) body cell in a multicellular eukaryote.
telophase
The final phase of mitosis during which new nuclei begin to form around the collections of chromosomes, the DNA begins to uncoil back into chromatin form, and the nucleoli re-form.
transformation
The process of a normal cell being converted into a cancerous cell that lacks regulatory control mechanisms. * Usually arises through serial mutations.
allele
A form of a gene. * Pairs of alleles indicate an organism's genotype, which, in turn, influences its phenotype.
amniocentesis
A type of genetic screening of a developing fetus common in prenatal care. * Involves the extraction of fetal cells from the uterus and extended analysis can determine various genetic diseases and disorders.A karyotype can be generated after several weeks and then analyzed to detect any chromosomal mutations and verify biological sex.
asexual reproduction
The form of reproduction requiring one parent to create two genetically identical offspring. * Does not allow for genetic variation.Examples include binary fission (bacteria), self-pollination (plants), and fragmentation and budding (animals).
autosome
Any chromosome found in the nucleus of a cell that is not a sex chromosome. * In humans, chromosome pairs 1 through 22 contain the bulk of the genes in the genome.
carrier
An individual who possesses a recessive allele but does not express it due to the presence of the dominant allele in the heterozygous genotype. * This allele is just as likely to be passed on to the offspring as the dominant allele in this situation, so the offspring might express the recessive phenotype if another recessive allele is inherited from the other parent.
character
A heritable trait that is observable. * Gregor Mendel studied such traits in his research on pea plants.
chiasma (pl. chiasmata)
The region at which two non-sister chromatids exchange genetic information through crossing over.
chorionic villus sampling (CVS)
A type of genetic screening of a developing fetus involving extraction of a sample of fetal tissue from the placenta. * A karyotype can be generated immediately and analyzed to detect any chromosomal mutations and to determine biological sex.
clone
An organism created from the DNA from an existing organism. * A lineage of genetically identical cells and/or organisms.
codominance
A type of genetic expression in which the heterozygous genotype is expressed as a combination of both homozygous phenotypes in the same organism. * Observed in flower color in petunias; the alleles for flower color are red (R) and white (W) and the heterozygous genotype (RW) produces a red-and-white flower.
complete dominance
The type of genetic expression witnessed by Gregor Mendel in his experiments with pea plants. * The expression observed when the homozygous dominant and heterozygous genotypes are indistinguishable and expressed as the dominant phenotype.Observed in stem height in pea plants; the alleles are tall (T) and dwarf (t), so Tt = tall.
crossing over
The process by which homologous chromosomes can intersect at compatible points along the chromatid and exchange genetic information.
cystic fibrosis
An autosomal recessive human disease causing buildup of mucus in the respiratory tract; fatal if untreated. * Attempted treatments with gene therapy seemed promising but have thus far been relatively unsuccessful.
dihybrid cross
The type of genetic cross in which the inheritance of two genes together are being analyzed. * If both parents are heterozygotes for both traits, then a 9:3:3:1 phenotype ratio is predicted.
diploid (2n) cell
Any cell that has two distinct sets of genetic information, one inherited from each parent. * A cell capable of undergoing mitosis and cytokinesis to produce more diploid cells.Characteristic of somatic cells.
dominant allele
The allele that is expressed in the phenotype over the recessive allele if found in the heterozygote combination.
epistasis
A type of genetic expression in which the expression of one gene blocks the expression of another gene. * An example is the interaction between a hair color gene and a hairless gene; if the hairless trait is expressed, the hair color trait will be irrelevant.
F₁ generation
The first filial (offspring) generation in a series of genetic crosses.
F₂ generation
The second filial (offspring) generation in a series of genetic crosses using the F₁ offspring as parents.
fertilization
The process of fusion of a sperm and egg to create a new, genetically unique diploid zygote.
gamete
A haploid sex cell (either sperm or egg) created through meiosis. * If fertilized, contributes to the creation of a new diploid zygote.
gene
A discrete sequence of covalently bonded DNA nucleotides at a given locus along a chromosome. * Codes for a protein and, thus, instructs the genetic expression of a trait.
genetics
The study of the inheritance of genetic traits through generations. * Gregor Mendel is considered the father of modern genetics.
genotype
The combination of alleles possessed by an organism for a given gene. * Determines the phenotype, although its expression can be influenced by the environment.
haploid (n) cell
A cell that has only one set of genetic information and half of what is typical for a diploid cell of that species. * A cell created through the process of meiosis of a diploid germ cell.Characteristic of gametes (sex cells).
heredity
The transmission of genetic traits from one generation to the next through reproduction.
heterozygous
A genotype containing two different alleles (hybrid). * If the trait demonstrates a completely dominant inheritance pattern, then the resultant phenotype is dominant.
homologous chromosomes
A pair of chromosomes found in the nucleus of an individual diploid cell that are the same size and shape, and that contain the same arrangement of genes. * One member of the pair is inherited from each parent.
homozygous
A genotype containing two alleles of the same type (pure breeding). * If both alleles are dominant, then the genotype is described as [XXX] dominant and the resultant phenotype is dominant.If both alleles are recessive, the genotype is described as [XXX] recessive and the resultant phenotype is recessive.
Huntington's disease
An autosomal dominant human disease causing degeneration of the nervous system and eventual death. * Associated with a late onset of symptoms, often after the individual has reproduced offspring and potentially passed along the disease.
hybridization
The crossing of two different sets of genetic instructions for a given character. * Creates a heterozygous offspring.
incomplete dominance
A type of genetic expression in which the heterozygous genotype is expressed as an intermediate between the two homozygous phenotypes. * Observed in flower color in snapdragons; the alleles for flower color are red (R) and white (W) and the heterozygote genotype (RW) produces a pink flower.
karyotype
The arrangement of all of an individual's chromosomes in homologous pairs. * In humans, spans from the largest autosome pair (1) to the smallest autosome pair (22) followed by the sex chromosome pair (23).
law of independent assortment
One of Gregor Mendel's principles of genetic inheritance. * Describes the notion that, for unlinked genes, the inheritance of one gene does not affect the inheritance of the other.This does not generally hold true for linked genes, because they're located on the same chromosome and are, thus, inherited together.
law of segregation
One of Gregor Mendel's principles of genetics. * Describes the way in which an individual's pair of alleles for a gene separates during gametogenesis and only one allele is randomly passed on to a gamete.
locus (pl. loci)
A specific location on a chromosome where a gene is found.
meiosis
The process of a single diploid germ cell undergoing two divisions to create four genetically unique haploid cells. * In the testes of males, produces four functional sperm (spermatogenesis).In the ovaries of females, produces one functional egg cell and three supportive polar bodies (oogenesis).
meiosis I
The first division that occurs during meiosis in which a diploid germ cell splits to create two haploid cells. * Involves the random separation of each homologous pair of chromosomes into two separate cells.Increases genetic variation due to crossing over and independent assortment.Called a reduction division (2n → n).
meiosis II
The second set of divisions that occurs during meiosis in which the two haploid cells created from meiosis I undergo a mitotic division to create four haploid cells. * Called a mitotic division of haploid cells because each chromatid is separated from its sister chromatid at the centromere and the duplicated information is allocated to separate cells.
monohybrid cross
The type of genetic cross in which the inheritance of only one gene is being analyzed. * If both parents are heterozygotes, then a 3:1 phenotype ratio and a 1:2:1 genotype ratio are predicted.
multifactorial
A phenotypic character that is influenced by multiple genes and multiple environmental influences.
norm of reaction
The range of phenotypes produced by a single genotype due to environmental influences.
P generation
The individuals involved in the parental (initial) generation in a series of genetic crosses. * Produces the F₁ generation.
pedigree
A graphical representation of the inheritance of a trait within a family (or related families) over multiple generations.
phenotype
The physical expression of an organism's genotype. * If the trait demonstrates a completely dominant inheritance pattern, the phenotype will be either dominant or recessive.
pleiotropy
A type of genetic expression in which one gene has multiple phenotypic effects. * An example is the sickle cell trait; possessing the heterozygous condition for sickle cell anemia can cause systemic symptoms.
polygenic inheritance
A type of genetic expression in which many genes influence the expression of a single phenotype. * Often results in quantitative characters, traits that can be described along a spectrum of expression.Examples include human height, skin color, and eye color.
Punnett square
A tool used to predict the genotype and phenotype frequencies of the offspring generated from a given cross. * Demonstrates the possible outcomes from random fusion of gametes.
quantitative characters
A heritable trait that varies over a gradient instead of being expressed in an either-or manner. * An example is height in humans (as contrasted to height in pea plants).
recessive allele
The allele that is masked by the dominant allele and not expressed if found in the heterozygous genotype.
recombinant chromosome
A chromosome that has experienced a crossing-over event and now has genetic information from two sources.
sex chromosomes
The pair of chromosomes that determines biological sex. * Inheritance varies across species; XX = female in mammals and fruit flies but male in birds.In humans, the 23rd pair of chromosomes in a karyotype.
sexual reproduction
The form of reproduction that requires the input of a haploid set of information from two parents to create a new diploid organism. * Allows for the introduction of genetic variation through processes such as crossing over, independent assortment, and random fusion of gametes.
sickle cell trait
An autosomal codominant human disease originally caused by a point mutation and involving the creation of abnormally shaped hemoglobin. * Individuals with one normal hemoglobin allele and one sickle cell allele will express sickle cell anemia (the less severe form) and will produce both normal and sickle-celled red blood cells.
somatic cell
Any diploid body cell. * Able to regenerate more like cells through the process of mitotic cell division.
synapsis
The process by which homologs come together and form a tetrad during prophase I of meiosis.
Tay-Sachs disease
An autosomal recessive genetic disease in which a defective enzyme allows lipids to accumulate in the brain. * Causes malfunctioning of the nervous system and eventual death, typically during infancy.A genetic marker has been identified so genetic counseling can help individuals determine their risk of passing on the trait to their offspring.
testcross
A type of cross conducted to determine the genotype of an individual who possesses the dominant phenotype. * By crossing the dominant phenotyped individual with a recessive individual and analyzing the offspring, you can determine whether the genotype in question is homozygous dominant or heterozygous.
tetrad
Used to describe the paired homologs that form during synapsis of prophase I of meiosis. * Once in this form, crossing over can occur.
trait
Any variation in a genetic character. * Versions of genetic expression.
true breeding
A plant that will only produce like plants when permitted to self-pollinate. * Homozygous for the trait being studied.
variation
Genetic differences among members of the same species. * Essential within a population for evolution to occur.
zygote
The diploid cell that results from the fertilization of a haploid egg cell by a haploid sperm. * Can undergo mitotic cell division to generate a multicellular embryo.
activator
A protein that encourages expression of a gene upon binding to a specific region of DNA.
aminoacyl-tRNA synthetases
An enzyme responsible for the synthesis of free amino acids to their respective tRNA molecule for use in translation (protein synthesis).
aneuploidy
The condition of having the incorrect number of chromosomes. * Often manifested as a monosomy (having a single chromosome instead of a pair) or a trisomy (having a chromosome triplet instead of a pair).
anticodon
The sequence of three nucleotides in a tRNA molecule that are complementary to an mRNA codon and are responsible for transferring the appropriate amino acid to the growing polypeptide chain.
antiparallel
The orientation of the two strands of DNA nucleotides within a DNA molecule. * One strand is positioned in the 3' → 5' direction and the other in the 5'→ 3' direction.
bacteriophage
A virus whose host range includes bacteria. * Also called a phage.
Barr body
A characteristic observed in the cells of mammalian females in which one of the two X chromosomes is inactivated. * Contributes to the blotchy expression of black and tan colors in female calico cats.
base-pair substitution
A type of point mutation in which one nucleotide is substituted for the appropriate nucleotide in a DNA strand. * The type of mutation responsible for the sickle-cell trait.
capsid
The protective outer coating in many viruses composed of protein.
cell differentiation
The developmental process of cells specializing in structure and function. * Occurs through differential gene expression.
chromosome theory of inheritance
The theory explaining that, although the inheritance of one trait does not generally affect the inheritance of another, genes are contained along discrete stretches of DNA (chromosomes) that actually represent the particulate nature of inheritance.
codon
The sequence of three RNA nucleotides in an mRNA transcript that codes for a specific amino acid.
conjugation
The process by which two bacteria make contact through a conjugation tube and exchange genetic information. * Acts to increase genetic variation in asexually reproducing organisms.
corepressor
A molecule that must bind to the inactive repressor to activate it, thus allowing it to bind to the operator and inhibit transcription. * In the trp operon, tryptophan is the corepressor.
crossing over
A type of genetic recombination that occurs during prophase I of meiosis. * Involves the reciprocal exchange of genetic information between non-sister chromatids of a homologous pair.
deletion
A type of chromosomal mutation in which a segment of a chromosome breaks off and is lost.
deletion
A type of gene mutation involving the removal of a nucleotide pair from a DNA strand. * Causes a shift in the reading frame.
deoxyribose
The type of pentose-sugar present in DNA nucleotides.
differential gene expression
In multicellular organisms, the expression of different sets of genes in different cells with the same genome. * Essential for differentiation and overall development.
DNA fingerprint
The pattern of DNA bands that results from the gel electrophoresis of certain regions of an individual's DNA treated with restriction enzymes. * Can be used to establish an individual's unique genetic identity, as in criminal identification in forensics and paternity determination.
DNA ligase
An enzyme involved with sealing up nicks in a DNA molecule by covalently bonding adjacent nucleotides in a condensation reaction.
DNA polymerases
The enzymes responsible for the synthesis of the new complementary strands of DNA during replication. * Act by reading the exposed nucleotide on the original strand and pairing it up with a free complementary nucleotide.
double helix
The structure assumed by the genetic material, DNA. * Comprised of two complementary strands of DNA nucleotides running in antiparallel orientation (one runs 3' → 5', the other 5' → 3').
Down syndrome
A human genetic disorder resulting from a nondisjunction mutation at the 21st chromosome. * Associated with the presence of a trisomy at the 21st chromosome.
Duchenne muscular dystrophy
An X-linked recessive human disorder that is characterized by the degeneration of muscle tissue.
duplication
A type of chromosome mutation in which a segment of a chromosome is copied and inserted into a homologous chromosome.
enhancer
The region along DNA that can bind with various control elements and regulates the expression of a gene.
epigenetic inheritance
Inheritance of traits based on factors other than what is coded for genetically. * Caused by changes in local micro-environmental factors, like histone modification and DNA methylation.
euchromatin
The form of chromatin that is less condensed and highly accessible for transcription. * Associated with acetylation of histone tails.
exon
An expressed sequence in the DNA gene that remains after RNA splicing. * Codes for amino acids in the resultant protein.
5' cap
The guanine nucleotide cap placed on the 5' end of the pre-mRNA transcript as part of RNA processing.
frameshift mutation
A point mutation that results in a shift in the reading frame of an mRNA transcript and, thus, disrupts all remaining codons in the transcript. * Results from an addition or deletion mutation.
gel electrophoresis
A biotechnology useful for separating small fragments of DNA generated through the action of restriction enzymes. * Relies on the negative charges on the DNA molecule and the differing lengths of the fragments to create a banding pattern of DNA.
gene cloning
The process of using restriction enzymes to splice out a desired gene, inserting it into a bacterial plasmid, and finally encouraging the bacterium to make copies of the gene (through DNA replication) and copies of the gene product (through protein synthesis).
gene therapy
A biotechnology employed in an effort to treat people afflicted with a genetic disease. * Attempts to replace malfunctioning genes with healthy versions.Bioengineered viruses often used as shuttles to transfer healthy genes to cells.
genetic engineering
Any process during which DNA is manipulated for the practical applications of human societies. * Includes gene cloning, reproductive cloning, and the creation of genetically modified organisms (GMOs).
genetic map
A sequential list of the genes (genetic loci) contained on each chromosome found in a species.
genetic recombination
Any method of increasing genetic variation in a species. * In sexually reproducing species, can be achieved through crossing over, independent assortment, and random fusion of gametes.In asexually reproducing species like bacteria, is achieved through conjugation and other transformation events.
genomic imprinting
The type of genetic expression in which expression of an allele depends on whether it was inherited from the male or female parent. * Occurs during the formation of gametes in which one allele from certain genes is silenced.
genomic library
A collection of thousands of DNA fragments, each containing a gene and housed within a cloning vector, representing the coding sequences within the entire genome. * Complementary DNA (cDNA) libraries are created for gene cloning that store intron-free versions of genes.
helicase
In DNA replication, the enzyme responsible for breaking the hydrogen bonds between complementary strands and exposing the nucleotide sequences along the two strands. * Allows DNA polymerase to then read the exposed codes and build two new complementary strands.
hemophilia
An X-linked recessive human genetic disorder that is characterized by the inability to form blood clots.
heterochromatin
The form of chromatin that is more highly condensed and not easily accessible for transcription. * Associated with DNA methylation.
histone
A protein that serves as the structural core for a nucleosome. * Allows DNA to wrap itself around the protein for DNA packing.Can be acetylated or methylated to affect access to the genes for transcription and, thus, to regulate genetic expression.
histone acetylation
Modifications made to DNA involving additions of acetyl groups to certain proteins of the histone proteins. * Affects genetic expression and may influence epigenetic inheritance.
