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A neurotransmitter released by neurons to excite an action potential or trigger a muscle to contract.


Protein filaments that, along with myosin, allow muscles to contract

adaptive radiation

The evolutionary process by which ancestral forms of an organism are diversified through adaptation to new environments.

allelic frequency

The frequency with which a particular allele for a certain characteristic appears among all possible alleles for that characteristic in a population

alternation of generations

The fluctuation between the diploid (sporophyte) and haploid (gametophyte) life stages that occur in plants.

amino acid

The monomer of a protein. A central carbon attached to an amino group (-NH2), a carboxyl group (-COOH), and a hydrogen atom (-H). The fourth group is variable and defines the amino acid's chemical identity.

analogous trait

A trait that is morphologically and functionally similar to that of a different species but that arose from a distinct, ancestral condition.


A vascular flowering plant in which seeds are enclosed inside protective ovaries, such as fruit or flowers. Angiosperms can be monocots or dicots.


Pollen-producing structure at the top of the stamen, the male reproductive organ of flowers.


The largest artery in the body; carries oxygenated blood from the left ventricle of the heart.

aphotic zone

part of the marine pelagic zone that begins 600 feet below the surface of the ocean; only chemosynthetic organisms, scavengers, and predators are able to survive in this habitat.


vessel that carries blood away from the heart and has thick, elastic, muscular walls that can dilate or contract to control blood pressure within the vessels. Blood in arteries is oxygenated, with the exception of the blood in the pulmonary artery

autonomic nervous system

the involuntary half of the peripheral nervous system; in two antagonistic parts: the sympathetic and parasympathetic nervous systems. Their interactions control smooth and cardiac muscle, glands, and organs and processes such as heartbeat, the movements of the digestive tract, and the contraction of the bladder


an organism that can produce the organic molecules and energy necessary for life through the processes of photosynthesis or chemosynthesis


one in a class of plant hormones that stimulates (among other things) cell elongation, secondary tissue growth, and fruit development


emulsifier of fats secreted by the liver and stored in the gallbladder for release in the small intestine


amount of living matter in a given ecosystem. Because only 10 percent of energy is transferred between trophic levels, the biomass of lower trophic levels is greater than the biomass of subsequent trophic levels: biomass of producers > biomass of primary consumers > biomass of secondary consumers > biomass of tertiary consumers.


particular geographic area with a common climate and characteristic plant and animal life; characterized by specific climax communities


liquid that carries nutrients and oxygen to the cells and carries carbon dioxide and nitrogenous wastes away


rigid structures composed of living cells rooted in a matrix of calcium, phosphate salts, and collagen fibers


center of the central nervous system; coordinates the processes of the body


lower terrestrial plant (often a moss or liverwort) that lacks a vascular system and is dependent on environmental moisture for reproductive and nutritive functions


asexual reproductive process in which a small portion of the cell membrane and cytoplasm receive a nucleus and pinch off from the parent cell


roughly spherical underground bud containing additional buds that can develop asexually into new plants

Calvin cycle

light-independent phase of photosynthesis, where carbon dioxide is fixed to a three-carbon compound used to form glucose; ATP and NADH are consumed in this cycle


tiny blood vessels able to branch through the body and deliver oxygen and nutrients to every cell.


central element of life

carrying capacity

maximum number of individuals in a population that can be sustained in a given environment


firm but flexible substance, found in regions of vertebrate skeletons, such as the ribs, that need to bend

cellular respiration

process in which the cell burns glucose to create ATP with the aid of oxygen; cells have two different methods of turning food into usable fuel: aerobic respiration and anaerobic respiration


complex carbohydrate that constitutes the cell walls of plants and protists


part of the brain that ensures that movements are coordinated and balanced


part of the brain that controls all voluntary movement, sensory perception, speech, memory, and creative thought

chemical cycles

cycles in which inorganic elements move through the biotic and abiotic aspects of an ecosystem. The two most important chemical cycles are the carbon and nitrogen cycles


synthesizing organic compounds by energy derived from chemical reactions rather than from the energy of the sun


rough polysaccharide that constitutes the cell wall of fungi and exoskeleton of arthropods

circadian rhythms

behavior cycles that depend on time of day

circulatory system

system of organs and blood that brings nutrients and oxygen to cells and carries away wastes.; in higher vertebrates, the system has a pulmonary and systemic circuit; pulmonary circuit carries blood to the lungs to be oxygenated, while the systemic circuit carries oxygenated blood to the body; vertebrates have a closed ______, while arthropods have an open one

climax community

combination of plant and animal forms that dominate mature ecological communities. Climax communities are unique and shaped by various factors, including temperature, rainfall, and soil acidity


phenomenon in which two alleles of the same gene are fully expressed in the phenotype when both are present in a heterozygote


animals that are unable to retain heat produced by metabolic activities. Also known as ectothermic. The metabolism of cold-blooded animals is greatly influenced by climate and temperature


many populations that interact in a given geographical locale constitute ecological communities. Communities exhibit particular interactions such as competition, symbiosis, predation, and food relationships; undergo ecological succession


struggle for survival between organisms or populations that use similar resources and occupy similar niches

contractile vacuole

An organelle often found in protozoa that pumps excess water out of the cell to keep the cell from bursting in a hypotonic environment (like freshwater).


