AP Biology Key Terms & Vocabulary

321 essential AP Biology terms, defined — aligned to the College Board CED.

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5' and 3' ends: The two distinct ends of a nucleic acid strand, named for the 5' phosphate and 3' hydroxyl on the sugar. New nucleotides are always added to the 3' end.
A
Activation energy: The energy barrier a reaction must overcome to proceed. Enzymes work by lowering this barrier.
Active site: The region of an enzyme that binds the substrate. Its shape and charge determine which substrate the enzyme can act on.
Active transport: Movement of molecules that requires energy input, often pumping substances against their concentration gradient (low to high).
Adaptive radiation: Rapid diversification of one lineage into many species as new habitats or niches become available, like Darwin's finches or Hawaiian Drosophila.
Adhesion: Water sticking to other surfaces. Combined with cohesion, it lets water climb against gravity, as in plant vascular tissue.
Allele: One of the alternative versions of a gene. Students often confuse a gene (the locus) with an allele (the specific variant at that locus).
Allele frequency: The proportion of a specific allele among all copies of a gene in a population. A change in allele frequency over generations is the definition of evolution.
Allopatric speciation: Speciation driven by geographic isolation that blocks gene flow, allowing populations to diverge independently. It is the most common mode of speciation.
Allosteric site: A binding location on an enzyme other than the active site. Molecules binding here can alter the enzyme's activity.
Alternative splicing: Joining exons in different combinations from one transcript to produce multiple mature mRNAs, and thus different proteins, from a single gene.
Amino acid: The monomer of proteins. Each has a central carbon bonded to a hydrogen, a carboxyl group, an amine group, and a variable R group.
Analogous structures: Features with similar function but different ancestry, arising through convergent evolution, like bird and insect wings. They do NOT indicate common ancestry.
Anaphase: The mitotic stage where sister chromatids separate and spindle fibers pull them toward opposite poles.
Aneuploidy: Having an abnormal number of chromosomes, usually from nondisjunction. It often causes new phenotypes and developmental disorders.
Anticodon: The three-nucleotide sequence on a tRNA that base-pairs with a complementary mRNA codon, ensuring the correct amino acid is added.
Antiparallel double helix: DNA's structure: two nucleotide strands running in opposite 5' to 3' directions, twisted together and held by hydrogen bonds between paired bases.
Antiparallel strands: The two DNA strands run in opposite 5' to 3' directions. This orientation forces continuous synthesis on one strand and discontinuous on the other.
Apoptosis: Programmed cell death, a controlled self-destruct triggered by signals. It removes damaged or unneeded cells and is a normal part of development and tissue maintenance.
Aquaporin: A channel protein that lets large quantities of water move rapidly across a membrane.
Artificial selection: Humans choosing which individuals reproduce based on desired traits. The mechanism is identical to natural selection; only the selecting agent (humans vs. environment) differs, and it usually reduces genetic diversity.
ATP: Adenosine triphosphate, the cell's main energy currency. Energy is released when its phosphate bond is broken, powering cellular work.
ATP synthase: The membrane-bound enzyme that synthesizes ATP from ADP and inorganic phosphate as protons flow through it down their gradient.
B
Bacterial transformation: Introducing foreign DNA into bacterial cells, often via a plasmid, so they express the new genes. A workhorse of genetic engineering.
Balancing selection: Selection that maintains multiple alleles at stable frequencies rather than fixing one, often via heterozygote advantage. It preserves genetic polymorphism.
Behavioral response: An organism's action triggered by an internal or external cue, like a deer shifting to nighttime grazing when it detects predator signals. It can occur without direct contact with the stimulus.
Biodiversity: The variety of species and functional roles in an ecosystem. Higher functional diversity generally increases resilience to disruption, but raw species count alone doesn't guarantee stability.
Biogeochemical cycle: The movement of matter (carbon, nitrogen, water, phosphorus) between biotic organisms and abiotic reservoirs. Matter cycles and is conserved, while energy only flows.
Biological species concept: Defines a species as a group capable of interbreeding in nature to produce viable, fertile offspring. It depends on actual reproductive isolation, not lab capability.
Biomagnification: The increasing concentration of a persistent toxin in tissues at successively higher trophic levels. Top predators accumulate the highest doses.