Human Genome Project
The effort to sequence every gene on each chromosome in the human genome. * Was coordinated with efforts to sequence the genomes of other representative organisms and completed ahead of schedule in 2003.
inducer
A specific molecule that inactivates the repressor protein in an [XXX]-ible operon. * In the lac operon, lactose acts as the [XXX].
insertion
The addition of an extra nucleotide into a DNA strand. * Causes a shift in the reading frame.
intron
An interrupting sequence in the DNA gene that results in excessive codons in the pre-mRNA transcript. * Removed during RNA splicing by spliceosome action.
inversion
A type of chromosome mutation in which a segment of a chromosome breaks off, inverts its orientation, and reattaches in the same chromosome.
lagging strand
During replication, the strand in the original DNA molecule along which DNA polymerase must build short Okazaki fragments of replicated DNA. * DNA ligase is used to seal up the gaps between fragments.
leading strand
During replication, the strand in the original DNA molecule along which DNA polymerase can continuously build a complementary strand.
linkage map
A type of genetic map designed around the frequencies of recombination rates due to crossing over in meiosis I. * The farther apart two genes are on a chromosome, the greater the likelihood that they will become unlinked during crossing over.
linked genes
Genes that are physically located on the same chromosome and are, thus, typically inherited together. * Can be unlinked through the process of crossing over, though this is random and unpredictable.
lysogenic cycle
The phase of viral replication in which the viral DNA becomes incorporated into the host cell's genome as a prophage but does not cause lysis. * All cells produced through cell division from the infected cell will carry the prophage DNA.Also called the temperate phase.
lytic cycle
The phase of viral replication in which the virus is immediately virulent. * The DNA of the virus is inserted into the host cell and is transcribed and translated by the host cell to synthesize new viruses that cause cell lysis.
map units
A unit used to measure the distance between genes on a chromosome. * One map unit is defined as representing a 1 percent recombination frequency.
mismatch repair
The process by which cells use special enzymes to recognize and replace mismatched DNA nucleotide pairs.
missense mutation
A type of base-pair substitution in which the new codon still codes for the same amino acid as the initial codon. * A neutral mutation in a coding gene.
mRNA (messenger RNA)
The form of RNA constructed through the process of transcription. * Represents a complementary copy of a DNA gene.A single-stranded linear molecule that can easily be modified by a spliceosome and read by a ribosome for translation.
nonsense mutation
A type of mutation that changes an amino acid to a stop codon, resulting in a shorter and (usually) dysfunctional protein.
nuclease
An enzyme involved in the hydrolysis of DNA and RNA molecules into nucleotides so that they may be reused in further metabolism.
nucleosome
The most basic unit of DNA packing consisting of DNA wound twice around a protein histone complex. * Stacked upon one another to more highly pack the DNA and form chromatin fibers.
operator
The site of regulation of transcription in prokaryotes. * When a repressor protein is bound, RNA polymerase cannot access the structural gene and transcription is inhibited.When no repressor protein is bound, RNA polymerase has access to the structural gene and transcription is permitted.
operon
The unit of regulation of genetic expression in prokaryotes. * Consists of a promoter, an operator, and a structural gene.
origins of replication
The points along a DNA molecule at which DNA replication begins. * Creates a replication fork in the DNA molecule where the two strands are separated.
plasmid
The small, circular pieces of DNA common in prokaryotes that confer special adaptive genes. * The F plasmid confers the ability to form a sex pilus and to exchange plasmids with other bacteria.The R plasmid confers various antibiotic resistance to the bacterium.
point mutation
Any mutation of DNA that involves a change in one nucleotide. * An insertion or addition involves the addition of an extra nucleotide into a DNA strand.
poly-A tail
The repetitive adenine tail placed on the 3' end of the pre-mRNA transcript as part of RNA processing. * Can consist of between 50 and 250 adenine nucleotides.
polymerase chain reaction (PCR)
A reaction conducted in a controlled laboratory setting that simulates DNA replication of a desired gene fragment. * Involves the denaturation of the double-stranded DNA molecule, the addition of primers, and then the addition of DNA polymerase and free DNA nucleotides to exponentially magnify the quantity of DNA present.
polyploidy
The condition of having more than two complete sets of chromosomes. * Involved with sympatric speciation in plants.
primary transcript
The pre-mRNA transcript created in the nucleus of eukaryotes before modifications like RNA splicing have occurred.
primer
In DNA replication, a short polynucleotide with a free 3' end that binds complementarily to the template strand and is elongated by the action of DNA polymerase. * Synthesized from nucleotides by the action of primase.
prions
Small pieces of infectious protein. * Examples of diseases caused by prions include mad-cow disease and kuru.
promoter
The region along a DNA molecule at the start of the gene at which RNA polymerase and other transcription factors bind to begin transcription.
prophage
The viral DNA when incorporated into the bacterial host's genome. * More generally called a provirus.
proto-oncogenes
Genes that produce regulatory proteins that stimulate normal cell growth and division. * When subject to mutation, can lose their regulatory function, become an oncogene, and lead to cancer.
purine
A type of nitrogenous base present in DNA and RNA containing two carbon-based rings. * Examples include adenine (A) and guanine (G).
pyrimidines
A type of nitrogenous base present in DNA and RNA containing one carbon-based ring. * Examples include cytosine (C), thymine (T), and uracil (U).
recombinant DNA
DNA from two separate species now part of the same molecule. * An example includes the genetic material possessed by a transgenic bacterium that has been bioengineered for gene cloning.
regulatory gene
A gene downstream of the structural gene under regulation. * When transcribed and translated, produces a regulatory protein that can bind to the operator region of an operon and block transcription of the structural gene.
repressor
Any protein that inhibits the expression of a gene. * In prokaryotes, a protein that binds to the operator region of an operon and inhibits transcription of the structural gene downstream.Created in either an active form in a repressible operon or an inactive form in an inducible operon.
restriction enzymes
Bacterial enzymes that are capable of recognizing specific sequences of DNA nucleotides (restriction sites) and making a cut between covalently bonded base pairs. * Some make jagged cuts and produce sticky ends; others make straight cuts and produce blunt ends.Useful for creating smaller, more manageable pieces of DNA to separate via gel electrophoresis.
restriction fragment length polymorphisms (RFLPs)
The DNA segments of varying lengths produced when a restriction enzyme cuts different samples of DNA at its restriction site. * Can be used to establish genetic markers for disorders such as Huntington's disease.
retrovirus
A virus that possesses RNA instead of DNA as its genetic material. * Also contains reverse transcriptase, an enzyme that synthesizes viral DNA from RNA so that the genetic instructions can become incorporated into the host cell's genome.Examples include HIV and influenza.
ribose
The type of pentose-sugar present in RNA nucleotides.
ribosome
The cellular organelle that coordinates the synthesis of a polypeptide during translation. * Consists of a large and small subunit composed of RNA and proteins.Reads each mRNA codon on the transcript and positions the tRNA with the corresponding amino acid so that adjacent amino acids can form a peptide bond.
ribozyme
An enzyme-like RNA molecule that catalyzes RNA splicing.
RNA polymerase
In transcription, the enzyme that binds to the promoter region of a gene, opens up the DNA molecule along a short stretch, reads the template strand, and builds a complementary mRNA transcript.
RNA processing
The modifications of the pre-mRNA transcript in eukaryotes. * Includes RNA splicing and the addition of the 5' cap and the poly-A tail.
RNA splicing
The removal of introns and the sealing together of remaining exons in the pre-mRNA transcript to form the mature mRNA transcript. * Alternative RNA splicing allows for regulation of gene expression as one pre-mRNA transcript is spliced in multiple ways to produce different protein products.
rRNA (ribosomal RNA)
The globular form of RNA that makes up the ribosome and consists of a large and small subunit. * Coordinates the translation of the mRNA transcript into a protein by reading each mRNA codon on the transcript and positioning the rRNA with the complementary anticodon and respective amino acid.
semiconservative model
The model describing the mechanism by which DNA replicates. * The original double-stranded molecule is opened up and each original strand acts as a template for the building of a new, complementary strand.Each resultant DNA molecule is half-old, half-new; it conserves half of the original molecule and builds the other half.
sex-linked gene
Any gene found on a sex chromosome. * X-linked genes are those located on the X chromosome and are significant in the genetics of mammals and fruit flies (where XX = female and XY = male).
single-strand binding protein
In DNA replication, the proteins that help stabilize the two single strands of DNA after they have been exposed by helicase but before they have been read and paired by DNA polymerase.
spliceosome
A complex structure that interacts with the ends of an intron and serves to splice it out and connect the adjacent exons.
sticky end
The single-stranded ends on a double-stranded DNA molecule after it has been cut by a restriction enzyme. * Useful in gene cloning because pieces of complementary DNA can easily be hydrogen-bonded together.
TATA box
The promoter important in the synthesis of the transcription initiation complex.
telomerase
An enzyme that catalyzes the lengthening of a chromosome along the telomere region. * Prevents the ends of chromosomes from becoming damaged after successive cell divisions.
telomeres
The protective ends of eukaryotic chromosomes comprised of tandemly repetitive DNA.
template strand
The strand of a DNA molecule that is read by RNA polymerase and used as the model to build the complementary mRNA transcript.
terminator
The sequence in a DNA strand at the end of a gene that signals the end of transcription to RNA polymerase.
topoisomerase
In DNA replication, the enzyme that functions to relieve strain in the DNA molecule just before the replication fork.
transcription
The first stage of protein synthesis in which a DNA gene is read and a complementary mRNA transcript is constructed. * RNA polymerase binds at the promoter, opens up the DNA molecule along a short segment, and builds a complementary RNA strand based on the DNA template.
transcription factor
A regulatory protein that binds to a region along DNA near a gene and enhances transcription of that gene.
transcription initiation complex
The complex assemblage of transcription factors and RNA polymerase required to be bound at the promoter before the initiation of transcription can occur.
transduction
The process by which a bacterium picks up new genetic information from another bacterium through a viral intermediate.
transformation
A means of genetic recombination in bacteria in which a bacterium picks up and expresses foreign DNA from other bacteria or from the external environment. * The term used to describe the change in genetic expression witnessed among the bacteria in Frederick Griffith's experiment.
transgenic
An organism containing genes from another species in addition to its own. * Examples include bacteria that are bioengineered with recombinant DNA for gene cloning purposes and genetically modified organisms (GMOs), like many plant foods.
translation
The second phase of protein synthesis during which the mRNA transcript is read by a ribosome and the appropriate tRNA molecules carry in the amino acids to build the protein.
translocation
A type of chromosome mutation in which a segment of a chromosome breaks off and attaches to a non-homologous chromosome.
transposons
A segment of DNA that can move within the genome by means of a DNA intermediate. * Retrotransposons move within the genome by means of an RNA intermediate, specifically an mRNA transcript of the retrotransposon DNA.Discovered by Barbara McClintock during her work with corn kernel color.
tRNA (transfer RNA)
The form of RNA that transfers the appropriate amino acid to the growing polypeptide chain. * A single-stranded molecule with several folds held together by hydrogen bonds between complementary nucleotides along the fold.Possesses an anticodon sequence at one end and an amino acid at the other; the anticodon is complementary to an mRNA codon.
tumor-suppressor genes
Genes that produce regulatory proteins that inhibit cell division and, thus, prevent uncontrolled cell growth. * When subject to mutation, can lose their regulatory function and lead to cancer.
vaccine
An attenuated pathogen or a mimic of a pathogen that stimulates the recipient's immune system to produce antibodies against the pathogen.
viral envelope
An additional protective membrane in some viruses that helps the virus incorporate into the host cell.
viroid
Small pieces of infectious nucleic acid typically found parasitizing plants. * An infectious unit smaller than a virus.
wild type
The phenotype typically observed in nature. * An example is the red-eye trait in Drosophila melanogaster; the white-eye trait is the result of a mutation and is much less common.
wobble
A characteristic of the third base in a tRNA anticodon that allows it to hydrogen-bond to more than one kind of nitrogenous base.
X-ray crystallography
A technique useful for analyzing the complex, three-dimensional molecular structure of large molecules like proteins and nucleic acids. * Employed by Rosalind Franklin in her efforts to determine the molecular structure of DNA.
adaptive radiation
A pattern of divergent evolution in which many species emerge from one ancestor due to an environment with a variety of niches. * An example includes the divergence of finches from the ancestral mainland finch as they populated each distant island of the Galapagos.
allometric growth
The variation in rates of development in various parts of the body in an effort to appropriately coordinate overall development of form. * A result of heterochrony.Observable in humans whose heads and trunks grow at a much slower rate than the appendicular skeleton.
allopatric speciation
Speciation that occurs because members of the same ancestral population have become isolated and exposed to different environmental conditions over time. * If reintroduced, members of the two new populations could not interbreed.
analogous structures
Similar structures in different species that may appear similar externally and serve similar functions but develop from different embryonic origins; evidence of convergent evolution. * Suggest similar environmental pressures resulting in similar adaptations among species in separate lineages.Examples include the wings of a bat, an insect, and a bird.
artificial selection
The process by which humans selectively breed organisms with desired traits and, thus, speed up divergent evolution. * Examples include the breeds of domestic dogs and cats and the domestication of wild mustard into vegetables like Brussels sprouts and broccoli.
average heterozygosity
The percentage of a population's individuals that possess a heterozygous genotype at a given locus.
balancing selection
Natural selection that maintains relatively stable frequencies of at least two phenotypic traits in a population (balancing polymorphism). * Ensures there is variation for evolution to act upon in a given population.
binomial nomenclature
The system of naming each discrete type of organism using its genus and species name. * Examples include E. coli, Tyrannosaurus rex, and Homo neanderthalensis.
biogeography
The distribution of fossils and populations of extant species over geographical ranges.
biological species concept
The definition of a species based on the ability of groups of populations of like organisms to reproduce viable offspring and the inability to do the same with members from other populations.
catastrophism
The hypothesis proposed by Georges Curvier that each stratum in sedimentary rock corresponds to a major catastrophic event in the Earth's history that caused a mass extinction.
clade
An evolutionary branch that includes all descendant species and their shared common ancestor. * The analysis of this is called cladistics.
cladogram
A graphical representation of the number of shared characteristics among closely related species.
cline
The gradual change in phenotype frequencies over a geographical range that mirrors the gradual change in environmental influences. * An example is observed in ear length of rabbits throughout North America; the farther north, the lower the surface-area-to-volume ratio of the ears, designed to minimize heat loss in colder climes.
convergent evolution
A pattern of evolution observed when groups of relatively unrelated organisms possess similar traits or morphs due to evolution in similar environments. * Not evidence of a common ancestor, but instead suggestive of a like response to similar environmental pressures.
descent with modification
The notion that a species makes discrete changes in the collection of traits over time, which accumulates in speciation. * Darwin's initial description for the process of evolution.
directional selection
Natural selection that favors individuals in a population with more extreme versions of a phenotype. * Graphically represented by a bell curve with a peak over the more extreme version of a phenotype.
disruptive selection
Natural selection that favors individuals with either one extreme of a phenotype or the other; the intermediate phenotype is selected against. * Graphically represented by a bimodal curve with two smaller peaks over each extreme version of a phenotype.
endemic
Describes a species that is only found in one discrete geographical location. * Examples include many species observed by Charles Darwin on the Galapagos Islands, like the giant tortoise, the marine iguana, and myriad finch species.
evolution
The change in the frequency of an allele in a population over time (microevolution). * The adaptive changes that accumulate in a species over time and that influence speciation (macroevolution).
evolutionary adaptation
The accumulation of traits that enhance survival and reproduction by organisms living in a particular environment.
fitness
The contribution that an individual makes to the gene pool of the subsequent generation. * A reflection of how well an organism survives and reproduces in a given environment due to its specific collection of genetic traits.
fossil
A trace of a long-dead organism, preserved most often in sedimentary rock. * Can take various forms including a cast, a mold, an imprint, and petrified tissue.The subject of study in paleontology.
fossil record
The extensive collection of data that displays much of the history of life on Earth through the remnants of organisms in sedimentary rock. * Analysis of the distribution of fossils over millions of years through geographic strata supports the theory of evolution via natural selection.
frequency-dependent selection
A decrease in the reproductive success of an organism of a given phenotype due to the increased frequency of that morph in the population.
gene flow
The movement of alleles into or out of a population through migration of individuals. * Immigration refers to individuals entering a population.Emigration refers to individuals leaving a population.
gene pool
The sum of all the genes present in a population at a given time.
genetic drift
Unpredictable changes in allele frequencies due to the very limited size of a population. * An example is the bottleneck effect, observed when the surviving population of a catastrophic event possesses nonrepresentative allele frequencies.Another example is the founder effect, observed when a small number of individuals from a population become isolated and populate a new area.
geographic variation
Observable differences in the gene pools of separate populations or in discrete subpopulations.
gradualism
The notion that evolutionary processes occur in slow, gradual increments over very long periods of time, yet can account for very profound change over the entire history of life on Earth.
