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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.


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.


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.


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.


The less organized, uncondensed form of DNA observed during interphase. * Allows for replication and transcription.


The organized, condensed form of DNA observed during mitosis and containing a discrete set of genes from the organism's genome.


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.


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.


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.


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.


Structures composed of keratin proteins used for anchoring two cells together into a strong sheet.


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.


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.


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.


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).


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.


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.


A molecule consisting of a carbohydrate covalently bonded to a protein. * Functions in cell-cell communication and recognition.


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.


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.


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.


A molecule that binds to the specific receptor site of another molecule.


A small, membrane-bound sac containing hydrolytic (digestive) enzymes. * Can fuse with vesicles and break down their contents.


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.


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.


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.


The region in a prokaryotic cell where the DNA is concentrated.


The structure within the nucleus that produces rRNA, an integral component of the ribosome.


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.


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.


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.


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.


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.


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.


The inner compartment of a chloroplast where the Calvin cycle (light-independent reactions) of photosynthesis occurs.


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.


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.


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.


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.


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.


Any negatively charged ion resulting from a neutral atom that has gained one or more electrons. * Examples include Cl⁻, PO₄³-, and SO₄²-.


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.


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.


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.


Any substance that speeds up a chemical reaction. * Biological catalysts are called enzymes.


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.


An organic substance that must bind to an enzyme to allow proper functioning. * Examples include most vitamins common to the animal diet.


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.


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.


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).


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.


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.


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.


The affinity of an atom to attract electrons in a covalent bond.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


The structure formed as two or more atoms bond. * The simplest forms are diatomic, as in O₂ and N₂.


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.


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.


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.


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.


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).


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 (∂-).


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.


The polymer form of a nucleic acid created as nucleotides covalently bond. * The form typical of DNA and RNA.

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