Bottleneck effect: A type of genetic drift where a population is drastically reduced for at least one generation, so survivors carry only a fraction of the original gene pool. Cheetahs are a real example.
C
Calvin cycle: The carbon fixation reactions of photosynthesis, occurring in the stroma. It uses ATP and NADPH from the light reactions to build carbohydrates from carbon dioxide.
Cancer: Uncontrolled cell division resulting from disrupted cell cycle regulation, such as failed checkpoints. Shows what happens when the cell cycle's brakes fail.
Carbohydrate: A macromolecule built from sugar monomers. Used for quick energy and structural support.
Carbon cycle: Recycling of carbon through photosynthesis, cellular respiration, decomposition, and combustion. Photosynthesis is the entry point that pulls atmospheric CO2 into organisms.
Carrying capacity: The population size (K) an environment's resources can sustain long-term. It is a dynamic equilibrium, not a hard ceiling; populations can overshoot it and then crash back.
Cell: The basic unit of life. Cells organize life's processes and provide the environment in which organelles operate.
Cell cycle: The highly regulated sequence of events that controls eukaryotic cell growth and division: interphase (G1, S, G2), mitosis, and cytokinesis.
Cell cycle checkpoint: An internal control point that verifies conditions are right before the cell cycle proceeds. The exam often tests the purpose and timing of these checkpoints.
Cell differentiation: The process by which a cell becomes specialized through expressing tissue-specific genes. Drives the formation of distinct tissues from identical cells.
Cell signaling: How cells send and receive information, either by direct contact or by releasing chemical signals. It coordinates growth, immune responses, and homeostasis across an organism.
Cell wall: A rigid outer boundary in bacteria, archaea, fungi, and plants. Provides structure, a permeability barrier, and protection against osmotic lysis.
Cellular respiration: The process that uses energy stored in biological macromolecules to synthesize ATP. Found in all forms of life.
Cellular response: The end result of a signal transduction pathway, such as cell growth, secretion of a molecule, gene expression, or apoptosis. The same signal can produce different responses in different cells.
Centromere: The constricted region joining sister chromatids and the attachment point for spindle fibers. Holds the copies together until anaphase.
Channel protein: A membrane protein that forms a pore allowing specific ions or molecules, such as sodium or potassium, to cross the membrane.
Chemiosmosis: The flow of protons back across a membrane through ATP synthase, driving the formation of ATP. Powers both photosynthesis and respiration.
Chemosynthesis: Carbon fixation powered by oxidizing inorganic compounds like hydrogen sulfide rather than sunlight. It supports deep-sea vent ecosystems where photosynthesis is impossible.
Chi-square test: A statistical test comparing observed offspring ratios to expected ratios to decide whether to reject the null hypothesis. Frequently calculated on the AP exam.
Chiasma: The point where non-sister chromatids of homologous chromosomes cross and exchange segments during prophase I. The physical site of crossing over.
Chlorophyll: The pigment in photosystems that absorbs light energy, boosting electrons to a higher energy level to begin the light reactions.
Chloroplast: The double-membrane organelle of photosynthesis, found in plants and photosynthetic algae.
Cholesterol: A steroid lipid embedded in animal cell membranes where it provides structural stability and regulates membrane fluidity.
Cladogram: A branching diagram showing relatedness based on shared derived characters, but with no time scale or measure of evolutionary distance, unlike a phylogenetic tree.
Codominance: When both alleles in a heterozygote are fully expressed, so the phenotype shows both, like AB blood type. Neither allele masks the other.
Codon: A sequence of three mRNA nucleotides that specifies one amino acid or a stop signal. The genetic code reads mRNA in these triplets.
Cohesion: Water molecules sticking to each other via hydrogen bonds. Why water forms droplets and produces surface tension.
Commensalism: A symbiotic interaction (+/0) where one species benefits and the other is neither helped nor harmed, like barnacles riding a whale.
Common ancestry: The principle that organisms share descent from shared ancestors. The universal genetic code is one of the strongest pieces of evidence that all life shares one common ancestor.
Community: Groups of interacting populations of different species in an area. Interactions between populations determine how each accesses energy and matter.
Compartmentalization: The use of internal membranes and organelles to partition a eukaryotic cell into specialized regions, minimizing competing reactions and increasing reactive surface area.