Hardy-Weinberg equilibrium
The notion that a large, randomly mating population that is experiencing no mutation, gene flow, or selection is not evolving. * Frequencies of alleles and genotypes, thus, remain constant from generation to generation.Modeled for allele frequencies by p + q = 1.Modeled for genotype frequencies by p² + 2pq + q² = 1.
heterochrony
Evolutionary change that affects the specific timing and/or rate of an organism's development.
heterozygote advantage
Observed when natural selection favors the heterozygote condition (through greater reproductive success) over either homozygote condition. * Serves to maintain variation in a gene pool.
homologous structures
Similar structures in different species that originate from a common ancestor. * May have very different external appearances with specialized functions, yet all have a shared developmental origin; evidence of divergent evolution.An example includes the forelimb of mammals like whale flippers, human arms, and bat wings.
ingroup
The group of taxa being studied cladistically that share some derived traits. * Helps establish evolutionary relatedness between species.
maximum parsimony
The notion that, when investigating various explanations for a phenomenon, the simplest explanation consistent with the data should be considered first. * Useful when establishing evolutionary relationships between species and in explaining evolutionary adaptations.
modern synthesis
The comprehensive understanding of evolutionary theory that integrates population genetics, statistics, biogeography, taxonomy, and paleontology.
molecular clock
A mechanism of determining evolutionary time, based on the notion that some parts of the genome change at relatively constant and predictable rates.
monophyletic
An evolutionary branch that includes one common ancestor and all its descendant species. * A true clade.
morphological species concept
The definition of a species based upon shared physical characteristics. * Used more exclusively for asexual organisms and before genetic information was known about different species.
mutation
A change in DNA that can contribute to evolution by creating a new trait and, thus, altering allele and genotype frequencies. * Often duplication mutations are responsible for important evolutionary change, as seen in the olfactory receptor genes of mammals.
natural selection
The concept explaining how nature (a particular set of environmental conditions and limiting factors present at a given time) determines the phenotypic traits present in a population or species that are best fit. * The mechanism by which biological evolution occurs through differential survival and reproduction of better-adapted individuals.
neutral variation
•The type of variation in phenotypes in a population that does not affect survival or reproduction and, thus, does not play a part in evolution.
outgroup
The taxon or group representing the point of comparison in a cladistic study. * Possesses the most distant evolutionary relationship relative to any of the ingroups.
paedomorphosis
The quality observed when an adult organism retains the juvenile features of an ancestral organism. * A result of heterochrony.Common in some salamanders that possess external gills.
paraphyletic
Groups of species that include the common ancestor and some, but not all, of the descendant species.
phylogenetic tree
A graphical representation of a hypothesis describing the relatedness of different species over evolutionary time. * Constantly undergoes revision as new evolutionary evidence is found and genetic evidence is uncovered.Darwin sketched the first known phylogeny while a naturalist on the H.M.S. Beagle.
phylogeny
The evolutionary history of a species or of a group of closely related species.
phylogram
A phylogenetic tree in which the lengths of the trunks and branches are relative to the number of genetic differences separating different species.
polyphyletic
Groups of species that include members of different ancestral lineages. * All members in the grouping do not share a recent common ancestor.
polyploidy
A chromosomal change in which an organism possesses more than two sets of chromosomes. * Autopolyploidy occurs when an individual from one species produces polyploid gametes and self-fertilizes.Allopolyploidy occurs when two different species interbreed and combine their chromosomes.
population
A discrete collection of individuals of the same species. * The smallest level of biological organization in which evolution can occur.
population genetics
The study of genetic changes in a population from generation to generation. * Examines changes in allele, genotype, and phenotype frequencies.
postzygotic isolation
A reproductive isolating mechanism that prevents a hybrid zygote from ever developing fully and/or successfully reproducing, and, thus, preserves the integrity of each species. * Reduced hybrid viability involves the production of hybrid offspring that are weak and often don't survive to maturity.Reduced hybrid fertility involves the production of viable but sterile hybrid offspring.
prezygotic isolation
A reproductively isolating mechanism that prevents a hybrid zygote from ever forming between members of different species, and, thus, preserves the integrity of each species. * Temporal isolation involves species that mate at seasonally different times.Mechanical isolation involves morphological differences that prevent successful mating between species.
punctuated equilibrium
The notion of evolutionary rates describing short bursts of evolutionary change followed by very long periods of time in which there is little or no change.
reproductive isolation
A condition prohibiting members of one species from reproducing with members of another species. * Acts to maintain the genetic integrity of a species.Includes prezygotic isolating mechanisms that prevent a hybrid zygote from ever forming and postzygotic isolating mechanisms that prevent the hybrid zygote from fully developing, surviving, and/or reproducing.
sedimentary rock
The type of rock composed of sand and other sediment most suitable for preserving fossils.
sexual dimorphism
The characteristic observed among many species in which males and females possess noticeably different morphs. * Often the male is more phenotypically ornate than the female due to intersexual selection; examples include the tails of peacocks and the manes of male lions.Alternatively, the typical body size of a male and female can be very different; an example includes the anglerfish.
sexual selection
The type of natural selection that directly or indirectly favors reproductively advantageous traits. * Intrasexual selection refers to direct competition between members of the same sex for access to members of the opposite sex for mating purposes.Intersexual selection refers to the effects of females choosing select males for mating based on their possession of certain desired traits.
speciation
The origin of a new species due to the accumulation of microevolutionary events over time. * Can be observed as a divergent evolution that occurs in a lineage, including the various species of extinct human species sharing the Homo lineage with extant Homo sapiens.
species
The most exclusive level of biological classification that includes all individuals that are capable of interbreeding and producing viable offspring.
species selection
The theory that the species that survives the longest and produces the most ancestors will direct the major evolutionary trends.
stabilizing selection
Natural selection that favors the intermediate phenotype and selects against the extreme phenotypes. * Graphically represented by a bell curve with an exaggerated peak and with decreased tail regions over each extreme phenotype.
sympatric speciation
Speciation that occurs in spite of a lack of a geographically isolating mechanism. * Common among plants due to polyploidy.
systematics
The study of the biodiversity and relatedness among organisms, both extinct and extant. * Molecular [XXX] focuses on similarities in gene and protein sequences.
taxonomy
The branch of biology concerned with the classification of all organisms into various taxa (levels of classification). * Binomial nomenclature based on an organism's genus and species names was developed by Carl Linnaeus.
uniformitarianism
The principle developed by Charles Lyell that explains that geological processes occurring today are the same as those that occurred at any point in the Earth's history.
vestigial structures
Structures that persist, usually in diminutive form, in a species even though the function has been lost. * Evidence of shared ancestry with species that have maintained the structural and functional integrity of the structure in question.Examples include the human appendix and the whale pelvis.
acoelomate
Any diploblastic organism lacking a body cavity. * Internal organs have direct contact with epithelia for exchange of materials.Examples include members of phylum Platyhelminthes.
algae
Any plantlike protist that is a photoautotroph and possesses photosynthetic pigments and a cell wall. * Green [XXX] (chlorophytes) contain the green pigments chlorophylls a and b, possess a cell wall composed of cellulose, store excess sugars as starch, and are likely the direct ancestors of land plants.
alternation of generations
The life-cycle pattern in which a diploid sporophyte form alternates with a haploid gametophyte form. * Common in plants and many algae.
alveolata
A diverse clade of protists characterized by membrane-bound sacs called alveoli. * Dinoflagellates are aquatic protists possessing two flagella that lie in a groove around the organism's surface; some are responsible for red tides.Ciliates are free-living protists possessing large macronuclei and smaller micronuclei and that use cilia to move and feed; an example includes Paramecium.
amniotes
Clade of tetrapods including reptiles, birds, and mammals and characterized by the presence of the amniotic egg. * Members of class Reptilia are ectothermic and include turtles, crocodiles, alligators, and extinct dinosaurs. Members of class Aves are endothermic and include all birds. Members of class Mammalia are endothermic and include monotremes, marsupials, and placental mammals.
anaerobes
An organism that can or must tolerate an oxygen-lacking environment. * Obligate anaerobes must exist in an oxygen-lacking environment.Facultative anaerobes can exist in either an oxygen-containing or an oxygen-lacking environment.
angiosperms
All seed plants that produce a flower containing the reproductive structures and possibly a fruit for seed protection and dispersal. * Characterized as sporophyte-dominant; the male gametophyte is independent within the pollen grain and the female gametophyte is dependent and comprises the embryo sac within the ovule.Includes the monocots and the dicots.
antibiotics
Medications useful in treating bacterial infections. * Mechanisms of action include interfering with the synthesis of bacterial cell walls, proteins, or DNA.Misuse and overuse have resulted in resistant bacteria (such as MRSA) that are very difficult to treat.
ascomycetes (sac fungi)
Fungi that produce sexual spores in a saclike ascus from a fruiting body called the ascocarp. * Their asexual stage involves production of asexual spores called conidia.Examples include the Neurospora bread mold, morels, and truffles.
basidiomycetes (club fungi)
Fungi that produce sexual basidiospores in the basidium of the fruiting body, called the basiodiocarp. * Examples include mushrooms and puffballs.
bilateral symmetry
An animal body plan with a single line of symmetry (the sagittal plane) dividing the organism into two mirror image halves. * Associated with cephalization, a well developed nervous system, and increased motility.Body plan consists of the ventral (front or stomach-side), opposite the dorsal (back or upper-side) and the anterior (head region) opposite the posterior (tail/foot region).
binary fission
An asexual form of reproduction in prokaryotes in which one parent cell splits to form two identical offspring cells, each with a copy of the parental genome. * Can occur in as quickly as 20 minutes in E. coli.
bryophytes
All nonvascular plants that are also very small and herbaceous. * Characterized as gametophyte-dominant; fertilized gametes produce a small, dependent sporophyte through mitosis.Includes the mosses (phylum Bryophyta), liverworts, and hornworts.
capsule
A sticky layer of carbohydrate or protein present in some prokaryotes external to the cell wall. * Functions in protection and adherence to other cells (such as during infection).
cephalization
The possession of a well-developed head region in an organism where the brain (or ganglia) and many sensory organs are clustered. * Associated with the bilaterally symmetrical body plan.
chemoautotroph
An organism that obtains energy by oxidizing inorganic molecules and requires CO₂ as a carbon source. * Examples include some extremophiles.
chemoheterotroph
An organism that consumes organic molecules to obtain carbon and ATP. * Examples include animals, fungi, protozoans, and many Eubacteria.
coelomate
Any triploblastic animal possessing a fluid-filled body cavity derived from the mesoderm layer during embryonic development. * Completely lined by the peritoneum.Examples include most bilaterally symmetrical animals like annelids, arthropods, and vertebrates.
commensalism
A symbiotic relationship between two organisms in which one organism benefits from the association while the other organism is neither helped nor harmed by it (+/0). * An example includes barnacles populating a whale flipper.
conjugation
In prokaryotes, a process of genetic recombination during which an F⁺ bacterium forms a sex pilus (conjugation tube) with an F⁻ bacterium and transfers a copy of its plasmid, thus conferring the fertility factor to the recipient. * In ciliates, the sexual-reproductive process during which haploid micronuclei are exchanged.
cyanobacteria
A group of photosynthetic Eubacteria that contain the pigments phycocyanin and phycoerythrin. * Used to be misclassified as "blue-green algae."
determinate cleavage
The type of cleavage pattern in which newly formed cells are predetermined and can only become a specific cell type. * A cell separated from the embryo cannot develop into another embryo and will die.Characteristic of most protostomes.
deuteromycetes (imperfect fungi)
Fungi that lack a known sexual form of reproduction. * Includes many molds and yeast.
deuterostome
A true coelomate whose blastopore develops into the anus during the gastrulation phase of embryonic development. * Also displays an indeterminate radial cleavage pattern and bilateral symmetry (echinoderms in the larval stage only).Examples include members of phyla Echinodermata and Chordata.
endospore
When a bacterial cell is exposed to harsh conditions, it produces this thick-coated, resistant structure for protection until the environment becomes more favorable.
euglenozoans
Any organism belonging to a diverse clade of protists that all contain a unique rod within their flagella. * Kitenoplastids include Trypanosoma, responsible for African sleeping sickness.Euglenids are mixotrophs, with two distinct flagella at one end, and include Euglena.
extremophiles
Archaebacteria that live in environments considered extreme to most other organisms. * Includes the thermophiles (heat-loving), halophiles (salt-loving), and methanogens (anaerobic and methane-metabolizing).
foraminiferans (forams)
Marine and freshwater protists that possess porous outer shells called tests, which are segmented and composed of calcium carbonate.
gametangium
The haploid reproductive structure within the gametophyte plant that produces haploid gametes through mitosis. * The male one is called the antheridium and produces sperm; the female one is called the archegonium and produces the eggs.
gametophyte
The multicellular haploid form of a plant or alga. * Created as cells within the sporophyte undergo meiosis to produce and release haploid spores; these germinate through mitosis to create the gametophyte.The dominant form in nonvascular plants like mosses.
germ layer
A major tissue layer produced from the gastrulation phase of embryonic development that gives rise to certain tissues and cells during organogenesis. * Diploblastic organisms possess two germ layers; triploblastic organisms possess three germ layers.
Gram-negative
When treated with a Gram stain, bacteria that stain pink due to their possession of a thin layer of peptidoglycan in their cell wall. * Usually more toxic than their Gram-positive counterparts.