The main component of the cytoplasm. It is a grayish, gel-like liquid containing the nucleus, organelles, and cytoskeleton.


A flowering plant (angiosperm) that possesses two cotyledons during embryonic development. Usually has taproots, flower parts in multiples of fours and fives, and branching veins in leaves.


A sugar compound consisting of two carbohydrate monomers.

ecological succession

The progression of plant life and attendant animal life in a given geographic location, from pioneer plant to climax community.


The study of the interactions and relationships of populations with each other and their abiotic environments.


A community of organisms and its abiotic environment.



electron transport chain

The final stage of aerobic respiration. The electron transport chain establishes an electrochemical gradient across the inner mitochondrial membrane that powers the synthesis of ATP in oxidative phosphorylation.

endocrine system

Control system of the body that functions by releasing hormones into the bloodstream.


Process by which liquids or small solid particles are taken into a cell in the form of small vesicles that are produced through the invagination of the cell membrane.


An interior skeleton found in vertebrates made of bone and cartilage.



energy pyramid

Energy in a community can be depicted as a pyramid of food or biomass. The availability of food, biomass, and energy from the trophic level of producers up through each subsequent level on the food web is approximately 10 percent of that available in the previous trophic level.

excretory system

The organ system that filters blood and removes nitrogenous wastes from the body in the form of urea or uric acid. In humans, the two kidneys are the vital organs of blood filtration. In annelids, nephridia fill the filtering role; Malpighian tubules do the same in arthropods. In humans, other important structures of the system are the ureters, the urinary bladder, and the urethra.


A rigid, chitinous protective structure that surrounds the bodies of arthropods and provides support.

facilitated diffusion

Diffusion of molecules that cannot pass through the cell membrane independently, but rather through permeable protein channels embedded in the membrane. Facilitated diffusion does not require outside energy.


A molecule that stores energy for harvest by the electron transport chain.


The second stage of anaerobic respiration, which produces the NAD+ necessary for glycolysis. There are two types of fermentation: alcoholic fermentation and lactic acid fermentation. Yeast engage in alcoholic fermentation, while muscle cells lacking oxygen produce lactic acid.


An organ that stores the bile produced by the liver and releases it to the small intestine during digestion.


A haploid plant or plant structure that produces haploid gametes through mitosis.


a simple cluster of nerve cells that acts as a coordinating center

gene flow

The movement of genes, within a population or between populations, through mating.

genetic code

The series of codons that make up an organism's DNA.


An artificial form of vegetative propagation in which parts of two young plants are joined together, first by artificial means and then by tissue regeneration.

Gram staining

A process by which components of bacterial cell walls are bound to Gram's stain. Depending on the amount of peptidoglycan in their cell walls, bacteria stain differently and are classified as Gram-negative or Gram-positive.

guard cells

Epidermal plant cells found in pairs surrounding the stomata of leaves. By increasing or decreasing their size, guard cells regulate gas exchange by opening and closing individual stoma.


A vascular nonflowering plant (commonly known as a conifer) in which seeds are "naked"—collected in a cone and not protected by an ovary. The dispersion of their spermatozoids often relies on wind.


The muscular organ that pumps blood through the circulatory system. Mammals and birds have a four-chambered heart, with a left atrium and ventricle and a right atrium and ventricle. The right half of the heart receives deoxygenated blood from the body and pumps it to the lungs, while the left half receives oxygenated blood from the lungs and pumps it to the body.


The genetic transmission of traits from parents to offspring, so that offspring resemble their parents. Traits transmitted this way are called hereditary traits.


Organisms that can only get the organic molecules and energy necessary for life through the consumption of other organic matter. In the food web, all consumers and decomposers are heterotrophs. Heterotrophs can be herbivores, carnivores, or omnivores.

hydrogen bond

A weak bond between hydrogen and a set of other elements, including oxygen. Hydrogen bonds are a subset of dipole-dipole interactions.