Competition: An interaction (−/−) where species or individuals vie for the same limited resource. It drives niche differentiation and character displacement.
Competitive exclusion principle: Gause's law: two species competing for an identical niche cannot coexist; the superior competitor eliminates the other. Coexistence requires niche partitioning.
Competitive inhibitor: A molecule that binds reversibly to an enzyme's active site, blocking the substrate from binding.
Complementary base pairing: The rule that adenine pairs with thymine (A-T) and cytosine with guanine (C-G) in DNA via hydrogen bonds; in RNA, adenine pairs with uracil (A-U).
Concentration gradient: A difference in solute concentration across a membrane. Gradients store potential energy and drive diffusion.
Conjugation: The direct cell-to-cell transfer of DNA between prokaryotes, often via a plasmid. Spreads genes, including antibiotic resistance.
Convergent evolution: When unrelated lineages independently evolve similar traits due to similar selective pressures. It can make distant species look related, misleading naive tree-building.
Coral bleaching: When heat-stressed corals expel their symbiotic zooxanthellae algae, which supply most of their energy, causing the coral to starve. It's a breakdown of a foundational mutualism.
Crossing over: The exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I. A major source of genetic diversity in gametes.
Cyclic AMP (cAMP): A classic second messenger made from ATP that relays and amplifies signals inside the cell. Often produced after a GPCR activates the enzyme that synthesizes it.
Cyclin: A regulatory protein whose levels rise and fall through the cell cycle. It partners with a kinase to push the cell past checkpoints.
Cyclin-dependent kinase (CdK): An enzyme that drives the cell cycle forward only when bound to a cyclin. The cyclin-CdK partnership is the master control of cell cycle progression.
Cytokinesis: Division of the cytoplasm into two daughter cells, by a cleavage furrow in animal cells or a cell plate in plant cells. It usually overlaps the end of mitosis.
D
Dehydration synthesis: The reaction that links two monomers by removing the equivalent of a water molecule (an H from one, an OH from the other), forming a covalent bond. This is how polymers are built.
Denaturation: The disruption of a protein's structure by changes in temperature, pH, or chemical environment, which can eliminate an enzyme's ability to catalyze reactions. Sometimes reversible.
Density-dependent factor: A limiting factor whose effect intensifies as population density rises, like disease, competition, and predation. These drive the braking effect in logistic growth.
Density-independent factor: A limiting factor that affects the same proportion of a population regardless of its size, like drought, fire, or floods.
Differential gene expression: When different cells express different subsets of genes, producing specialized cell types from one genome. The basis of cell differentiation.
Diffusion: The passive net movement of molecules from high to low concentration until evenly distributed.
Dihybrid cross: A cross tracking two genes at once, used to test independent assortment. Produces a 9:3:3:1 phenotypic ratio when genes assort independently.
Diploid: Having two sets of chromosomes (2n), one from each parent, as in most body cells. Meiosis reduces diploid cells to haploid gametes.
Directional selection: Selection that shifts a population's trait mean toward one extreme, like pesticide resistance becoming common. One end of the trait range is consistently favored.
Disruptive selection: Selection that favors both extremes and removes intermediates, splitting a population's trait distribution. It's the only selection mode that can directly lead to speciation.
Divergent evolution: When populations adapting to different environments accumulate phenotypic differences, often producing new species. Adaptive radiation is divergent evolution that's especially rapid.
DNA fingerprint: A pattern of DNA fragments unique to an individual, used in forensic identification and comparing samples. Produced by sequencing or fragment analysis.
DNA polymerase: The enzyme that builds new DNA strands in the 5' to 3' direction, adding complementary nucleotides to a template. It requires an RNA primer to start.
DNA replication: The semiconservative copying of DNA before cell division, where each old strand templates a new one. Ensures hereditary information is passed on accurately.
DNA sequencing: Technology that determines the exact order of nucleotides in a DNA molecule, often yielding a DNA fingerprint for comparing samples.
Dominant allele: An allele whose trait appears in the phenotype even when only one copy is present, masking the recessive allele.
E
Ecological efficiency: The roughly 10% of energy transferred from one trophic level to the next, with 90% lost as heat and metabolism. This thermodynamic limit caps food chains at about 4-5 levels.