Gram-positive
When treated with a Gram stain, bacteria that stain dark purple due to their possession of a thick layer of peptidoglycan in their cell wall. * Usually less toxic than their Gram-negative counterparts.
gymnosperms
All seed plants that are vascular but whose seeds lack protective flower structures. * Characterized as sporophyte-dominant; the male gametophyte is independent within the pollen grain and the female gametophyte is dependent within the megasporangium.Examples include cycads, ginkgos, gnetophytes, and conifers.
heteromorphic
The condition present within plants and some algae in which the sporophyte and gametophyte forms differ in morphology.
hypha (pl. hyphae)
The filamentous structure comprising the body of a fungus and containing cell walls composed of chitin. * If septa (cross walls) are present, then cells are separate and characterized as septate; if septa are lacking, then cells are continuous and multinucleate and characterized as coenocytic.<b></b>Haustoria are specialized hyphae in parasitic fungi that penetrate their host's tissues.
indeterminate cleavage
The type of cleavage pattern in which newly formed cells are not predetermined based on their location in the developing embryo and can develop into many cell types. * A cell separated from the embryo can develop into another embryo; cells remaining in the initial embryo will compensate for the missing cell.Characteristic of deuterostomes and allows for the formation of identical twins.
invertebrates
Organisms belonging to any animal phyla other than Chordata and lacking a vertebral column. * Includes the major phyla Porifera (sponges), Cnidaria (jellyfish and corals), Platyhelminthes (flatworms), Nematoda (roundworms), Annelida (segmented worms), Arthropoda (arthropods), Mollusca (molluscs), and Echinodermata (echinoderms).
isomorphic
The condition present within plants and some algae in which the sporophyte and gametophyte forms possess the same morphology even though they differ in chromosome content.
karyogamy
The second stage of sexual reproduction in fungi involving the union of the nuclei of the two parent mycelia to produce a new diploid cell.
lichen
The collective body formed from the mutually symbiotic relationship between a fungus and either a photosynthetic alga or a cyanobacterium.
metamorphosis
A developmental phase in some animals that transforms the larva, a free-living, sexually immature form of the organism, into a sexually mature adult. * Notable in many insects (for example, caterpillar to butterfly) and amphibians (for example, tadpole to frog).
mutualism
A symbiotic, interdependent relationship in which both organisms involved benefit from the association (+/+). * An example includes an angiosperm and its pollinator.
mycelium (pl. mycelia)
The interwoven mass of collective hyphae found within the substrate. * Function in the secretion of digestive exoenzymes and the absorption of organic molecules and other nutrients.
nucleoid region
The area in a prokaryotic cell where the circular DNA molecule is located. * Numerous small, circular plasmids might exist outside of this region and provide nonessential but often adaptive genes.
obligate aerobe
An organism that must be in an oxygen-containing environment and that requires O₂ for cellular respiration.
parasitism
A symbiotic relationship between two organisms, in which one organism involved benefits from the association while the other organism is harmed (+/-). * An example includes a pathogenic bacterium and its human host.
pathogenic prokaryotes
Parasitic bacteria that cause disease in a specific host organism. * Some secrete exotoxins, toxic substances that can continue to cause specific effects even in the absence of the bacterium.Others produce endotoxins, toxic cell membrane components of some Gram-negative bacteria that are only released when the bacteria die.
peptidoglycan
The carbohydrate-protein complex characteristic of the cell walls of Eubacteria. * A thick layer is associated with Gram-positive bacteria; a thin layer is associated with Gram-negative bacteria.
photoautotrophs
An organism that obtains energy by using light to fix CO₂ into glucose. * Examples include plants, algae, and cyanobacteria.
photoheterotrophs
An organism that must consume organic molecules to obtain carbon but utilizes light to obtain ATP. * Examples include some marine prokaryotes.
plasmogamy
The first stage of sexual reproduction in fungi involving the union of the cytoplasm of the two parent mycelia. * A mycelium is heterokaryon if its parental nuclei have not yet fused and are distributed randomly; it is dikaryon if each set of parental nuclei pair off in each cell for some time before fusing.
protist
Any eukaryote that is generally unicellular (except in many algae) and is not a true plant, fungus, or animal. * Many are photosynthetic and plantlike (algae); others are motile, heterotrophic, and animal-like (protozoans); and few are heterotrophic and fungus-like (slime molds).
protostome
A true coelomate whose blastopore develops into the mouth during the gastrulation phase of embryonic development. * Also displays a determinate spiral cleavage pattern (with some exceptions, as in the lophophores) and bilateral symmetry.Examples include members of phyla Arthropoda, Mollusca, and Annelida.
pseudocoelomate
Any triploblastic organism possessing a fluid-filled body cavity between the endoderm and mesoderm layers; cavity does not develop from the mesoderm layer during embryonic development. * Examples include members of phyla Nematoda and Rotifera.
pseudopodium (pl. pseudopodia)
An extension in the cell membrane of an amoeba that is created through cytoplasmic streaming and is used for moving and feeding.
radial cleavage
The type of cleavage pattern in which the planes of cell division are oriented in either a parallel or perpendicular fashion relative to the polar axis, producing aligned tiers of cells. * Characteristic of deuterostomes during early embryonic development.
radial symmetry
An animal body plan with multiple planes of symmetry, represented by the radius extending from the center of the oral surface to the outer body surface. * Associated with sessile organisms or bottom-feeders.Possessed by members of phyla Porifera, Cnidaria, and Ctenophora.
radiolarians
Marine protists containing a fused test composed of silica. * Possess pseudopodia called axopodia that radiate from the central body.
seedless vascular plants
All plants that possess specialized vascular tissue containing cells with lignin yet lack the specialized seed as a reproductive structure. * Characterized as sporophyte-dominant with an independent multicellular gametophyte produced through germination of the haploid spore.Include the ferns that are characterized as homosporous and possessing sori, clusters of spores under the leaves.
slime molds
Fungus-like protists that are absorptive heterotrophs. * Plasmodial ones are unicellular and multinucleate, produced by multiple mitotic divisions without cytokinesis, and form a mass called a plasmodium.Cellular ones are predominantly haploid and possess fruiting bodies responsible for asexual reproduction.
spiral cleavage
The type of cleavage pattern in which the planes of cell division are oriented obliquely relative to the polar axis, producing cells that sit in the grooves between cells above and below. * Characteristic of most protostomes during early embryonic development.
sporangium
The diploid reproductive structure within the sporophyte plant that produces haploid spores through meiosis. * Homosporous plants produce one type of spore; heterosporous plants produce two types of spores.The microsporangium produces microspores that create and contain the male gametophyte; the megasporangium produces the megaspore that creates and contains the female gametophyte.
sporophyte
The multicellular diploid form of a plant or alga. * Created when a gamete from the haploid gametophyte form is fertilized with another gamete; through mitosis and differentiation this reaches the mature form.The dominant form in angiosperms, gymnosperms, and seedless vascular plants.
stramenopila
A diverse clade of protists characterized by flagella with fine, hairlike projections on their surface. * Oomycetes are characterized by a diploid existence and cell walls made of cellulose; examples include the water molds and mildews.Diatoms are unicellular algae characterized by a glass-like wall composed of silica.Brown algae are multicellular and include kelp and other seaweeds.
thallus (pl. thalli)
The plantlike body of a seaweed lacking true roots, stems, and leaves. * Instead possesses a rootlike holdfast, a stemlike stipe, and a leaflike blade.Floats often are used to hold the stipe upright and help the blades float close to the surface to receive sunlight.
three-domain system
A newer system of classification using three larger taxa called domains to further classify the kingdoms of life. * Domain Archea includes all Archaebacteria, domain Bacteria includes all Eubacteria, and domain Eukarya includes all eukaryotes (that is, Protista, Plantae, Fungi, and Animalia).
transduction
A process of genetic recombination in prokaryotes during which a bacteriophage transfers a piece of bacterial DNA from the genome of its previous host to the successive bacterial host.
transformation
A process of genetic recombination in prokaryotes during which a bacterium picks up and expresses foreign DNA from its environment. * The process observed by Frederick Griffith during his experiments with pneumonia-causing bacteria and mice that led to the understanding that DNA, not protein, is the genetic material.
vascular plants
Plants that contain specialized conductive vascular tissues called xylem and phloem. * Includes the seedless vascular plants (such as lycophytes and pterophytes/ferns) and seed plants (such as gymnosperms and angiosperms).
vertebrates
Organisms belonging to phylum Chordata and subphylum Vertebrata that possess a vertebral column supporting and protecting the nerve cord. * Includes the major classes Chondrichthyes (cartilaginous fishes), Osteichthyes (bony fishes), Amphibia (amphibians), Reptilia (reptiles), Aves (birds), and Mammalia (mammals).
zygomycetes
Fungi that produce sexual zygospores of either + or - mating type from the zygosporangium. * An example includes the black bread mold.
abscisic acid (ABA)
A plant hormone that antagonizes the action of the growth hormones (for example, auxin). * Functions in the inhibition of growth, the promotion of seed dormancy, and the closing of stomata during water stress.
adventitious
The characteristic describing a plant organ that grows from an unexpected location.
aggregate fruit
An aggregate fruit structure that develops from many separate carpels on a single flower. * Examples include strawberries and blackberries.
annual
A type of angiosperm (flowering plant) that completes a reproductive cycle from germination to flowering to reproduction to death in a single year. * Examples include cereal grains, legumes, and wildflowers.
apical dominance
The characteristic observed in plants that encourages growth upward toward the sun in the shoot system and downward with gravity in the roots over lateral growth outward in either the root or the shoot system.
apical meristem
The region of active mitosis in a plant concentrated near the tips of the roots and the shoots. * Results in primary growth, the increase of the length of the stem and roots.
apoplast
The continuum of cell walls and extracellular spaces between adjacent plant cells. * The route that some water and dissolved solutes take when moving from the soil toward the root vascular cylinder; stops at the endodermis and is rerouted to the symplast.
auxin
A major plant hormone highly concentrated in the apical meristems of shoots and roots and responsible for cell elongation. * Functions in promoting phototropism and gravitropism, enhancing apical dominance, and encouraging root and stem growth.
axillary bud
Any growth of new tissue at the node of a stem where new branching can occur. * Usually dormant in a young shoot.
bacteroid
The collective form that nitrogen-fixing Rhizobium bacteria take when living within the nodule of a legume root. * Surrounded by a vacuole formed by the plant root cell.
bark
All tissues external to the vascular cambium in a woody plant including the secondary phloem and periderm.
carpel (pistil)
The collective female reproductive structure of the flower that consists of the stigma, style, and ovary. * The sticky stigma receives the pollen during pollination and the pollen travels down the necklike style to reach the ovary, where the egg is housed.
cation exchange
The process by which H<font face="Arial Super">+</font> ions in the soil displace mineral cations from clay particles, making them available to the plant for absorption.
circadian rhythms
Physiological cycles with a duration of approximately 24 hours and under internal control, not in response to environmental stimuli. * Examples include sleep movements in the leaves of some bean plants.
collenchyma
One of three generalized plant cell types; alive at functional maturity and characterized by irregularly thick primary cell walls and the absence of secondary walls. * Functions in flexible support of the plant body.
companion cell
A nonconductive cell adjacent to a sieve tube member that retains its nucleus and controls the activity of that sieve tube. * Assists in the active loading and unloading of sugar into and out of the sieve tube.
cytokinins
A group of plant hormones produced in the roots and transported throughout the plant to stimulate cytokinesis. * Function in the control of cell division and apical dominance and in the slowing of the aging process.
dermal tissue
One of three general plant tissue types; comprises the outer covering of the plant in either the epidermis (in herbaceous plants) or the periderm (in woody plants). * Functions in protection of the plant from the external environment and in regulating gas exchange and transpiration through specialized guard cells.
dicot
A clade of angiosperms (flowering plants) that features two cotyledons (seed leaves). * Characterized by flower parts in multiples of four or five, netlike leaf venation, vascular bundles arranged in a ring in the stem, and a central stele of xylem surrounded by phloem within the root.
dioecious
A type of plant that contains either male or female reproductive structures on an individual organism. * Analogous to separate sexes present in animals with distinct males and females.
dormancy
The state of seeds after dissemination and before germination during which the embryo experiences suspended growth and development and a very low metabolic rate. * Conditions required to break dormancy for germination vary from species to species, some requiring a stable environment and others requiring a specific environmental cue (such as fire).
double fertilization
The process in angiosperm reproduction in which the pollen grain contributes one sperm nucleus to fertilize the egg and another to combine with the two polar nuclei to create the triploid endosperm.
embryo sac
The female gametophyte in angiosperms that develops within the ovule and consists of the egg, three antipodal cells, two polar nuclei, and two synergid cells. * Site of the development of the seed after fertilization.
endodermis
The innermost layer of cells of the root cortex that surrounds the root stele. * Lined with a waxy Casparian strip, which forces water from the apoplastic route to the symplastic route.
endosperm
The triploid nutritive tissue present in the seeds of angiosperms that develops from the fusion of the two polar nuclei in the embryo sac and one of the sperm nuclei from the pollen grain. * Designed to develop with the embryo and provide sustenance for the seedling.
epicotyl
In an angiosperm seed, the embryonic axis above the point of cotyledon attachment. * Terminates in the embryonic shoot tip.
epidermis
The outer protective tissue layer in herbaceous plants comprised of tightly packed dermal tissue. Secretes a waxy cuticle to waterproof the shoot system and prevent desiccation. Replaced by the periderm in a mature woody plant.
epiphyte
A specialized type of plant that grows upon the branches of trees in the canopy. Its roots never make contact with the soil and instead just anchor the plant to its host. Absorption of water and minerals occurs primarily across leaf cells.
essential nutrient
A nutrient that an organism must absorb or ingest that is necessary for metabolism but cannot be synthesized. Macronutrients are needed in large quantities and include all the major building blocks of organic monomers and potassium, calcium, and magnesium. Micronutrients are required in small doses, are often toxic in excessive quantities, and include chlorine, iron, and manganese.
ethylene
A plant hormone released as a gas, often in response to stress. * Functions in fruit ripening, the avoidance of mechanical stress through the triple response, leaf abscission, and apoptosis.
flower
The reproductive structure in angiosperms that consists of the reproductive stamens and carpels and the sterile petals and sepals. * Considered complete if it contains all four basic floral organs and incomplete if it lacks one or more.
fruit
Nutrient-rich, mature ovary of a flower that functions in protection and dispersal of the dormant seed.
gibberellins
A group of plant hormones concentrated in young plants and emerging organs. * Functions in the promotion of seed and bud germination, flowering, and the growth of the root and stem.
gravitropism (geotropism)
A plant movement in response to the force of gravity, regulated by the hormone auxin, and significant in the germination of a seedling. * Shoots exhibit negative gravitropism; roots exhibit positive gravitropism.
ground tissue
One of three general plant tissue types; comprises the bulk of the plant body and includes any cells that are not vascular or dermal derivatives. If internal to the vascular tissue, it's called pith; if external to the vascular tissue, it's called cortex. Functions in support, storage, and metabolism (for example, photosynthesis).
guard cell
The cells that flank each stoma and regulate gas exchange and transpiration. * The active transport of K<font face="Arial Super">+</font> into the cell causes water to follow and the cell to become turgid, so the stoma opens.
heat-shock protein
Proteins produced by plants in response to heat stress that act to minimize or prevent denaturation of enzymes and other proteins.
hypocotyl
In an angiosperm seed, the embryonic axis just below the point of cotyledon attachment. * Terminates in the radicle, the embryonic root.
imbibition
The act of a seed experiencing a vast influx of water due to its low water potential. * Results in the breaking of dormancy and the initiation of seedling germination.
lateral meristem
The region of active mitosis in woody plants along the length of the stem where primary growth has stopped. Includes the vascular cambium (which adds new vascular tissue) and the cork cambium (which replaces the epidermis with periderm tissue). Results in secondary growth, the increase in the thickness of the stems and roots.
lateral roots
Roots arising from the pericycle of established roots that branch out horizontally from the taproot. * Function to increase the surface area of the roots in the soil for maximum support and absorption.
leaf
One of three major plant organs; generally broad and flat to maximize sunlight absorption and gas exchange. Typically composed of a stem-like petiole, a flattened blade for photosynthesis, and internal vascular tissue called veins. Modifications to the typical leaf include spines, tendrils, bracts, and storage leaves.
loam
Very fertile soil composed of roughly equal parts of sand, clay, and silt. * The fine nature of some particles provides a very large surface area for retention of nutrients; the coarse nature of other particles provides for sufficient air spaces for oxygen and soil drainage.
meristem identity gene
Regulatory genes in plants that code for transcription factors that regulate the conversion of indeterminate vegetative meristems into differentiated flowering meristems.
mesophyll
The ground tissue layer in leaves between the upper and lower epidermis. Palisade mesophyll includes the upper, elongated cells containing many chloroplasts. Spongy mesophyll includes the lower, loosely arranged cells punctuated by leaf veins and air spaces.
mineral nutrient
An essential chemical nutrient absorbed by a plant through its roots as a dissolved mineral ion. * An example in plants includes the nitrate ion (NO<font face="Arial Sub">3</font><font face="Arial Super">-</font>).
monocot
A clade of angiosperms (flowering plants) that features a single cotyledon (seed leaf). * Characterized by flower parts in multiples of three, parallel leaf venation, scattered vascular bundles in the stem, and vascular bundles arranged in a ring in the root.
monoecious
A type of plant that contains both male and female reproductive structures on an individual organism. * Analogous to hermaphroditic animals that possess both male and female sex organs.