A common biochemical reaction in which the bond between two molecules is split by the addition of a water molecule; breaks down polymers and dimers


soluble in water; these substance are either polar or ionic

hydrostatic skeleton

A fluid skeleton in many soft-bodied invertebrates, including annelids, that allows an organism to change shape but not volume.


having greater concentration of solute than another solution


major gland in the brain that is the bridge between the endocrine and nervous systems


having less concention of solute than another solution


learning that occurs during a sensitive or critical period in the early life of an individual and is irreversible for the length of that period


hormone secreted by the pancreas that reduces blood sugar levels; lack this hormone can result in diabetes


The phase of the eukaryotic cell cycle in which the cell prepares for division, primarily by replicating its DNA. After interphase, the cell enters mitosis.

intertidal zone

The most shallow zone in a marine habitat. Periodically dry or wet with the changing tides. Algae, sponges, mollusks, starfish, and crabs inhabit this zone. Also called the littoral zone.


solutions containing equal concentrations of solute


organ of blood filtration in humans


broadest category of biological taxonomy


An evolutionary theory (proved false by Darwin) stating that species change over time by the use and disuse of structures and the inheritance of acquired traits.

learned behavior

Behavior that an organism picks up over the course of its life. Three types of learned behavior are habituation, conditioning, and associative learning.


made the first observations of bacteria and protozoa using single-lens microscopes of his own design


Connective tissue between bones

linked genes

Genes that are located close together on the same chromosome. Linked genes will not undergo independent assortment during gamete formation, constituting an exception to the law of independent assortment. Crossing-over will disrupt the linkage of two genes on the same chromosome if they are far enough apart.

lymphatic system

A means of returning blood fluid, lymph, that has escaped from capillaries back into the bloodstream. Defended against foreign bodies by lymphocytes.

medulla oblongata

Part of the brain responsible for the control of involuntary functions such as breathing, cardiovascular regulation, and swallowing.


The internal tissue of a leaf between the epidermal cells; specialized for photosynthesis. Contains the palisade and spongy layer.


Inorganic molecules required by the body to carry out life processes. Important minerals are iron, a necessary component of hemoglobin; iodine, which is essential for making thyroid hormone; and calcium, which is required by the bones and for many cellular processes.


Structures that create movement in an organism by contracting under a stimulus from a neuron. There are three types of muscle: skeletal, which is responsible for voluntary movement; smooth, which is responsible for involuntary movement; and cardiac, which makes up the heart.


An error in the sequence of nucleotides in DNA or RNA that in turn affects the production of proteins. There are two main types of mutations: substitution mutations and frameshift mutations. A substitution mutation occurs when one nucleotide is replaced by another; these mutations can range from ineffectual to drastic, depending on how the new nucleotide changes the protein coded for. Frameshift mutations occur when a nucleotide is either inserted or deleted into the code; these mutations are always drastic and often fatal, since an insertion or deletion will affect every codon in a particular genetic sequence by throwing the entire three-by-three codon frame out of whack.

myelin sheath

A structure that speeds the movement of action potentials along the axon of a neuron. The sheath is built of Schwann cells, which wrap themselves around the axon of the neuron, leaving small gaps in between known as the nodes of Ranvier.


Protein filaments that, along with actin, allow muscles to contract.


An energy-carrying coenzyme produced by glycolysis and the Krebs cycle. NADH carries energy to the electron transport chain, where it is stored in ATP.

natural selection

The theory, first proposed by Darwin, which holds that organisms produce as many offspring as possible, which compete for limited resources. Organisms' characteristics vary, and certain characteristics will allow organisms to survive and reproduce more effectively. These adaptive characteristics will be more prevalent in subsequent generations. Natural selection is the engine of evolution, choosing the most fit genes to pass from one generation to the next.


A blood filtration and excretory organ characteristic of segmented worms.


Tiny, tubule structures responsible for the filtering of blood in the kidneys of vertebrates.

neritic zone

The medium depth zone of the marine biome. Extends to 600 feet beneath the water's surface and sits on the continental shelf, hundreds of miles from any shore. Algae, crustaceans, and fish inhabit this region.


the unique role a population plays in a community; includes all characteristics that define the way a population exists in a community, from where the members live to what they eat, when they sleep, and how they reproduce.

nitrogenous base

adenine, thymine, guanine, and cytosine in DNA; RNA is composed of adenine, uracil, guanine, and cytosine


A longitudinal rod of cells that forms in the least developed chordates and in embryonic stages of more developed chordates.

olfactory epithelium

Region near the top of the nasal cavity with chemoreceptors and neurons that inform the sense of smell.


