abiotic

Nonliving materials in the environment—such as elements, sunlight, and soil—that influence and are influenced by living (biotic) entities on the planet.

acetylcholine

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

acids

Hydrogen ion (H⁺) donors. Acids are very important in the chemical reactions of life because they are highly reactive. Acids have pH values below 7. They are the opposite of bases.

actin

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

active site

The part of an enzyme that interacts with, or binds to, a substrate.

active transport

The movement of molecules across a cell membrane from a region of lower concentration to a region of higher concentration. Because active transport involves moving the molecule against the natural flow of the concentration gradient, the process requires energy.

adaptive radiation

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

adenosine triphosphate (ATP)

The energy storage molecule for the cell. ATP consists of an adenosine molecule bonded to three phosphate groups. Each phosphate bond contains energy; by breaking these bonds, the cell can get the energy it needs for chemical reactions. Cells build ATP during cell respiration, using the raw material of glucose.

adrenal glands

Two glands, the adrenal cortex and adrenal medulla, located on the kidney.

aerobic respiration

A form of cell respiration requiring oxygen (as opposed to anaerobic respiration, which does not need oxygen). Aerobic respiration is much more efficient than anaerobic respiration; it produces 36 ATP for every molecule of glucose. Aerobic respiration proceeds in three stages: glycolysis, the Krebs cycle, and the electron transport chain.

allele

A specific form or possible version of a gene having multiple versions. Alleles may be dominant or recessive.

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.

anaerobic respiration

A form of cell respiration that does not use oxygen (as opposed to aerobic cell respiration). Anaerobic respiration is less efficient than the aerobic variety and produces just 2 ATP per molecule of glucose. Anaerobic respiration has two stages: glycolysis and fermentation.

analogous trait

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

anaphase

The stage of mitosis in which sister chromosomes are separated and pulled to opposite ends of the cell by microtubules; the fourth stage of the first meiotic division (meiosis I), during which maternal and paternal homologous pairs are separated on microtubules; the fourth stage of the second meiotic division (meiosis II), during which either maternal or paternal sister chromatids are separated on microtubules.

angiosperm

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

anther

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

anticodon

The sequence of three nucleotides on tRNA that pairs with a codon of mRNA at the A site of a ribosome during translation.

antigen

A protein coat on the surface of red blood cells; a red blood cell may have a protein coat of type A, B, or AB. If the cell has no antigens, it is called type O. The presence of a foreign antigen in a body will cause blood to clot.

aorta

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

aphotic zone

Literally, zone without light. The aphotic zone is part of the marine pelagic zone and begins 600 feet below the surface of the ocean. Only chemosynthetic organisms, scavengers, and predators are able to survive in this habitat.

artery

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. The autonomic nervous system is 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.

autosome

Any chromosome that is not a sex chromosome. Humans have 44 autosomes, in 22 homologous pairs. The two sex chromosomes are the twenty-third pair of chromosomes.

autotroph

An organism that can produce the organic molecules and energy necessary for life through the processes of photosynthesis or chemosynthesis. Autotrophs do not rely on other organisms for food. In a food web, autotrophs are producers.

auxin

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

base

An ion or compound that removes H⁺ ions from solution. Often bases are substances that release hydroxide ions (OH^–). Bases have pH values above 7. They are the opposite of acids.

bile

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

binary fission

Asexual reproduction found in prokaryotes in which a cell divides into two equal daughter cells by a nonmitotic process.

biomass

The 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.

biome

A particular geographic area with a common climate and characteristic plant and animal life. There are six major terrestrial biomes and two aquatic biomes. The six terrestrial biomes are tropical rain forest, savanna, desert, temperate deciduous forest, taiga, and tundra. The two aquatic biomes are marine and freshwater. Each biome is characterized by specific climax communities.

blood

The liquid that carries nutrients and oxygen to the cells and carries carbon dioxide and nitrogenous wastes away. The liquid fluid of blood is called plasma. Red blood cells contain hemoglobin, an iron-containing protein that binds oxygen. White blood cells fight disease. Platelets clot to prevent extreme blood loss resulting from injury.

bone

Rigid structures composed of living cells rooted in a matrix of calcium, phosphate salts, and collagen fibers. Bones are the primary component of most vertebrate skeletons.

brain

The center of the central nervous system. The brain coordinates the processes of the body. It is composed of various distinct regions, all of which have different functions, including the cerebrum, cerebellum, medulla oblongata, and hypothalamus.

bryophyte

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

budding

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

buffer

Solutions that resist change in pH even when acids and bases are added.

bulb

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. Also called the Calvin-Benson cycle or the dark reactions.

capillary

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

carbon

The central element of life. Carbon has the ability to form bonds with up to four other elements or molecules at the same time.

carrying capacity

The maximum number of individuals in a population that can be sustained in a given environment. As populations become increasingly concentrated, competition for food and space, predation, and disease all determine carrying capacity.

cartilage

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

cell

The smallest unit of life, consisting of a solution of organic molecules enclosed by a plasma membrane.

cell cycle

A process in which cells reproduce. First the cell replicates its DNA and then divides into two daughter cells. The two main phases of the cell cycle are interphase and mitosis.

cell membrane

The phospholipid bilayer that surrounds all cells, regulating the passage of molecules in and out of the cell.

cellular respiration

The 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.

cell theory

The doctrine that every living organism is composed of cells and that all cells come only from other preexisting cells.