multiple fruit
A single fruit structure formed from the fusion of multiple segments produced from the carpels from many clustered flowers (those displaying influorescence). * Examples include pineapples and figs.
mycorrhizae
The mutually beneficial, symbiotic relationship between a fungus and plant roots. The fungus helps increase the surface area of the root for increased absorption of nutrients; the plant provides sugars. If the fungal hyphae extend between the cell walls of the root, then it is called ectomycorrhizae; if instead the hyphae grow within the root cells, then it is called endomycorrhizae.
nitrogen-fixing bacteria
Soil-dwelling prokaryotes that convert atmospheric nitrogen (N<font face="Arial Sub">2</font>) into ammonia (NH<font face="Arial Sub">3</font>) through the process of nitrogen fixation. * Possess the enzyme nitrogenase that catalyzes the successive reduction of nitrogen.
nodule
The hollowed-out region within the root epidermal cells of a legume within which symbiotic nitrogen-fixing Rhizobium bacteria live.
organ identity gene
Homeotic genes in plants that code for transcription factors that regulate the development of floral organs from an emerging leaf using positional information.
ovule
The female reproductive structure within the ovary responsible for production of the megaspore through meiosis and then its development into the egg and the other structures in the embryo sac.
parenchyma
One of three generalized plant cell types; alive at functional maturity and characterized by thin primary walls and the absence of secondary walls. * Functions in metabolic activities (for example, photosynthesis) and storage of organic products.
perennial
A type of angiosperm (flowering plant) that completes many reproductive cycles over the course of a long life span and usually dies via infection or environmental trauma. * Examples include trees, shrubs, and many grasses.
pericarp
The thickened, often edible wall of a fruit produced from the mature ovary.
pericycle
The outermost layer of the root vascular cylinder from which later roots emerge.
petal
A sterile modified leaf that serves to attract animal pollinators to the flower and encourage pollination.
phloem
The type of vascular tissue in plants composed of living cells at functional maturity and designed to carry out the transport of sugars from the source (leaves or herbaceous stems) to the sink (either roots for storage or other cells for use).
photoperiodism
A physiological response observed in plants in response to photoperiod. Short-day plants only flower when the photoperiod is below a critical length; long-day plants only flower when the photoperiod is above a critical length. Vernalization (lengthy exposure to cold temperatures) is required to initiate flowering in other plants.
phototropism
A plant movement in response to the stimulus of light and action of the hormone auxin. * Plants exhibit positive phototropism when they bend their stems and reposition leaves to move toward the sun.
phytochrome
Photoreceptors that absorb mostly red wavelengths of light and regulate myriad plant responses to light through kinase activity. * Involved with seed germination, control of flowering, and shade avoidance.
pollination
The act of pollen (male gametophyte) transferring to the stigma of a flower. * Initiates sexual reproduction in gymnosperms and angiosperms if followed by fertilization.
proton pump
The most significant transport protein in plant cells that uses ATP to move H<font face="Arial Super">+ </font>ions against the gradient. * Responsible for the maintenance of a proton gradient and, thus, a membrane potential.
root
One of three major plant organs. Functions in anchoring the plant into the soil, absorbing water and minerals, and often storing excess sugars. Modifications on the basic root include prop roots, storage roots, strangling aerial roots, and buttress roots.
root cap
The protective layer of cells just beneath the root apical meristem. * Aids movement through the soil by secreting a polysaccharide slime.
root hairs
The thin extensions of root epidermal cells that act to increase the surface area of the root and maximize absorption of water and dissolved minerals.
root pressure
The upward push of xylem sap due to the influx of water into the root vascular cylinder. Results from the accumulation of mineral ions in the root xylem at night when transpiration is minimal. Guttation can result when more water enters the leaf than transpires from it, forcing liquid water to exude from the tips of leaves.
root system
One of two major plant organ systems including the roots and other below-ground structures. Functions to anchor the plant, absorb water and minerals, and store excess food. A taproot system consists of one major vertical root from which many smaller lateral roots branch. * A fibrous root system consists of a mat of thin roots extending in all directions just below the soil surface.
sclerenchyma
One of three generalized plant cell types; dead at functional maturity and characterized by thick, lignin-impregnated secondary cell walls. * Functions in rigid support of the plant body, including structures such as sclerids and fibers.
seed coat
The protective outer covering in angiosperm seeds formed from the integuments of the ovule. * Prevents desiccation during seed dormancy.
self-incompatible
The characteristic of some plants that prevents self-fertilization by rejecting their own pollen.
sepal
A sterile modified leaf that encloses and protects a flower bud.
shoot system
One of two major plant organ systems containing the stems, the leaves, and any other above-ground modifications of those organs.
sieve tube member
The tubelike structure that permits that flow of phloem sap (dissolved sugars) from the site of photosynthesis to the rest of the plant body. * A sieve plate is an end plate positioned at the end of each sieve tube containing perforations for flow of materials.
simple fruit
A fruit structure consisting of one complete fruit that developed from a single carpel or from several fused carpels. * Examples include most fruits, from apples and bananas to peas and lemons.
spore
A haploid cell produced through meiosis in the sporangia of a sporophyte plant. In angiosperms, germinates through mitosis to produce the multicellular gametophyte within the ovary. Microspores are smaller spores produced by the microsporangium in the anthers. * Megaspores are larger spores produced by the megasporangium in the ovule of the ovary.
stamen
The collective male reproductive structure of the flower that consists of the filament and the anther. The anther contains microsporangia that produce microspores that develop into pollen. The filament props up the anther for maximum access by pollinators.
stele
The aggregate vascular tissue in stems and roots present either in one large vascular cylinder (for example, dicot root) or in smaller discrete vascular bundles (for example, dicot stem).
stem (shoot)
One of three major plant organs. Functions to prop up the photosynthetic leaves toward the sun and contains vascular tissue for circulation of nutrients. Consists of alternating nodes (branching sites) and internodes (lengths along the stem lacking branches). * Modifications on the typical stem include stolons, rhizomes, bulbs, and tubers.
stoma (pl. stomata)
Pores concentrated on the underside of many plant leaves that permit gas exchange and transpiration. * Regulated by the action of guard cells that surround each opening.
symplast
The continuum of cytoplasm between adjacent plant cells connected via plasmodesmata. * The route that some water and dissolved solutes take when entering the root vascular cylinder from the soil.
terminal bud
Any growth of new tissue at the terminal end of a shoot. * Usually active in young shoots.
thigmotropism
The type of plant movement in response to the touch stimulus. * Observed in the growth of vines and the rapid turgor movements in the sensitive Mimosa plant.
tonoplast
The membrane that surrounds the large central vacuole in mature plant cells.
topsoil
A mixture of soil particles derived from rock, living organisms, and humus (partially decayed organic materials). * The uppermost horizon (distinct soil layer).
tracheids
One form of xylem vessels that are long and thin, with tapered end plates containing pits (regions lacking a secondary wall that allow for continuity of water flow from cell to cell). * Characteristic of all vascular plants.
transfer cell
A specialized companion cell possessing numerous ingrowths in its cell wall to enhance the transfer of solutes from apoplast to symplast.
translocation
The movement of sugars throughout the plant body via phloem tubes. * Typical movement is from leaves (a source, a net producer of sugar) to cells throughout the stem and finally to the roots for storage (a sink, a net consumer of sugar).
transpiration
The process by which water evaporates out of the stomata of the leaves. Generates a pull on the remaining water molecules in the leaf's mesophyll cells and indirectly on the water in the xylem tubes throughout the body of the plant. Drives the ascent of xylem sap.
tropism
Any plant movement toward or away from a stimulus. * Movement toward a stimulus is a positive tropism; movement away from a stimulus is a negative tropism.
turgor pressure
The force placed against the cell wall after the influx of water and the swelling of the cell. * If the pressure is high, the cell is turgid.
vascular tissue
One of three major tissue types in plants consisting primarily of elongated, tubular cells designed for the transport of materials throughout the plant body. * Includes specialized cells of the xylem and phloem.
vegetative reproduction
Asexual reproduction in plants, including fragmentation. * Utilized in techniques like grafting and cloning.
vessel elements
Xylem vessels that are shorter and wider with large perforations on the end plates for more efficient flow of xylem sap. * Characteristic of all angiosperms and some gymnosperms and seedless vascular plants.
water potential
A measure of the combined effects of solute potential and the physical pressure of water exerted on the cell wall of plants. * Determines the direction of water flow (from regions of higher water potential to regions of lower water potential).
xylem
The type of vascular tissue in plants composed of dead cells at functional maturity. * Arranged in a tubular fashion and designed to carry out the transport of water and dissolved materials from the roots to the rest of the plant body.
xylem vessel
A tubelike structure created from vessel elements aligned end to end. * Permits the flow of water and dissolved minerals through the plant body.
zone of cell division
The region near the tip of the root where active mitosis occurs. * Includes the root apical meristem and its derivative cells.
zone of elongation
The region in the root just above the zone of cell division in which newly divided cells elongate. * Results in growth of the length of the root and, thus, extension farther into the soil.
zone of maturation
The region in the root just above the zone of cell elongation in which newly elongated cells begin to mature structurally and functionally through differentiation.
absorption
The third stage in the digestive process that involves transporting small organic molecules and other nutrients from the lumen into the body's cells.
accessory digestive glands
Structures that are not directly part of the alimentary canal but that produce and secrete digestive products into the canal. * Include the salivary glands in the mouth (secrete salivary amylase for starch digestion), the pancreas (secretes pancreatic juices into the small intestine), the liver (produces bile and secretes it into the small intestine), and the gallbladder (stores excess bile).
acclimatization
A physiological adjustment to an environmental change. * Examples include the seasonal change in coat color (from white to brown) in the arctic fox and the use of individual baggies to introduce new fish to the established temperature of a fish tank.
action potential
The rapid change in membrane potential in a neuron involving the selective opening and closing of Na<font face="Arial Super">+ </font>and K<font face="Arial Super">+</font> channels. * Occurs as a sequence of depolarization until the threshold is reached &#8594; continued depolarization during the rising phase &#8594; repolarization during the falling phase &#8594; hyperpolarization during the undershoot phase.
active immunity
Immunity conferred by natural exposure to a specific pathogen or by vaccination (or immunization); involves the actions of an individual's lymphocytes and memory cells. * Contrasts with passive immunity, in which antibodies from an infected individual are transferred to an individual who has not been exposed to the pathogen.
adrenal gland
Paired endocrine glands that are positioned just above the kidneys. The adrenal medulla region produces epinephrine and norepinephrine to control the fight-or-flight response to stress. The adrenal cortex region produces corticosteroids, which help the body cope with chronic stress (including glucocorticoids, which increase blood glucose levels).
anaphylactic shock
A systemic, life-threatening, allergic response that involves widespread mast cells' degranulation, rapid vasodilation of peripheral blood vessels, and a sudden and dangerous decrease in blood pressure. * Epinephrine is used to therapeutically counteract this response.
atrioventricular (AV) node
Located in the septum between the right atrium and ventricle. * Responsible for sending a nervous impulse via the bundle branches and Purkinje fibers for contraction of the ventricles and movement of blood out of the heart.
autoimmune disease
Any disease that results from the body's inability to recognize self and the initiation of an immune response against the body's own tissues. * Examples include lupus, rheumatoid arthritis, and insulin-dependent diabetes mellitus.
B lymphocyte (B cell)
A lymphocyte that has receptors composed of two heavy chains and two light chains linked by disulfide bridges and specific to a given antigen. * If triggered, participates in humoral immunity (antibody-mediated immunity) by encouraging the proliferation of specific plasma cells and their successive production of antibodies.
bile
The substance produced by the liver, stored by the gallbladder, and secreted into the small intestine via the hepatic duct. * Contains bile salts that function in the emulsification of fat globules into fat droplets in preparation for further digestion by lipases.
blood
A fluid connective tissue consisting of aqueous plasma, erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (cell fragments important in clotting). * The medium by which nutrients and wastes circulate around the organism internally.
blood pressure
The pressure exerted on the walls of a blood vessel. * Clinically measured in humans in the brachial artery of the arm and presented as the systole (ventricular contraction pressure) over the diastole (ventricular relaxation pressure).
blood vessel
A tube within which blood travels in an animal's circulatory system. Arteries are under high pressure and are elastic and contractile; veins are under low pressure and possess valves to prevent backflow. Capillary beds are the site of exchange of nutrients and wastes. * Blood travels from aorta &#8594; arteries &#8594; arterioles &#8594; capillaries &#8594; enuoles &#8594; veins &#8594; vena cava.
cardiac cycle
One complete round of contraction and relaxation&#8212;and, thus, pumping and filling&#8212;of the heart. * The number of heartbeats per minute is called the heart rate, which can be measured by feeling one's pulse (rhythmic arterial contractions).
cardiovascular disease
Any disorder of the heart and/or blood vessels. * Includes heart attack (cardiac muscle death due to coronary artery blockage), stroke (death of brain tissue due to blockage of the arteries in the brain), atherosclerosis (hardening of the arteries due to arterial plaque buildup), and hypertension (high blood pressure).
CD4
A surface protein present on most helper T cells and memory helper T cells that encourages binding between the T cell and the antigen-presenting cell during activation. * <I>CD8</I> is a similar surface protein present on the surface of cytotoxic T cells that enhances the interaction between it and the target cell.
circulatory system
The assemblage of structures including the circulatory medium, a series of tubes, and a pump to allow internal movement of substances. Characterized as open if the hemolymph is not always contained within vessels and instead bathes organs directly in the hemocoel and sinuses. Characterized as closed if blood is always contained in vessels and exchange only occurs at the capillaries.
clonal selection
Antigen-triggered cloning of a specific B or T lymphocyte, encouraging the cloning of respective memory cells and effector cells that also target that specific antigen. If this is occurring for the first time for this pathogen, it is called the primary immune response. If an individual has been exposed to the same pathogen before, this is called secondary immune response.
complement system
Proteins that are activated by the presence of invading microbes and stimulate the lysis of the invading cell. * Interferons are proteins secreted by cells infected with a virus that encourage neighboring cells to take protective antiviral actions.
complete digestive tract
A tube with two openings, specialized structures, and a directional flow of food and digestive products. * In mammals, involves food and digestive products moving from mouth &#8594; pharynx &#8594; esophagus &#8594; stomach &#8594; small intestine &#8594; large intestine (colon) &#8594; rectum &#8594; anus.
connective tissue
One of four animal tissue types composed of cells arranged or scattered in a particular extracellular matrix that function in binding and support. Characterized as loose (LCT), fibrous (FCT), adipose, cartilage, bone, and blood. Fibers can be collagenous, elastic, or reticular.
cytotoxic T cell
The T cells actively involved in cell-mediated immunity by destroying infected, cancerous, or foreign cells. Triggered as antigens and cytokines activate the cell and encourage proliferation of memory cytotoxic T cells and active cytotoxic T cells. Helper T cells are activated by antigens presented by dendritic cells and secrete cytokines that encourage cytotoxic T cell activity.
diabetes mellitus
A chronic disease resulting from an excess of blood sugar due to dysfunctional or insufficient levels of insulin. Type I diabetes, caused by inheritance of a gene, results from an autoimmune disorder that destroys the alpha cells of the pancreas. Type II diabetes, caused by obesity and/or physical inactivity, results from insufficient levels of insulin and/or insensitive insulin receptors.
diaphragm
The skeletal muscle that is positioned between the thoracic and abdominal cavity. * Contracts during inspiration and acts to increase the volume of the thoracic cavity.
digestion
The second stage of the digestive process, involving the breakdown of large organic polymers found in food into small organic molecules that can be absorbed. Classified as mechanical if it involves the physical breakdown of food using teeth and other specialized structures. Classified as chemical if it involves the breakdown of food into smaller molecules using hydrolytic enzymes.
elimination
The final stage in the digestive process. * Involves the removal of waste products and any material left unabsorbed from the lumen to the external environment.
endocrine system
The assemblage of endocrine glands that produce and, via neurosecretory cells, secrete hormones, chemical messengers that act on specific target cells and initiate a response. * In humans, the endocrine glands include the hypothalamus, pituitary gland, thyroid gland, parathyroid glands, pancreas, adrenal glands, gonads, and pineal gland.
