AP Biology key terms

A bond in which electrons are shared unequally between atoms, giving one end of the molecule a partial positive charge and the other a partial negative charge. This uneven sharing lets water dissolve many substances.

A weak attraction between a partially positive hydrogen atom in one molecule and a partially negative atom, usually oxygen or nitrogen, in another. Individually weak, these bonds collectively give water its unusual properties.

The tendency of molecules of the same kind to stick together, driven in water by hydrogen bonding. Cohesion helps water move upward through plant vessels and creates surface tension.

The tendency of molecules of one kind to stick to molecules of a different kind. In plants, water adhering to vessel walls helps counter gravity as water rises.

The resistance of a liquid's surface to being broken or stretched, caused by cohesive forces pulling surface molecules inward. It allows some small organisms to rest on water.

A reaction in which water is used to break a covalent bond, splitting a large molecule into smaller ones. Digestion of proteins and polysaccharides works this way.

A reaction that joins two smaller molecules by removing a water molecule, forming a new covalent bond. It builds polymers such as proteins and starch from their subunits.

A small repeating subunit that can be linked to identical or similar units to build a larger molecule. Amino acids and nucleotides are examples.

A large molecule made of many smaller subunits joined by covalent bonds. Proteins, nucleic acids, and polysaccharides are biological examples.

A lipid with a phosphate-containing polar head and two nonpolar fatty acid tails. In water these molecules self-assemble into bilayers that form cell membranes.

Describing a molecule that has both a water-loving region and a water-fearing region. This dual nature drives membrane lipids to form bilayers in aqueous environments.

The description of membranes as a dynamic two-dimensional fluid of lipids in which proteins, cholesterol, and carbohydrates are embedded and can move laterally.

The property of a membrane that allows some substances to cross freely while restricting others. Small nonpolar molecules pass easily; ions and large polar molecules need transport proteins.

Passive movement of a substance across a membrane down its concentration gradient with the help of a channel or carrier protein. No energy input is required.

Movement of a substance across a membrane against its concentration gradient, requiring energy usually supplied by ATP. The sodium-potassium pump is a classic example.

A difference in the amount of a dissolved substance between two regions. Particles tend to move from higher to lower concentration, and cells exploit this to do work.

The net movement of particles from a region of higher concentration to one of lower concentration due to random motion. It requires no energy and continues until equilibrium is reached.

The diffusion of water across a selectively permeable membrane from a region of higher water concentration to one of lower water concentration. It drives water balance in cells.

A measure of the tendency of water to move from one area to another, combining the effects of solute concentration and physical pressure. Water moves toward lower water potential.

The idea that mitochondria and chloroplasts originated as free-living bacteria engulfed by an ancestral host cell. Evidence includes their own circular DNA and double membranes.

An organelle bounded by an inner and outer membrane where aerobic respiration produces most of a cell's ATP. Its folded inner membrane increases the surface area for energy reactions.

A plant organelle containing chlorophyll and stacked internal membranes where photosynthesis converts light energy into chemical energy stored in sugars.

A protein, often, that speeds up a reaction by lowering its activation energy without being consumed. Each enzyme binds specific substrates at its active site.

The minimum energy that colliding reactants must possess for a reaction to occur. Enzymes reduce this barrier, allowing reactions to proceed at body temperature.

The specific region of an enzyme where substrate molecules bind and are converted to products. Its shape determines which reactions the enzyme can catalyze.

Loss of a protein's functional three-dimensional shape due to heat, extreme pH, or other stress, disrupting bonds that maintain structure. Denatured enzymes usually lose catalytic activity.

Adenosine triphosphate, the cell's main energy currency. Breaking its terminal phosphate bond releases energy that powers endergonic cellular work such as transport and synthesis.

The set of reactions that harvest energy from organic molecules, mainly glucose, to produce ATP. In the presence of oxygen it includes glycolysis, the citric acid cycle, and oxidative phosphorylation.

The first stage of cellular respiration, occurring in the cytoplasm, in which glucose is split into two pyruvate molecules with a net gain of ATP and NADH. It does not require oxygen.

The process by which plants, algae, and some bacteria use light energy to convert carbon dioxide and water into organic compounds, releasing oxygen. It occurs in chloroplasts.

The series of steps by which a cell converts an extracellular signal into a specific cellular response, often through a receptor and relay molecules. It amplifies and integrates incoming information.

A protein that binds a specific signaling molecule and changes shape in response, triggering a cellular response. Receptors may sit in the membrane or inside the cell.

A regulatory mechanism in which the output of a process affects its own activity. Negative feedback reduces the output to restore balance; positive feedback amplifies it.

The maintenance of a relatively stable internal environment within an organism despite external changes. It relies on sensors, control centers, and effectors working through feedback.

A type of nuclear division that halves the chromosome number and produces genetically variable gametes. Two successive divisions follow one round of DNA replication.

The exchange of corresponding chromosome segments between homologous chromosomes during prophase I of meiosis. It creates new combinations of alleles on a chromosome.

The random orientation of homologous chromosome pairs at the metaphase plate in meiosis I, so alleles of different genes are distributed to gametes independently. It increases genetic variation.

One of two or more alternative versions of a gene that occupy the same location on a chromosome. Different alleles can produce different traits or phenotypes.

The specific combination of alleles an organism carries for a given gene or set of genes. It interacts with the environment to produce observable traits.

The observable characteristics of an organism, such as appearance, physiology, or behavior. It results from the genotype interacting with environmental factors.

The synthesis of an RNA molecule from a DNA template, carried out by RNA polymerase. In eukaryotes it occurs in the nucleus before the RNA is processed.

The process in which ribosomes read messenger RNA codons and assemble the corresponding sequence of amino acids into a polypeptide. It occurs in the cytoplasm.

The overall process by which information in a gene directs the production of a functional product, usually a protein. It can be regulated at transcription, RNA processing, translation, or after translation.

A cluster of genes in bacteria that are transcribed together and controlled by a single regulatory region. It allows coordinated responses to environmental conditions.

A change in the nucleotide sequence of DNA. Mutations are the original source of new alleles and therefore of genetic variation in populations.

The process by which individuals with inherited traits that improve survival and reproduction leave more offspring, so those traits become more common over generations.

An organism's relative contribution of offspring to the next generation compared with other individuals in the population. It measures reproductive success, not strength.

An inherited trait that increases an organism's ability to survive and reproduce in its environment, arising over generations through natural selection.

A random change in allele frequencies in a population, most pronounced in small populations. It is not driven by the trait's usefulness.

The movement of alleles between populations when individuals migrate and breed. It tends to reduce genetic differences among populations.

The maximum population size of a species that an environment can sustain indefinitely given available resources. Populations often grow logistically toward this limit.

Population growth at a constant per-individual rate, producing a rapidly accelerating curve. It occurs when resources are abundant and limiting factors are absent.

A feeding position in a food web, such as producer, primary consumer, or secondary consumer. Energy available generally decreases at each higher level.