cell wall

A rigid structure that surrounds the outer membrane of some cells and helps maintain their shape. In plants the cell wall contains cellulose; in fungi it contains chitin; in prokaryotes it typically contains peptidoglycan.

cellulose

A complex carbohydrate that constitutes the cell walls of plants and protist molds.

central nervous system (CNS)

The brain and the spinal cord. The CNS acts as the central command center of the body. Mostly made up of interneurons.

centriole

A structure in the cell that plays an important role in cell replication. During prophase, the centrioles migrate to the poles of the cell and form the mitotic spindle, which allows the chromosomes to be organized and split when the cell divides.

cerebellum

Part of the brain. Makes sure that movements are coordinated and balanced.

cerebrum

Part of the brain. Controls all voluntary movement, sensory perception, speech, memory, and creative thought.

chemical cycles

The 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.

chemosynthesis

Synthesizing organic compounds by energy derived from chemical reactions rather than from the energy of the sun. Chemosynthetic organisms are autotrophs.

chitin

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

chlorophyll

A pigment located within a chloroplast that absorbs light in plant cells, helping to convert light energy into biological energy through the process of photosynthesis.

chloroplast

A double-membrane-bound organelle that contains chlorophyll and is found in plant cells. Chloroplasts are responsible for mediating photosynthesis.

chromatin

The stringy web of genetic material and histone proteins found in the nucleus of eukaryotic cells. During cell division, each strand of DNA coils to form a chromosome.

chromosome

A physical structure composed of a single long strand of DNA (and associated proteins), containing along its length many genes. The human genome consists of 46 chromosomes contained within the nucleus of each cell.

cilia

Short, hairlike projections found on eukaryotic cells that can help the cell move or can sweep food particles toward the mouth.

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. The pulmonary circuit carries blood to the lungs to be oxygenated, while the systemic circuit carries oxygenated blood to the body. Vertebrates have a closed circulatory system, while arthropods have an open system.

citric acid cycle

See Krebs cycle.

climax community

A 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.

codominance

A phenomenon in which two alleles of the same gene are fully expressed in the phenotype when both are present in a heterozygote. Blood type is an example of codominance.

codon

A three-nucleotide sequence in a DNA or mRNA molecule. Each codon specifies a single amino acid.

coenzyme

A compound that regulates activity by binding to an enzyme to tell it when to catalyze a reaction.

cold-blooded

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.

community

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

competition

The struggle for survival between organisms or populations that use similar resources and occupy similar niches. Interspecific competition refers to competition between populations that may drive a population out of a community or push it to evolve a different niche to reduce competition. Intraspecific competition refers to competition between individuals of the same species.

consumer

Consumers are heterotrophic organisms within the food web of a community. In the trophic levels of the food chain, primary consumers consume producers, secondary consumers consume primary consumers, and tertiary consumers consume secondary consumers.

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).

corpus luteum

After releasing its ovum (ovulation), the follicle becomes the corpus luteum, which produces progesterone for the continued buildup of the uterine wall.

crossing-over

The exchange of genetic information between homologous chromosomes during meiosis I. Crossing-over can disrupt the normal linkage between genes on the same chromosome.

cytokinesis

The final part of mitosis, in which a cell with duplicated contents splits into two independent cells.

cytoplasm

The entire content of the cell outside the nucleus, including the membrane-bound organelles and the cytosol.

cytoskeleton

A system of protein filaments found throughout the cytoplasm of eukaryotic cells, which provides structural support for the cell. The cytoskeleton also helps with the movement of organelles within the cell. It is composed of microfilaments and microtubules.

cytosol

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

Darwin, Charles

English naturalist (1809–1882) who proposed the modern theory of evolution through natural selection. Darwin traveled aboard the HMS Beagle to the Galápagos Islands, where his revolutionary observations took shape.

decomposer

Organisms that consume waste products and dead organic material and constitute part of the food web, which also includes producers and consumers. Also called saprophytes. Decomposers liberate inorganic elements such as nitrogen and carbon and allow those elements to move back into their respective chemical cycles. Examples of decomposers are bacteria and fungi.

dehydration synthesis

A common biochemical reaction in which a new compound is formed by the joining of two compounds to release water. Occurs in the synthesis of polysaccharides and polypeptides. The reverse of hydrolysis.

deoxyribonucleic acid (DNA)

A type of nucleic acid polymer built from sugar-phosphate backbones and nitrogenous bases. DNA’s sugar, deoxyribose, has one fewer oxygen atom than ribose, found in RNA. The nitrogenous bases adenine, cytosine, guanine, and thymine are used in DNA.

dicot

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.

diffusion

The transport or natural drift of molecules traveling from an area of higher concentration to an area of lower concentration. Diffusion does not require outside energy from the cell.

digestive system

The system of organs that converts food to usable nutrients through mechanical and chemical breakdown. Important components of the system are the alimentary canal, glands, esophagus, stomach, small intestine, large intestine, and rectum.

diploid number

The total number of chromosomes present in a somatic cell. The diploid number is twice the haploid number. In humans, the diploid number is 46.

disaccharide

A sugar compound consisting of two carbohydrate monomers.

dominant

Refers to an allele that controls the phenotype even when a different allele is also present, as in a heterozygote. Can also refer to the trait or phenotype produced by a dominant allele. Also known as Mendel’s law of dominance, based on Gregor Mendel’s observations that when two purebred individuals with different forms of the same trait are mated, only one of the two forms appears in the first generation of offspring. Mendel called the apparent form dominant and the suppressed form recessive.