endoskeleton
An internal supportive and protective structure comprised of a framework of smaller components that connect at movable regions called joints and assist in movement of the organism. * Composed of cartilage in class Chondrichthyes; composed mainly of bone in other major vertebrate classes.
epithelial tissue
One of four animal tissue types composed of tightly packed cells attached to a basement membrane that function as a protective lining and barrier. Characterized as simple if arranged in a single layer or stratified if arranged in multiple layers. Can be cuboidal (cube-like), columnar (elongated), or squamous (flattened) in shape.
essential nutrients
Nutrients that an organism must consume and cannot synthesize, including energy-giving nutrients and certain vitamins and minerals. When an organism lacks one or more essential nutrients, malnourishment results. Undernourishment results when an organism is calorie-deficient; overnourishment results when an organism has excessive stored calories.
estivation
A physiological state during extended periods of very hot temperatures and decreased water supplies during which activity and the metabolic rate slow considerably.
excretion
The removal of nitrogenous metabolic waste products. Occurs through the process of filtration, reabsorption, secretion, and excretion. Involves the production of concentrated urine by the human kidney. * Invertebrate structural adaptations include the use of flame-bulb systems, metanephridia, and Malphigian tubules.
exoskeleton
An external supportive and protective structure comprised of layers of secreted tissue. * Contains chitin in arthropods and must be molted to allow for growth.
filtration
The process of components of blood being forced into the excretory tubule for eventual reabsorption or excretion. * The filtrate forms and becomes increasingly concentrated as necessary substances are selectively reabsorbed into the bloodstream; is finally excreted from the body.
ganglion (pl. ganglia)
A cluster of neurons in the central nervous system that function together to help communicate and coordinate information to the peripheral nervous system. * Common in arthropods, annelids, and molluscs.
gas exchange
The process of taking up necessary O<font face="Arial Sub">2</font> and releasing metabolic wastes like CO<font face="Arial Sub">2</font>. Takes place passively across a respiratory surface. The gases are supplied externally in the respiratory medium (for example, air for terrestrial animals and water for aquatic animals).
gastric juices
A combination of fluids secreted by the stomach including hydrochloric acid (HCl), the inactive enzyme pepsinogen, and mucus. The HCl lowers the pH to activate pepsinogen to pepsin for protein digestion. When mixed with the bolus of food from the esophagus, creates chyme.
gastrovascular cavity
A sac with a single opening that functions in digestion and distribution of nutrients throughout the organism. * The site of digestion for organisms in phyla Cnidaria and Platyhelminthes.
gill
The respiratory surface present in marine and freshwater fishes and many invertebrates specialized in the extraction of dissolved O<font face="Arial Sub">2</font> from the water. * Fishes utilize a highly adaptive countercurrent exchange mechanism to maximize the absorption of O<font face="Arial Sub">2</font> because it is in much lower concentrations in water relative to air.
gill circulation
The circuit common in fishes that brings oxygen-poor blood from the heart to the gills to pick up additional dissolved oxygen from the water, and moves oxygen-rich blood away from the gills and into the systemic circuit.
glandular epithelium
Epithelial cells specialized for the absorption and/or secretion of a particular product. * An example includes the cells of the respiratory tract that secrete a mucous membrane.
glia
Cells in the nervous system that serve an essential supportive role to neuron functioning. * Include the specialized Schwann cells, astrocytes, oligodendrites, and radial glia.
heart
Pump composed of cardiac muscle and generating its own electrical impulse for regular contraction and propulsion of blood throughout the vessels. * Any upper chamber is called the atrium (pl. atria); any lower chamber is called the ventricle.
heterotrophy
The mode of energy acquisition found in all animals involving the consumption of organic molecules. Involves suspension, substrate, fluid, and bulk feeding mechanisms. Includes herbivorous, carnivorous, and omnivorous diets.
hibernation
A physiological state during extended times of very cold temperatures and low food supplies during which the metabolic rate slows dramatically, the average body temperature lowers, and heart and respiratory rates decrease.
hydrostatic skeleton
A form of internal support in many invertebrates that utilizes fluid held under pressure in a closed body cavity. * The major skeletal component in most worms and cnidarians and permits peristaltic locomotive movements.
hypothalamus
A vertebrate endocrine gland in the brain that secretes products that control the action of the neighboring pituitary gland. * Function is significant in the integration of the endocrine and nervous systems.
immunoglobin
Secreted antibodies that are structurally similar to B cell receptors but lack the structure that allows them to be embedded into a membrane. * Composed of two identical heavy chains and two identical light chains bound by disulfide bridges.
inflammatory response
A second-line, nonspecific immune response triggered by the release of chemicals from injured or infected cells. * Histamine, stored in mast cells, triggers vasodilation and increased permeability of capillaries and leads to localized redness, swelling, and heat.
ingestion
The first stage of the digestive process involving the mechanics of eating and the initial consumption of organic macromolecules.
integration
The interpretation of external stimuli via sensory input by special processing centers of the central nervous system.
interstitial fluid
The internal, fluid environment of vertebrates that surrounds the body's cells and is used in the exchange of substances and homeostasis. * Regulators use negative feedback to carefully monitor and adjust internal conditions in a variable external environment; conformers allow their internal conditions to be variable and reflective of the environment.
lateral line system
An adaptation possessed by fishes and aquatic amphibians consisting of mechanoreceptors positioned along the length of the body that detect movements made in the water by other organisms.
leukocytes (white blood cells or WBCs)
A diverse group of blood cells specialized in immune response. * Include phagocytic WBCs that consume pathogens and natural killer cells that patrol the body for infected cells and release a chemical into the cells that encourage apoptosis (cell death).
limbic system
A region in the mammalian forebrain that interacts with the cerebral cortex to interpret emotions. * Composed of the amygdala and the hippocampus.
long-term memory
The ability of central nerve system cells to hold and retain information over an extended period of time, generally the organism's lifetime. * Requires coordination with the hippocampus.
lung
The site of respiration in terrestrial vertebrates comprised of tubes leading from the nose and mouth that transport air to the alveoli where O<font face="Arial Sub">2</font> and CO<font face="Arial Sub">2</font> are exchanged. * Upon inspiration, air travels from mouth &#8594; pharynx &#8594; trachea &#8594; bronchus &#8594; bronchioles &#8594; alveolus.
lymphatic system
A system of vessels of varying size that is responsible for reabsorbing fluid lost from the circulatory system and returning it via the lymph fluid. * Lymph nodes are regions located along the lymph vessels that filter lymph fluid and assist in immune defense.
major histocompatibility complex (MHC)
A set of genes that code for MHC cell surface proteins that function in antigen presentation to T cells. Class I MHC molecules are presented after a cell becomes injured or cancerous and trigger cytotoxic T-cell response. Class II MHC molecules are presented on macrophages, dendritic cells, and B cells and trigger helper T-cell response.
Malphigian tubules
The excretory structure utilized by insects and other terrestrial arthropods. Composed of outpockets of the digestive tract that remove nitrogenous wastes from the hemolymph and function in osmoregulation.
membrane potential
The difference in charge between the cell's cytoplasm and the cell's extracellular fluid due to the differential distribution of ions. * Maintained via the work of the sodium-potassium pump and changes through the selective opening and closing of gated ion channels.
memory cells
Long-living cells of the immune system that preserve the antigens for a specific pathogen on its surface. * Exist for both B cells and T cells.
metabolic rate
The sum of all of the energy-requiring reactions occurring within an organism in a given length of time. * Is relatively high in endotherms, which carefully regulate their internal temperature; is relatively lower in ectotherms, which gain their heat energy from the external environment.
metanephridium (pl. metanephridia)
A tubular excretory system utilized by Annelids that terminates in nephridiopores that empty wastes into the external environment. * Functions in excretion and osmoregulation.
motor neuron
One of three major types of neurons that function in the communication of a nervous impulse to a specific effector cell to initiate a desired response. * Can connect to muscles to trigger movement or a gland to stimulate secretion of a product.
muscle tissue
One of four animal tissue types composed of specialized cells that can shorten (contract) and allow for movement. * Characterized as skeletal (voluntary), smooth (involuntary), and cardiac (heart).
nephron
The functional unit of the vertebrate kidney. Functions by forcing the filtrate from glomerulus &#8594; Bowman's capsule &#8594; proximal tubule &#8594; loop of Henle &#8594; distal tubule &#8594; collecting duct &#8594; ureter. Antidiuretic hormone (ADH) encourages increased permeability along the distal tubule and, thus, reabsorption of water.
nervous system
The assemblage of structures responsible for the reception of stimuli, integration of the meaning of the stimulus, and coordination of the appropriate response. In vertebrates, composed of the brain and spinal cord in the central nervous system and the network of nerves in the peripheral nervous system. In invertebrates, can exist as a nerve net, nerve ring, ganglia, and/or a nerve cord.
nervous tissue
One of four animal tissue types composed of specialized cells that function in the sensing of stimuli and the transmission of nervous impulses for coordination of a response.
neuron (nerve cell)
The functional unit of the nervous system. Composed of dendrites, the cell body, and an elongated axon, and terminates at the synapse. Sensory neurons receive information from the external environment; interneurons connect neurons together in the central nervous system; motor neurons initiate a response in the organism.
neurotransmitter
Chemical messengers that transmit the nervous message from the presynaptic neuron to the postsynaptic neuron. Secreted via exocytosis of vesicles at the synaptic terminal of the presynaptic neuron. Common examples include acetylcholine (ACh), dopamine, and serotonin.
nitrogenous waste products
As a product of protein digestion, amine groups are produced and those in excess must be excreted. Ammonia (NH<font face="Arial Sub">3</font>) is the most toxic and must be flushed with large volumes of water. Urea is less toxic and is flushed with a moderate amount of water. * Uric acid is the least toxic and can be released with minimal water.
organ
Structures consisting of different tissues organized for a specialized function. Mesenteries (sheets of connective tissue) allow their suspension in body cavities. In mammals, most are arranged in the upper thoracic and the lower abdominal cavities.
organ system
The coordination of specific organs for a specialized function. * Examples include the integumentary, muscular, skeletal, nervous, endocrine, reproductive, digestive, urinary/excretory, respiratory, circulatory, and immune systems.
osmoregulation
The means by which an organism controls its internal water and solute levels in a dynamic external environment. Marine invertebrates tend to be osmoconformers, while marine vertebrates are osmoregulators. Examples include the excretion of excess salt by marine fishes and the excretion of large volumes of water in dilute urine in freshwater fishes.
pancreas
An endocrine gland composed of clusters of endocrine cells called the islets of Langerhans. Alpha cells produce glucagon (which raises blood glucose level); beta cells produce insulin (which lowers blood glucose level). Also has digestive function in the production and secretion of pancreatic juices that are released into the small intestine.
parathyroid glands
Endocrine glands located near the thyroid that release parathyroid hormone (PTH), which acts to increase the blood calcium level.
peripheral nervous system (PNS)
The network of cranial nerves, spinal nerves, and ganglia that transmit information to and from the central nervous system (CNS). The somatic division transmits information to and from skeletal muscles in response to external stimuli. The autonomic division transmits information to and from smooth and cardiac muscles in response to internal stimuli.
peristalsis
The mechanism of rhythmic waves of contraction that propel materials through the alimentary canal (digestive tract). Involves the alternating contraction of circular and longitudinal muscles. Small, circular sphincter muscles separate successive chambers (organs) for controlled movement.
phagocytic leukocytes
Neutrophils function in the destruction of pathogens in infected tissues. Monocytes function by moving into tissues where they develop into macrophages. Eosinophils work collectively to surround and destroy multicellular parasites. * Dendritic cells are responsible for the ingestion of pathogens and the stimulation of acquired immunity.
pineal gland
A small endocrine gland in the brain that produces melatonin for the regulation of biological rhythms, including sleep.
pituitary gland
A vertebrate endocrine gland in the brain that acts as the "master gland" by controlling the activity of other endocrine glands. The anterior pituitary consists of endocrine cells that produce and secrete tropic hormones, nontropic hormones, and growth hormones. The posterior pituitary is an extension of the hypothalamus and releases antidiuretic hormone (ADH) and oxytocin.
prostaglandins (PGs)
Local regulators composed of modified fatty acids that function in various reproductive and immune responses.
protonephridium (pl. protonephridia)
A tubular excretory system utilized by flatworms consisting of flame bulbs that move the filtrate into the tube and nephridiopores in the body wall through which the excretory product exits the organism.
pulmonary circuit
The circuit common in avian and nonavian reptiles and mammals that brings oxygen-poor blood to the lungs to pick up oxygen via the lung capillaries and delivers oxygen-rich blood to the heart for transfer to the systemic circuit. * In amphibians, a specialized pulmocutaneous circuit also connects with the skin capillaries, a significant contributor to overall respiration.
reflex arc
A network of nerves that constitutes an automatic response to a stimulus that bypasses the brain. * The knee-jerk reflex is familiar to humans; the response is quick because the sensory message is transmitted only to motor neurons in the quadriceps muscles and to interneurons in the lower spinal cord.
refractory period
The time it takes to reestablish resting potential after the hyperpolarization of an action potential and before another action potential may be transmitted.
respiratory pigment
Proteins present in the blood that bind to and transport O<font face="Arial Sub">2</font> and CO<font face="Arial Sub">2</font> to the body's cells. Hemoglobin is utilized by vertebrates and contains iron as its oxygen-binding element, coloring the blood red. Myoglobin is a specialized respiratory pigment common in vertebrate skeletal muscle and found in high concentrations in diving mammals.
Rh factor
A red blood cell antigen that can cause destruction of the fetal blood cells by the mother during pregnancy. * Along with the ABO blood group antigens, is used to describe one's blood type (such as, A- or O+).
saltatory conduction
The means by which the electrochemical message is quickly transmitted through a jumping fashion along the length of the axon. * Due to the myelin sheath secreted by neighboring Schwann cells, only the noninsulated portions of the axon have to undergo an action potential, and the message travels much faster.
sarcomere
The functional unit of skeletal muscle contraction spanning from Z line to Z line and composed of overlapping actin (thin filaments) and myosin (thick filaments). * When the distance from Z line to Z line shortens, the muscle contracts; when the distance lengthens, the muscle relaxes.
sensory neuron
One of three major types of neurons that functions in the detection of external and internal stimuli. * Receptor types include mechanoreceptors (touch), pain receptors, thermoreceptors (heat), chemoreceptors (chemicals), olfactory receptors (smell), electromagnetic receptors, and photoreceptors (light).
short-term memory
The ability of central nervous system cells of the frontal lobe to hold information for a temporary period of time until the information becomes irrelevant.
signal transduction pathway
A molecular mechanism that links a chemical or mechanical stimulus to a specific cellular response via the action of enzymes stimulated by a second messenger.
sinoatrial node (SA) node (pacemaker)
Located in the upper-right atrium and responsible for generating the heart's own electrical impulse and coordinating contraction. * Signals released cause the atria to simultaneously contract, pushing blood into the ventricles.
skeletal muscle
One of three types of muscle tissue characterized by striations and attached to bones; responsible for voluntary movements. * Each muscle is composed of a bundle of muscle fibers, each fiber is composed of smaller myofibrils, and each myofibril is composed of myofilaments composed of actin and myosin.
sliding-filament model
The mechanism describing the contraction of skeletal muscle. Triggered by the release of acetylcholine (ACh) from a motor neuron that signals the sarcoplasmic reticulum to release Ca<font face="Arial Super">2+</font> that, in turn, binds to troponin and unblocks tropomyosin so the myosin cross-bridge can form. ATP allows the power stroke so that the actin is pulled over the myosin toward the midline of the sarcomere.
systemic circulation
The circuit common in all vertebrates that brings oxygen-rich blood from the capillaries of the respiratory exchange surfaces to the body's cells, and delivers oxygen-poor blood back to the heart for transfer to the gill, pulmocutaneous, or pulmonary circuit.
