Audio Glossary
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- acid A chemical compound that can give up a hydrogen ion. Compare base and buffer.
- acid rain Rainfall with a low pH. Acid rain is a consequence of the release of sulfur dioxide and other pollutants into the atmosphere, where they are converted to acids that then fall back to Earth in rain or snow.
- acrosome The front tip of a sperm, which contains enzymes that digest the outer covering of the unfertilized ovum.
- actin A protein found in the cytoskeleton, in muscle tissue, and in bacterial flagella.
- actin filament One of the two types of filaments that make up muscle, consisting of two molecules of the protein actin. Compare myosin filament.
- action potential An electrical signal generated by the flow of ions across the plasma membrane of a neuron. Action potentials are self-sustaining and can travel down a neuron in only one direction.
- activation energy The small input of energy required for a chemical reaction to proceed.
- active carrier protein A protein in the plasma membrane of a cell that, using energy from an energy storage molecule such as ATP, changes its shape to transfer a molecule across the plasma membrane. Compare passive carrier protein.
- active immunity The immunity of an organism to a pathogen that depends on the production of antibodies specific to that pathogen by the organism's own body. Compare passive immunity.
- active site The specific region on the surface of an enzyme where substrate molecules bind.
- active transport Movement of molecules that requires an input of energy. Compare passive transport.
- adaptation A characteristic of an organism that improves that organism's performance in its environment.
- adaptive evolution The process by which natural selection improves the match between organisms and their environment over time.
- adaptive radiation An evolutionary expansion in which a group of organisms takes on new ecological roles and forms new species and higher taxonomic groups.
- adenosine triphosphate See ATP.
- adrenal gland One of a pair of endocrine glands that sit atop the mammalian kidney and which release the hormones epinephrine and norepinephrine.
- adrenaline See epinephrine.
- aerobic Of or referring to a metabolic process or organism that requires oxygen gas. Compare anaerobic.
- aerobic respiration A general term used to describe a series of oxidation reactions that use oxygen to produce ATP.
- allele One of several alternative versions of a gene. Each allele has a DNA sequence different from that of all other alleles of the same gene.
- allele frequency The proportion (percentage) of a particular allele in a population.
- allopatric speciation The formation of new species from populations that are geographically isolated from one another. Compare sympatric speciation.
- altruism A behavior that benefits another individual but has a cost to the individual performing the behavior.
- alveolus (pl. alveoli) A small sac in the mammalian lung where gas exchange takes place.
- amino acid An organic compound that has an amino group, a carboxyl group, and a variable R group attached to a single carbon atom. Proteins are polymers of amino acids.
- amniocentesis A procedure in which a needle is inserted through the abdomen into the uterus to extract a small amount of amniotic fluid from the pregnancy sac that surrounds the fetus; this fluid contains fetal cells that can be used to test for genetic disorders.
- amnion A sac filled with a watery fluid that surrounds and protects the developing vertebrate embryo.
- amplification The chain of events set in motion when the binding of a single hormone molecule to a receptor activates thousands of proteins in the target cell.
- anabolic See biosynthetic.
- anaerobic Of or referring to a metabolic process or organism that does not require oxygen gas. Compare aerobic.
- analogous Of or referring to a characteristic shared by two groups of organisms because of convergent evolution, not common descent. Compare homologous.
- anaphase The stage of mitosis during which sister chromatids separate and move to opposite poles of the cell.
- anchoring junction A protein structure that acts as a “hook” between two animal cells or between a cell and the extracellular matrix.
- androgen One of a class of steroid hormones that stimulate cells to develop the characteristics of maleness. The primary androgen is testosterone. Compare estrogen.
- angiosperms The flowering plants, a group that includes most plants on Earth today; named for the protective tissues covering the plant's embryo in the seed. Compare gymnosperms.
- Animalia The kingdom made up of animals, multicellular eukaryotes that have evolved specialized tissues, organs and organ systems, body plans, and behaviors.
- annual A plant that completes its entire life cycle in one year.
- antagonistic Of or referring to the opposing effects of two hormones that together serve to regulate a process.
- antenna complex An arrangement of chlorophyll molecules in the thylakoid membrane of a chloroplast that harvests energy from sunlight.
- antibody A protein that is produced by a B cell and binds specifically to a particular antigen.
- anticodon A sequence of three nitrogen bases on a transfer RNA molecule that can bind to a particular codon on an mRNA molecule. Compare codon.
- antigen A characteristic membrane protein or molecule produced by an invading pathogen that is recognized by particular lymphocytes and the antibodies produced by those lymphocytes.
- anus The opening of the digestive system through which undigested food and other solid wastes leave the body.
- apical dominance Inhibition of the growth of lateral buds by the apical meristem.
- apical meristem A region of rapidly dividing cells in the tips of plant branches and roots that gives rise to new stem and root tissues.
- apoptosis Programmed or intentional cell death.
- appendicular skeleton A collective term for the bones of the arms, legs, and pelvis. Compare axial skeleton.
- aquaculture Managed agricultural systems in which fish and shellfish are cultivated for human consumption.
- aquifer An underground body of water that is sometimes bounded by impermeable layers of rock.
- Archaea A domain of microscopic, single-celled prokaryotes that arose after the Bacteria.
- artery A blood vessel that carries blood from the heart. Compare vein.
- arthropods A group of animals characterized by a hard exoskeleton; includes millipedes, crustaceans, insects, and spiders.
- artificial selection A process in which only individuals that possess certain characteristics are allowed to breed; used to guide the evolution of crop plants and domestic animals in ways that are advantageous for people.
- asexual reproduction The production of genetically identical offspring without the exchange of genetic material with another individual. Compare sexual reproduction.
- atmospheric cycle A type of nutrient cycle in which the nutrient enters the atmosphere easily. Compare sedimentary cycle.
- atom The smallest unit of a chemical element that still has the properties of that element.
- atomic mass number The sum of the number of protons and neutrons found in the nucleus of an atom of a particular chemical element.
- atomic number The number of protons found in the nucleus of an atom of a particular chemical element.
- ATP Adenosine triphosphate, a molecule that is commonly used by cells to store energy and to transfer energy from one chemical reaction to another.
- atrioventricular (AV) node A signaling center in the heart that relays the signal for cardiac muscle to contract from the sinoatrial node to the ventricles.
- atrium (pl. atria) A chamber in a heart that receives blood from the body and pumps it into the ventricle. Compare ventricle.
- auditory canal The tubular opening leading from the ear pinna to the eardrum.
- autoimmune disease A disease caused when the body destroys some of its own tissues that the immune system misidentifies as foreign tissues.
- autosome Any chromosome that is not a sex chromosome. Compare sex chromosome.
- auxin A plant hormone that causes apical dominance and has other important effects, such as controlling the way plants bend toward sunlight and stimulating the differentiation of vascular tissue.
- axial skeleton A collective term for the skull, the spinal column, and the ribs. Compare appendicular skeleton.
- axon An extension of a neuron that carries action potentials through an animal's body.