Ecological succession: The sequential change in community composition over time after a disturbance, progressing toward a stable climax community.
Ecosystem resilience: An ecosystem's ability to absorb disruption and recover. It rises with biodiversity and functional redundancy; monocultures are efficient but fragile.
Electrochemical gradient: The combined concentration and charge difference across a membrane. Establishing it requires energy, and its stored energy can power transport and ATP synthesis.
Electron transport chain: A series of membrane proteins that pass electrons through oxidation-reduction reactions, pumping protons to build an electrochemical gradient across a membrane.
Endocytosis: An energy-requiring process where the cell takes in large molecules or particles by folding the membrane inward to form vesicles.
Endomembrane system: The connected group of membrane-bound organelles (ER, Golgi, lysosomes, vacuoles, vesicles, nuclear envelope, plasma membrane) that modify, package, and transport molecules.
Endoplasmic reticulum: An organelle network that supports cell shape and handles intracellular transport. Rough ER (with ribosomes) makes proteins; smooth ER handles lipid synthesis and detoxification.
Endosymbiosis: The theory that mitochondria and chloroplasts evolved from once free-living prokaryotic cells engulfed by an ancestral host cell.
Endosymbiotic theory: The idea that mitochondria and chloroplasts were once free-living bacteria engulfed by a host cell. Their circular DNA, 70S ribosomes, double membranes, and binary fission support it.
Energy flow: The one-way movement of energy through an ecosystem, entering as sunlight or chemical energy and ultimately lost as heat. Unlike matter, energy is never recycled.
Enhancer: A regulatory DNA sequence, often far from the gene, that boosts transcription when bound by activator proteins. Can lie upstream or downstream of the start site.
Enzyme: A protein that acts as a biological catalyst, speeding up reactions by lowering their activation energy without being consumed.
Enzyme-substrate complex: The temporary structure formed when a substrate binds an enzyme's active site, the step that allows catalysis to occur.
Epigenetic change: A reversible modification of DNA or histones that alters gene expression without changing the DNA sequence. Can be influenced by the environment.
Eutrophication: Nutrient overload (nitrogen, phosphorus) that fuels algal blooms; when the bloom dies, decomposer respiration depletes dissolved oxygen, killing aquatic life by hypoxia, not direct toxicity.
Evolutionary fitness: Measured by reproductive success, not strength or size. The fittest organism is the one that leaves the most offspring in a given environment.
Exocytosis: An energy-requiring process where internal vesicles fuse with the plasma membrane to release large molecules out of the cell.
Exon: A coding segment of mRNA that is retained and spliced together during processing. Exons carry the information used to build the protein.
Exponential growth: Unrestrained growth modeled as dN/dt = r_max·N, producing a J-shaped curve. It assumes unlimited resources, so growth accelerates as the population gets larger.
F
Facilitated diffusion: Passive movement of large or charged substances through transport or channel proteins, still moving down the concentration gradient with no energy input.
FADH2: An electron carrier that delivers electrons from the Krebs cycle to the electron transport chain.
Feedback mechanism: A loop where the output of a process loops back to influence that same process. It keeps internal conditions stable or pushes them rapidly toward a new state.
Fermentation: A pathway that lets glycolysis continue without oxygen, producing organic molecules such as alcohol or lactic acid.
Fertilization: The fusion of two haploid gametes that restores the diploid chromosome number and creates a new allele combination in the zygote. A key source of variation.
Fluid mosaic model: The model describing the membrane as a flexible 'fluid' of phospholipids with a 'mosaic' of embedded proteins, cholesterol, glycoproteins, and glycolipids that can drift laterally.
Founder effect: A type of genetic drift where a few individuals colonize a new area, carrying a non-representative sample of alleles. Rare alleles can become common purely by chance.
Frameshift mutation: A mutation from inserting or deleting nucleotides that shifts the reading frame, altering every codon downstream. Often more disruptive than point mutations.
Functional redundancy: Having multiple species that perform the same ecological role, so the loss of one is buffered by others. It is the main reason diverse ecosystems resist single-point failures.
G
G protein-coupled receptor (GPCR): A common type of cell-surface receptor in eukaryotes that changes shape when a ligand binds, activating a G protein inside the cell. A huge fraction of drugs target GPCRs.