T lymphocyte (T cell)
A lymphocyte that has receptors composed of one heavy chain and one light chain bound by a disulfide bridge and specific to a given antigen. * Includes helper T cells and cytotoxic T cells and participates in cell-mediated immunity.
thermoregulation
The means by which an organism controls its internal temperature in a dynamic external environment. Often involves either vasodilation (increasing the diameter of peripheral blood vessels) or vasoconstriction (decreasing the diameter of peripheral blood vessels). Specific examples include countercurrent heat exchange and evaporative cooling.
threshold potential
The voltage potential that the membrane of a neuron must reach in order to transmit an action potential. * Typically is measured around -55mV.
thyroid gland
An endocrine gland located in the ventral surface of the trachea that releases calcitonin (lowers the blood calcium level) and thyroxine (regulates metabolic activity).
tracheal system
The respiratory system common in insects and composed partially of large pores called tracheae that open to the external environment.
urinary system (excretory system)
The collection of structures responsible for the formation and excretion of nitrogenous wastes. * In vertebrates, composed of the kidneys (which produce urine), the ureters (which move the urine), the urinary bladder (which stores the urine), and the urethra (which releases the urine from the body).
villus (pl. villi)
Large folds in the lining of the intestine that increase the surface area available for absorption. The epithelial lining also contains small folds called microvilli on its surface that further increase the surface area. A capillary bed and lacteal (lymph vessel) provide for exchange and circulation of materials.
acrosome
The vesicle at the tip of a sperm cell that helps the sperm penetrate the egg for fertilization. * Is discarded as the sperm approaches the egg during the arcosomal reaction, an action contributing a fast block to polyspermy.<b>
</b>asexual reproduction
Reproduction that only involves one parent and creates genetically identical offspring. Some form present in all kingdoms of life. Usually relies solely on mitotic division (or binary fission if prokaryotic) to produce offspring.
blastocyst
An embryonic stage of mammals characterized by a hollowed-out ball of cells created through invagination of the morula. * In humans, produced one week after gestation.
blastula
The hollow sphere of cells present at the end of cleavage during early embryonic development. * Contains an internal fluid-filled cavity called the blastocoel.
budding
An asexual mode of reproduction during which the offspring result from outgrowths of the parent body. * Common in many sponges (phylum Porifera) and tunicates (phylum Chordata, subphylum Tunicata).
bulbourethral gland
The pair of small accessory reproductive glands in males responsible for the production of a clear mucus that neutralizes acidic urea remaining in the urethra. * Secreted fluid can also contain some sperm in the pre-ejaculate, contributing to the high failure rate of the withdrawal method of contraception.
cervix
The neck of the uterus opening up to the vagina in the female reproductive tract.
cleavage
The process of early embryonic development characterized by successive cell divisions without cell growth. * Responsible for the transformation of the zygote into a cluster of cells.
cloaca
The common external opening to the reproductive, urinary, and digestive tracts. * Present in vertebrates (except most mammals).
contraception
The prevention of pregnancy in humans. * Common mechanisms employed include barrier methods (such as condoms) and hormonal methods (such as birth-control pills).
corpus luteum
The tissue that forms from a collapsed ovarian follicle after release of the ovum during ovulation. * Functions in the production and secretion of progesterone.
cortical reaction
The exocytosis of enzymes produced by cortical granules in the egg cytoplasm during fertilization. * Results in the development of the fertilization envelope and represents a slow block to polyspermy.<b>
</b>ectoderm
The outer germ layer present in diploblastic and triploblastic animals during embryonic development. * Contributes to the formation of the nervous system, the integument, and sensory structures.
ejaculation
The release of sperm from the epididymis of the male during copulation, often accompanied by orgasm.<b>
</b>embryo
The early developmental phase of an organism resulting from cleavage of the zygote and successive gastrulation. * In humans, becomes the fetus from the ninth week of gestation until birth.
endoderm
The inner germ layer present in diploblastic and triploblastic animals during embryonic development. * Contributes to the formation of the digestive system, the respiratory system, and the urinary bladder.
endometrium
The inner lining of the uterus rich in blood vessels and into which the developing embryo implants.
epididymis
The coiled tubes in the male reproductive tract just beyond the seminiferous tubules in the testes where sperm mature and become specialized for motility.
estrous cycle
A reproductive cycle in female mammals (except higher primates) during which the endometrium, if lacking an implanted embryo, is reabsorbed rather than shed. * Sexual activity occurs only at mid-cycle during estrus.
external fertilization
The method of fertilization in some animals in which the female releases eggs into an aqueous environment where they are fertilized upon the release of sperm by males. * Common in coral (phylum Cnidaria, class Anthozoa) and some bony fishes (phylum Chordata, subphylum Vertebrata, class Osteichthyes).
fission
An asexual method of reproduction during which the parent separates into two or more offspring of approximately equal size. * Common in sea anemones (phylum Cnidaria, class Anthozoa).
follicle
A structure on the ovary that contains the maturing egg before ovulation and secretes estrogens.
fragmentation
An asexual mode of reproduction during which the parent breaks into several pieces, some or all of which develop into independent offspring. Followed by regeneration, the regrowth of lost body fragments. Common in many sponges (phylum Porifera) and corals (phylum Cnidaria, class Anthozoa).
gamete
A haploid reproductive cell generated through gametogenesis, a specialized type of meiosis. * The female reproductive cell is the ovum (egg); the male reproductive cell is the spermatozooan (sperm).
gastrula
The three-layered, cup-shaped embryonic phase present in some animals. * Created through gastrulation of the blastula as cells migrate and change shape.
gestation (pregnancy)
The phase following fertilization during which the embryo develops internally within the female reproductive tract. * Can be terminated through abortion.
gonad
A reproductive structure responsible for the production of gametes. * In females, the ovaries; in males, the testes.
hermaphroditism
A form of sexual development in which the adult organism possesses both male and female sex organs. Allows for two fertilization events during reproduction in which both parents are fertilized. Common in earthworms (phylum Annelida, class Oligochaete).
in vitro fertilization
The use of reproductive technologies to assist in the pregnancy of human females. * An embryo grown in the lab is inserted into the womb of another female for development until birth.
induction
The interactions among embryonic cells that influence their differentiation process by triggering differential gene expression. * Mediated by surface-to-surface interactions or chemical signals.
internal fertilization
The method of fertilization in some animals in which the male deposits sperm into or near the female's reproductive tract where fertilization of the egg(s) will eventually occur. * Common in most vertebrates (phylum Chordata, subphylum Vertebrata) and some insects (phylum Arthropoda, class Insecta).
mammary glands
Exocrine glands in mammalian females that function in the production and secretion of milk through lactation. * Although possessed by mammalian males, the low levels of estrogen during development prevent the proper formation of the secretory apparatus and the fat deposits.
menstrual cycle (uterine cycle)
A reproductive cycle in females of higher primates during which the endometrium, if lacking an implanted embryo, is periodically shed in a bloody discharge. If instead a pregnancy is detected, this phase is suspended. It is suspended permanently after menopause. Comprised of the proliferative phase, the secretory phase, and the menstrual-flow phase.
mesoderm
The middle germ layer present in triploblastic animals during embryonic development. * Contributes to the development of the muscular system, the endoskeletal system, and the urogenital system.
morula
A cluster of cells produced after the initial zygote has undergone five to seven rounds of cell division.
neural tube
A tube of cells running along the dorsal side of the vertebrate embryo adjacent to the notochord. * Gives rise to the central nervous system.
notochord
The long, flexible rod running longitudinally along the dorsal side of the vertebrate embryo. * Gives rise to the vertebral column.
oocyte
A cell that is the precursor to the ovum in females. * The primary oocyte is a diploid cell in prophase I of meiosis that can be hormonally triggered to develop into an ovum; the secondary oocyte is the haploid cell after meiosis I of oogenesis that will eventually develop into the ovum.
organogenesis
The development of specific organs from the respective tissue layer in the embryo.
ovarian cycle
A reproductive cycle in females characterized by the eventual release of the ovum from an ovarian follicle in response to fluctuations of reproductive hormones. * Comprised of the follicular phase and the luteal phase.
ovary
The female gonad that functions in the production of ova and reproductive hormones.
oviduct (fallopian tubes)
The pair of cilia-lined tubes in the female reproductive tract along which the ova travel after ovulation. * In invertebrates, the pathway between the ovaries and the vagina; in vertebrates, the pathway between the ovaries and the uterus and the site of fertilization.
ovulation
The process of the release of a mature ovum from the ovary triggered by hormones. * In humans, a follicle releases one egg during each menstrual cycle.
ovum (egg)
The haploid female reproductive cell in sexually reproducing organisms. Possesses all the structures necessary to initiate embryonic development once fertilized. Created through the process of oogenesis, during which unequal cytokinesis results and also produces three small polar bodies.
parthenogenesis
A form of reproduction that involves the production of offspring from a female parent through the growth and development of a haploid egg. In invertebrates (for example, class Insecta and Daphnia), results in haploid offspring that produce eggs without meiosis. In vertebrates (such as some reptiles), includes the duplication of the X chromosome to create diploid female offspring.
pattern formation
The organizing of cells into specific, three-dimensional structures as part of morphogenesis and organogenesis.
penis
The male reproductive organ that functions to transfer semen to the female during copulation and internal fertilization. * Composed of several layers of highly vascularized, spongy erectile tissue that becomes engorged with blood during sexual excitation.
pheromone
A chemical messenger released by an organism that influences the behavior and/or physiology of another organism of the same species. * Often functions in attracting a mate for reproduction.
placenta
A structure in the pregnant uterus responsible for housing and nourishing the developing fetus with the maternal blood supply. * Formed from the uterine lining and the embryonic membranes.
prostate gland
The large accessory reproductive gland in males responsible for the production of a thin, milky fluid. Uses several small ducts to directly secrete its product into the urethra where it mixes with the other components of semen. Functions in the nourishment of sperm and the provision of anticoagulant enzymes needed for motility within the female reproductive tract.
regeneration
In asexual reproduction, the regrowth of lost body parts following fragmentation. * In some animals, the regrowth of lost appendages in response to injury (for example, a sea star regrows a lost arm).
scrotum
The sac created through a fold in the body wall that contains the testes. * Houses the testes externally at 2&#176;C cooler than body temperature for optimal sperm production.
semen
The combination of sperm and the secretions of several accessory glands released from the male reproductive tract during orgasm.
seminal vesicles
The accessory reproductive glands in males responsible for the release of a thick, yellowish, and alkaline fluid. * Responsible for 60 percent of the fluid content of semen and functions in nourishment of the sperm and lubrication.
sequential hermaphroditism
A specialized form of hermaphroditism in which an organism begins its reproductive life as one sex and eventually develops into the other. * Common in some bony fishes (phylum Chordata, subphylum Vertebrata, class Osteichthyes).
sexual reproduction
Reproduction that involves the combination of genetic material from two parents to create a genetically unique offspring. Some forms present in many protists, most fungi, and all plants and animals. Usually relies on meiotic divisions to produce haploid gametes from diploid germ cells.
somites
Paired blocks of mesoderm tissue positioned near the notochord in the vertebrate embryo. * Give rise to the vertebrae and skeletal muscle.
spermatozooan (sperm)
The haploid male reproductive cell in sexually reproducing organisms. Created through the process of spermatogenesis. Typically possesses a flagellum for locomotion in an aqueous environment to reach the egg.
testes
The male gonads responsible for the production of sperm in response to testosterone. * Consist of highly coiled seminiferous tubules surrounded by Leydig cells, where androgen hormones are produced.
urethra
The tube that delivers stored urine from the bladder to the opening of the penis or vagina for release from the body.
uterus (womb)
The internal chamber in the female reproductive tract where fertilization and/or development of the embryo occurs. * A thick, muscular organ that can expand during pregnancy to accommodate the growth of the embryo.
vagina
The organ in the female reproductive tract that receives the penis and into which ejaculation of semen occurs. * In mammals, also serves as the birth canal.
vas deferens
The muscular tubes in the male reproductive tract along which sperm are propelled. * Connects the epididymis to the urethra.
yolk
The portion of the fertilized egg where nutrients are stored. * Mostly concentrated at the vegetal pole region of the egg, away from the animal pole where the polar bodies are clustered.
zygote
A diploid, fertilized egg resulting from the fusion of two haploid gametes. * Through cell divisions during the cleavage process, initiates the development of the multicellular embryo.
agonistic behavior
Any behavior involving a contest that will determine the recipient of a resource, such as a mate or access to food. * Often involves actual or threats of fighting and fleeing, and is useful in the maintenance of a social hierarchy.
altruism
Any behavior that seems to reduce the individual's fitness while conferring increased fitness to another individual. * An example includes a primate spending time and energy on grooming another.
associative learning (classical conditioning)
The type of acquired learning that involves associating one stimulus with another. * First described by Ivan Pavlov, who measured the salivation response in dogs upon hearing a bell they associated with feeding.
behavior
The quality and quantity of any animal activity. * Includes outwardly obvious actions, such as chasing prey or participating in mating rituals, as well as less obvious actions like releasing pheromones to attract a mate.
behavioral ecology
The study of animal behavior, including the evolution of behavior and its contributions to increased reproductive fitness. * Includes ethology, the study of animal behavior in natural conditions (that is, observations made "in the field").
coefficient of relatedness
The probability that a given gene possessed by one individual will also be shared by another individual from a common parent or ancestor. * Among siblings and between parent and offspring, the value = ½; among first cousins, the value = (½)<font face="Arial Super">3</font> = ?.
cognition
The ability of the nervous system of an animal to receive, process, store, and use information received by its sensory receptors. * The study of cognition and the connection between nervous-system information and animal behavior is called cognitive ethology.
cognitive map
A representation of the way in which an animal's nervous system organizes spatial relations between objects in an animal's environment.
communication
Any form of animal behavior that involves the transmission of, reception of, and response to signals.
culture
Any transmission of information regarding rituals, activities, and skills from one generation to the next. * The culture of one population of a species may be different from that of a neighboring population.
fixed action pattern (FAP)
A sequence of behaviors exhibited by an animal that will be carried to completion once initiated. * An example includes yawning behaviors in many animals and the mating dances of some birds.
foraging
Any behavior involving the recognition of, searching for, capturing of, and consumption of food.
habituation
A simple type of learning involving a loss of responsiveness or sensitivity to a stimulus when the continual application of the stimulus results in a neutral affect. * An example includes crows that become accustomed to the presence of a scarecrow.
Hamilton's rule
A principle explaining the natural selection for an altruistic behavior if the negative cost of any altruistic act is offset by the perceived benefits to the recipient of the act. * Actual benefit to the other is viewed proportional to the coefficient of relatedness between the two individuals.
imprinting
A type of learned behavior acquired during a critical sensitive period that is influenced by a significant innate component. * In filial imprinting, the young offspring mimic the behaviors and characteristics of a parent, as when young ducklings learn to follow an individual they associate as the parent.
inclusive fitness
An assessment of evolutionary fitness that includes an individual's contributions to the evolutionary success of close genetic relatives and their offspring.
innate behavior
Behavior that is developmentally fixed and under strong genetic influence. * Examples include the herding behaviors in sheepdogs and the homing behaviors in many animals.
kin selection
A phenomenon significant to inclusive fitness that explains altruistic behavior between related individuals. * Useful in explaining the evolutionary pressures that would favor an adult female who adopts an orphaned relative.
kinesis
Any change in activity of an animal in response to a stimulus, but the response is nondirectional relative to the stimulus. * An example includes the effect of ambient temperature on the rate of movement of many insects.
landmark
Any physical structure in the environment utilized by an animal for spatial learning during navigation. * An example includes a bird using a tall building to assist it in migration.
learning
A change in behavior in response to experience. * Ranges from simple forms under strong genetic influence (for example, imprinting and habituation) to complex forms involving multiple, long-term interactions (for example, social learning and culture).
mate choice copying
A behavior in response to social learning in which an individual copies the mate-choice behavior of other individuals.
monogamous
A type of mating behavior that involves a significant level of pair-bonding and an enduring relationship. * Examples of animals exhibiting this behavior include many birds and a few insects, fish, and mammals.
operant conditioning (trial-and-error learning)
A type of associative learning that involves an animal learning to associate one of its behaviors with a reward or punishment and then either repeating or avoiding that behavior. * Used in training domesticated animals to be house-broken.
optimal foraging theory
The basis for a cost-benefit analysis of animal feeding behavior. * Helps explain why some organisms will risk potential exposure to a predator if the expected payoff in food is high.
polygamous
A type of mating structure that involves a single individual of one sex and multiple individuals of the opposite sex. * One male in mating relationships with several females is exhibiting polygyny; one female in mating relationships with several males is exhibiting polyandry.
promiscuous
A type of mating behavior that occurs without significant pair bonding or an enduring relationship. * Examples of animals exhibiting this behavior are far-ranging, from corals to arthropods and fish to mammals.
reciprocal altruism
Any altruistic behavior between unrelated individuals that is explained by the expectation of reciprocated actions that will eventually benefit the altruist. * An example includes the sharing of food between domestic cats in the same household.
sign stimulus
An external sensory stimulus that triggers a fixed action pattern in an animal. * An example includes the color red in male stickleback fish, which triggers aggressive attacks by other males (the reproductively viable male's belly turns red).
signal
A behavior in one animal that initiates a change in the behavior of another animal. * An example includes a warning call initiated by an individual that causes others to flee.
social learning
Any learning that occurs after observations of another individual and their interactions with others. * Essential in the transmission of culture and highly developed in most primate groups.
sociobiology
Term coined by E. O. Wilson to indicate the study of the social behavior of humans and other social animals based on evolutionary theory.
spatial learning
A change in the behavior of an animal in response to experience regarding the spatial surroundings in the environment. * Important in the locating of nesting sites, potential mates, and food resources.
taxis
Any movement in an animal toward or away from a stimulus. * An example includes chemotaxis, the movement of an individual toward a mate or food due to the detection of a chemical substance in the environment.
abyssal zone
The deepest benthic zones of the ocean that are relatively unexplored. * Often characterized by deep-sea hydrothermal vents, very hot, dark, O<font face="Arial Sub">2</font>-poor areas associated with areas of volcanic activity and inhabited by chemoautotrophic prokaryotes.
age structure
The relative number of individuals of each age in a population.
aphotic zone
The level in a body of water below the photic zone where there is insufficient light to support photosynthesis.
aposematic coloration
Bright warning coloration present in some animals that also possess sufficient physical or chemical defenses against potential predators. * Examples include the poison dart frog and the skunk.