Process by which water naturally travels from an area of high water concentration to low water concentration.

oxidative phosphorylation

Part of the electron transport chain. A process occurring in the mitochondria that results in the formation of ATP from the flow of electrons across the inner membrane to bind with oxygen.


A digestive organ that releases enzymes into the small intestine. Also an endocrine gland that regulates glucose levels in the blood by the release of insulin or glucagon from specialized cells called islets of Langerhans.


Four small glands embedded on the posterior surface of the thyroid gland. The parathyroid glands produce a hormone that regulates the level of calcium in the bloodstream.


Asexual reproduction in which an unfertilized gamete (usually female) produces female offspring. Parthenogenesis vastly increases the speed at which a population can grow, though it results in a loss of genetic diversity among members of the population.

pelagic zone

The open-ocean zone at the greatest depth in a marine habitat. This zone is divided into a photic (down to 600 feet below the water's surface) and aphotic zone.

peptide bond

The bond between the amino acids in a protein. Formed by dehydration synthesis.


The rolling motion of smooth muscle that moves food along the alimentary canal. Includes the passage from the esophagus to the stomach, the churning action of the stomach, and the passage through the small intestine.


Vascular tissue composed of cells that are living at maturity; transports the products of photosynthesis throughout the plant body.

photic zone

Literally, zone with light. The photic zone is part of the marine pelagic zone and extends to 600 feet below the surface of the ocean. Photosynthetic plankton as well as bony fish, sharks, and whales inhabit this zone.


An organism's response to the length of day and night within a 24-hour period (photoperiod); in many plants, this phenomenon determines when flowering occurs.


The process by which plants and other autotrophic organisms convert light energy into organic materials, such as glucose.


The evolutionary relationships of a genetically similar group of organisms.


Circular DNA molecules found in prokaryotes.


The male gametophyte of gymnosperms and angiosperms.


A group of interbreeding organisms in a particular locale exhibiting a unique set of characteristics such as patterns of growth and reproductive strategies.


Term that refers to one organism eating another. Predation covers both carnivorous and herbivorous consumption.


Autotrophic organisms such as plants, plankton, and chemosynthetic bacteria that are able to synthesize organic compounds using energy from the sun or chemical reactions. Producers do not have to consume other organisms to attain energy and are the foundation of every food web.


Temporary cytoplasmic protrusions of ameboid cells that function in movement and food uptake by phagocytosis.


three-carbon end product of glycolysis; raw material of the Krebs cycle

reproductive isolation

The inability of individuals within a species to create offspring with members of any other species

respiratory system

The organ system responsible for the intake of oxygen and diffusion of that gas into the blood and the elimination of carbon dioxide from the body. Important structures of the system are the pharynx, larynx, trachea, bronchi, bronchioles, and lungs. Alveoli in the lungs are the location of gas exchange with the blood. The movement of the muscular diaphragm allows the lungs to inhale and exhale.


The part of a plant beneath the soil; responsible for collecting water and minerals from the soil, storing nutrients, and securing the plant to the ground. Can be fibrous or a taproot.

root hair

An outgrowth of a plant root that provides an increased surface area for the absorption of water and dissolved minerals from the soil.


Slender horizontal stem that can form new plants via specialized nodes.

semicircular canal

Fluid-filled structure within the ear that can detect balance.


Green, leaflike structure that encloses and protects the unopened flower bud.

somatic nervous system

One half of the motor system of the peripheral nervous system. Responsible for voluntary, or conscious, movement. Neurons in this system target skeletal muscles and release the neurotransmitter acetylcholine.


The development of a species through evolution. A species forms when its members become reproductively isolated from all other organisms. Speciation can occur through geographic separation that eliminates gene flow or through adaptive radiation.


Usually unicellular and microscopic, spores are produced by protist molds, fungi, and plants and are able to develop into new individuals. Spores are able to survive without food or water for long periods. Most fungi spend part of their life cycle as hyphae and part as spores.


A diploid plant or plant structure that produces haploid spores through meiosis.

stabilizing selection

When selection pressures favor the average form of a trait.


The male reproductive organ of the flower, consisting of an anther and filament.


The top part of the pistil, where pollen grains are received.


The shaft of the pistil that leads from the stigma down into the ovary.


The starting material that will undergo chemical change in a chemical reaction facilitated by an enzyme.


A type of interaction within a community that falls into one of three categories: a parasitic relationship benefits one organism and hurts the other; a commensal relationship benefits one and does not affect the other; a mutualistic relationship benefits both organisms.


The gap between two neurons, spanning the space between the axon of one and the dendrites of the other. In order to pass an impulse across a synapse, neurons must release neurotransmitters.


The study of biological classification.

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