Batesian mimicry
A form of mimicry in which a harmless species mimics another species that is potentially harmful to predators. * An example includes the harmless milk snake that mimics the coloration of the venomous coral snake.
benthic zone
The bottom surface of any aquatic environment. Called the marine benthic zone (ocean floor) in marine environments. The community of organisms living in this zone constitutes the benthos.
biodiversity hot spot
A small geographic location considered to have an extremely high level of endemic species and high levels of endangered and/or threatened species. * The Hawaiian Islands represent one current example.
biogeochemical cycle
Any of the various nutrient circuits in an ecosystem involving abiotic and biotic factors. * Includes the water, nitrogen, carbon, and phosphorous cycles.
biological magnification
A trophic process by which ingested substances become increasingly concentrated at higher trophic levels as they accumulate through feeding.
biomass
The total dry weight of matter a certain species comprises in an ecosystem. * It's generally highest in the producers and decreases throughout the various levels of consumers present in an ecosystem.
biome
Any of the major regional ecosystems of the Earth characterized by a certain assemblage of organisms and a relatively predictable annual temperature and precipitation profile.
bioremediation
The use of living organisms and their natural metabolic activities to help detoxify and restore a damaged ecosystem. * An example includes the use of bacteria to clean up oil spills.
biosphere
The level of biological organization that includes all of the ecosystems existing in all of the biomes on Earth. * The most inclusive level of biological organization.
canopy
The uppermost region of vegetation in a terrestrial environment. * In dense forests, can block much of the sunlight from reaching the forest floor and contributes to evolutionary adaptations such as epiphytes.
carrying capacity
The maximum number of individuals in a population that can be supported by available environmental resources. * Symbolized as <I>K.
</I>chaparral
The type of scrubland biome in coastal, temperate regions that is characterized by mild winters; long, hot summers; and dense, low-lying evergreen shrubs.
character displacement
The tendency for traits to be more divergent between two species in sympatric populations than between the same two species in allopatric populations.
climate
The set of abiotic conditions constituting weather in a specific area. Includes factors such as temperature, water availability, sunlight levels, and wind levels in a given area. The term microclimate describes very small-scale variations in climate (for example, the conditions under a fallen tree). * The term macroclimate describes the often seasonal, large-scale, regional climate events.
cohort
A defined subgroup within a population belonging to the same age group. * A type of group studied and compared in population ecology.
commensalism
A (+/0) symbiotic relationship in which one organism benefits from the interaction while the other individual is not affected. * An example is a tree and a bird using a natural hollow as a nest; the tree is apparently unaffected by the presence of the bird.
community
The level of biological organization that includes several populations of organisms living in a common geographic area. More inclusive than the population level; less inclusive than the ecosystem level. The study of communities in relation to the environment is called community ecology.
competitive exclusion
The notion that during prolonged interspecific competition, one species will use recourses more efficiently and will, thus, have a competitive advantage over the other species. * Results in the eventual elimination of the less-efficient population from the community.
coniferous forest (taiga)
The terrestrial biome located just south of the tundra that is characterized by evergreen forests of cone-bearing trees and long, cold winters. * Includes the largest organisms on Earth (by biomass) in the giant redwoods and the tallest organisms on Earth in the coastal redwoods.
conservation biology
The multidisciplinary study focused on the effort to sustain biodiversity in an ecosystem or throughout the biosphere.
consumer
Heterotroph that relies on the producers or other consumers for food. Primary consumers are herbivores that ingest the autotrophs. Secondary consumers are carnivores or omnivores that rely on the primary consumers and possibly the producers for food. * Tertiary consumers are carnivores or omnivores that rely on the primary and/or secondary consumers and possibly the producers for food.
coral reef
A major marine biome found in the tropics, supported by the calcium carbonate secretions of corals, and characterized by high biodiversity, warm waters, and high nutrient levels. * Algae constitute the major producers of the ecosystem; corals, sponges, echinoderms, crustaceans, and many fishes occupy this habitat.
cryptic coloration
Coloration that camouflages an organism in its typical environment. * An example includes the leaf bug that mimics not only the coloration but also the general structure of a leaf.
demography
The statistical study of births and deaths in a population. * Represented partially through a life table (a summary of a population's mortality).
density
In population ecology, the number of individual organisms per unit area. Often estimated through the mark-recapture method. Across the Earth, human population density varies greatly from the very dense, vertically organized cities to the sparsely populated, open lands of the tundra.
desert
The terrestrial biome characterized by very low precipitation and inhabited by organisms possessing very specialized adaptations to prevent desiccation (for example, C<font face="Arial Sub">4</font> and CAM plants).
detritivore (decomposer)
A consumer in an ecosystem that relies on detritus (dead and decaying organic matter) as a food source.
dispersal
The distribution of individual organisms within the geographic boundaries of the population.
dispersion
In population ecology, the distribution of individual organisms within a geographic population boundary. * Areas with relatively uniform dispersion are associated with higher levels of animal territoriality.
disturbance
A force that disrupts normal community functioning and usually results in the removal of individuals from the community. Can be abiotic, as in fire or flood, or can be biotic, the result of human action. The intermediate disturbance hypothesis states that communities experiencing moderate disturbance will demonstrate more divergence than those under high or low levels of disturbance.
dominant species
The species in a community that has the highest biomass or greatest abundance, and whose role helps to control the distribution of other species in the community.
ecological footprint
A method for estimating the human carrying capacity of Earth by calculating the amount of land and water required to produce all of the resources necessary to support the population, recycle nutrients, and absorb wastes.
ecological niche
The sum of a species's use of environmental resources. * The means by which a particular individual makes a living utilizing abiotic and biotic resources.
ecological succession
The transition in the species composition of a biological community over time. Primary succession refers to the establishment of a biological community in a barren area not previously inhabited by organisms and lacking true soil. Secondary succession refers to the reestablishment of a biological community after a major disturbance has vastly reduced the species diversity.
ecology
The study of the interactions between organisms and their environment. * Includes the examination of biotic (living) and abiotic (nonliving) factors.
ecosystem
The level of biological organization that includes a community of organisms and the abiotic factors relevant in the environment. More inclusive than the community level; less inclusive than the biosphere level. The study of the energy flow and nutrient cycling in an ecosystem is called ecosystem ecology.
ecotone
The gradual transition from one ecosystem to another across a geographic region.
endangered species
A species facing extinction in all, or a significant portion, of its range. * Threatened species are species likely to become endangered in the near future.
estuary
The region where a freshwater river meets an ocean or sea. * Characterized by a halocline (a gradation of salt concentrations).
eutrophic lake
A lake ecosystem characterized by low O<font face="Arial Sub">2</font> levels, high nutrient levels, and high biological productivity.
eutrophication
The process by which nutrients, especially nitrogen and phosphorous, accumulate in a body of water and result in the increased growth of producers like algae.
evapotranspiration
The total evaporation of water from soil and from transpiration by plants.
exponential population growth
The geometric increase in population numbers in an ideal, nonlimiting environment. Represented by a J-shaped population-growth curve. Observed during the first rounds of reproduction in bacterial colonies in a Petri dish.
extinction vortex
The gradual process of potentially irreversible extinction, resulting from positive-feedback loops of genetic drift and inbreeding.
facilitator
A species that positively influences the survival and/or reproduction of other species in a biological community.
greenhouse effect
The warming of the Earth due to the trapping of heat by CO<font face="Arial Sub">2</font> and other greenhouse gases in the upper atmosphere.
host
The organism in a parasitic relationship that is harmed by the interaction, usually by having its nutrients depleted or otherwise experiencing dysfunction due to the presence of the parasite.
interspecific interactions
Relationships between members of different populations (and, thus, different species) in a community. * Includes interspecific competition, in which individuals from different populations compete for limited resources in an ecosystem.
intertidal zone
The shallow region in the ocean where land and water meet. * Characterized by high levels of algae and tide-pool organisms like arthropods and echinoderms.
introduced species
A nonnative species in a particular ecosystem, likely a result of human action. * Can become invasive if lacking natural controls in the new environment.
invasive species
Species, often introduced into the environment by humans, that overrun a particular ecosystem and negatively affect survival and/or reproduction of other species. * If kept unchecked, these species can contribute to the localized extinction of endemics and other native species.
<B>K-</B>selection
The phenomenon present in some populations where a typical life history is characterized by the production of relatively few offspring with a good chance of survival.
life history
A summary of the major series of events that an individual experiences from birth through reproduction until death. Called iteroparity when adults repetitively produce a large number of offspring until death. Called semelparity when adults have a single reproductive opportunity to produce a large number of offspring.
limnetic zone
In a lake ecosystem, the deeper, open-surface, well-lit waters farther from shore.
littoral zone
In a lake ecosystem, the shallow, well-lit waters close to shore.
logistic population growth
The type of growth in a population in which rapid initial growth eventually slows and plateaus as the population reaches carrying capacity. * Represented by an S-shaped population-growth curve.
minimum viable population (MVP)
The smallest population level required for a species to survive and reproduce enough to sustain the population generationally. * The determination of this value may involve a population viability analysis to establish the effective population size (the number of successfully breeding individuals).
M&#252;llerian mimicry
A form of mimicry in which two separate unpalatable species mimic one another. * An example includes the aposematic yellow and black stripes of many stinging wasps and bees.
mutualism
A (+/+) symbiotic relationship in which both organisms benefit from the interaction. * Examples include the relationship between flowering plants and their pollinators and the interaction between humans and their usual intestinal microflora.
neritic zone
The shallow region of the ocean overlying the continental shelf.
oceanic pelagic biome
The vast regions of open ocean far from shore mixed by ocean currents and characterized by plankton in the photic zone and scattered free-swimming and migratory animals.
oligotrophic lake
A lake ecosystem characterized by high O<font face="Arial Sub">2</font> levels, low nutrient levels, and few phytoplankton.
organismal ecology
The study of the complex ways in which an organism handles daily environmental challenges for survival.
parasite
The organism in a parasitic relationship that benefits from the interaction, usually by depleting nutrients or otherwise affecting normal functioning in the host. * An endoparasite (such as a tapeworm) lives within the host body; an ectoparasite (such as a flea) lives outside of the host body.
parasitism
A (+/-) symbiotic relationship in which one organism benefits (the parasite) and the other is harmed (the host). * A specialized form is parasitoidism, in which the parasite lays eggs on the living host, which eventually provides food for the developing larvae.
pathogen
A disease-causing agent. * Includes organisms such as protists, fungi, and just 1 percent of bacteria.
photic zone
The thin upper level in a body of water where there is sufficient light available for photosynthesis. * Primary productivity is concentrated here through the action of algae and other phytoplankton.
population ecology
The study of populations of organisms in relation to the environment. * Of concern are issues of age structure and population density and distribution.
precautionary principle
A guiding principle in making decisions in a given environment with respect to the potential risks associated.
predation
An interspecific interaction in which one species, the predator, eats the other species, the prey. * Although the individual prey is harmed, the prey population may be helped through the reduction of weak, old, and/or diseased individuals.
primary producers
Autotrophs, usually photoautotrophs, that are responsible for carbon fixation and the production of food in an ecosystem.
primary production
A quantification of the amount of light energy converted into food (chemical energy) by the actions of autotrophs in an ecosystem. Gross primary production (GPP) is the total primary production in an ecosystem. Net primary production (NPP) is the GPP in an ecosystem less the amount of energy required by the producers for respiration.
production efficiency
The percent of chemical energy stored in food that is not used for respiration by an organism.
resource partitioning
The effective sharing of resources by members of different populations in a community due to the unique nature of each species' ecological niche.
reproductive table
An age-specific summary of the reproductive rates in a population.
restoration ecology
The application of ecological principles in an effort to restore a damaged ecosystem to sustainable levels and to promote localized biodiversity. * May involve biological augmentation, the use of organisms to add deficient nutrients to a damaged ecosystem.
river
A flowing body of water generally formed by the convergence of many stream tributaries. * Vary greatly in size, temperature, and water quality.
R-selection
The phenomenon present in some populations where a high reproductive rate characterizes the typical life history.
savanna
The type of grassland biome in tropical regions that is characterized by warm temperatures year-round, scattered individual trees, large grazing and migratory herbivores, and distinct seasons driven by changes in rainfall.
secondary production
The quantity of chemical energy from food consumed by heterotrophs that is actually converted into new biomass.
species diversity
A measure of the species' richness and relative abundance of individual species in a biological community.
stream
A flowing body of water that is generally small, clear, and cold. * Can contribute to the formation of a river.
survivorship curve
A representation of age-specific mortality in a population that demonstrates the number of members of each cohort that are still alive at each age.
sustainable development
Planning by human societies that involves incorporating local and global ecosystem needs for long-term prosperity.
temperate broadleaf forest
The terrestrial biome located throughout mid-latitude regions and characterized by high levels of annual rainfall; vast forests of large, broadleaf deciduous trees; hibernating mammals; and migratory birds.
temperate grassland
The terrestrial biome characterized by fields of low-lying grasses, seasonal precipitation, hot summers and cold winters, and large grazing animals.
thermocline
A stratum of quick change in the temperature of a body of water, especially prevalent moving from the surface toward the bottom.
tropical rain forest
The terrestrial biome situated near the equator that is characterized by high levels of precipitation; warm temperatures year-round; dense, highly stratified plant life due to competition for light; and very diverse animal life.
trophic efficiency
The percentage of production that is transferred to each successive level in a trophic structure via feeding relationships. * Maximum efficiency estimated to be 10 percent.
trophic structure
A model representing the feeding relationships in an ecosystem, which establishes the flow of energy and the cycling of nutrients. * A specific flow of food from producer to the highest-order consumer is called a food chain; the interconnected food chains in an ecosystem are called a food web.
tropics
Regions that exist between the latitudes of 23.5&#176; north and south of the equator. * Closest to the sun year-round, so receives the greatest light intensity and temperature.
tundra
The very cold terrestrial biome characterized by the presence of permafrost and very limited plant growth. * Called arctic tundra at the northernmost biomes; called alpine tundra at high altitudes.
turnover
The seasonal mixing of lake waters as a result of changing water-temperature profiles. * Results in the movement of O<font face="Arial Sub">2</font>-rich water from the surface to lower levels and nutrient-rich water in the opposite direction.
wetland
An ecosystem representing a transition from an aquatic ecosystem to a terrestrial one. * Soils are either constantly or periodically soaked with water and support aquatic plant populations.
zero population growth (ZPG)
A time of stability in a population when the per capita birth rate equals the per capita death rate. * Represented by a plateau in a population-growth curve.