G0 phase: A non-dividing state a cell can enter and leave in response to cues. Many specialized cells, like neurons, rest here long-term.
G1 phase: The first growth phase of interphase where the cell is metabolically active and duplicates organelles and cytosolic components before DNA synthesis.
G2 phase: The growth phase after DNA replication where the cell produces proteins and ATP and duplicates centrosomes in preparation for mitosis.
Gamete: A haploid reproductive cell (egg or sperm) produced by meiosis. Two gametes fuse during fertilization to form a diploid zygote.
Gel electrophoresis: A technique that separates DNA fragments by size and charge as they move through a gel in an electric field. Smaller fragments travel farther.
Gene expression: The overall process of turning a gene's information into a functional product, mainly through transcription and translation. Regulated at many levels.
Gene flow: The movement of alleles between populations via migration. It keeps populations genetically similar, so stopping gene flow is often the first step toward speciation.
Gene mapping: Estimating the distance between linked genes in map units, based on how often crossing over separates them. The farther apart, the more recombination.
Genetic code: The nearly universal set of rules that maps each mRNA codon to an amino acid. Its shared use across organisms is evidence of common ancestry.
Genetic diversity: The level of allele variation in a population. Populations with low diversity are more at risk of decline or extinction because they're less able to withstand new pressures.
Genetic drift: Random change in allele frequencies due to chance sampling, not selection. Its effect is strongest in small populations, which is why endangered species lose diversity fast.
Genetic engineering: Techniques used to analyze and manipulate DNA and RNA, including PCR, gel electrophoresis, and bacterial transformation.
Genetic linkage: When two genes sit close together on the same chromosome and tend to be inherited together, violating independent assortment. Recombination frequency reveals it.
Genetic recombination: The creation of new allele combinations, largely through crossing over and independent assortment during meiosis. It mixes parental traits in offspring.
Genotype: The set of alleles an organism inherits for a gene or genes. It can be homozygous or heterozygous and underlies the observable phenotype.
Glycolysis: The biochemical pathway that splits glucose to form ATP, NADH, and pyruvate. Occurs in the cytosol and does not require oxygen.
Golgi complex: A stack of flattened membrane sacs that folds, chemically modifies, and packages newly made proteins for trafficking to their destinations.
Gradualism: A model where evolution occurs slowly and steadily over hundreds of thousands to millions of years, producing gradual transitional change.
Granum: A stack of thylakoids within the chloroplast. The light reactions of photosynthesis occur here.
GTP cap: A modified guanine cap added to the 5' end of eukaryotic mRNA that helps the ribosome recognize and bind the transcript.
H
Haploid: Having a single set of chromosomes (1n), as in gametes. Fertilization combines two haploid sets to restore the diploid number.
Hardy-Weinberg conditions: The five requirements for no evolution: large population, no migration, no mutation, random mating, and no selection. They're never fully met, which is the point of a null model.
Hardy-Weinberg equation: p² + 2pq + q² = 1 (with p + q = 1), relating genotype frequencies to allele frequencies. p² and q² are the homozygotes; 2pq is the heterozygote (carrier) frequency.
Hardy-Weinberg equilibrium: A null model describing a non-evolving population where allele frequencies stay constant. Deviation from its predictions signals that evolution is occurring.
Heat of vaporization: The energy needed to evaporate a liquid. Water's is high, which makes evaporative cooling (like sweating) effective for shedding heat.
Helicase: The enzyme that unwinds and separates the two DNA strands at the replication fork, exposing template strands for copying.
Heterotroph: An organism that obtains energy by consuming organic matter from other organisms, including carnivores, herbivores, omnivores, decomposers, and scavengers.
Heterozygote advantage: When the heterozygous genotype has higher fitness than either homozygote, as with sickle cell carriers resisting malaria. It maintains otherwise harmful alleles in a population.
Heterozygous: Having two different alleles for a gene. The dominant allele usually masks the recessive one in simple Mendelian inheritance.
Homeostasis: The maintenance of a stable internal environment despite external changes, achieved largely through feedback. Required for enzymes and cells to function properly.
Homologous chromosomes: A matching pair of chromosomes, one maternal and one paternal, carrying the same genes at the same loci. They pair up and separate during meiosis I.
Homologous structures: Features with the same underlying anatomy inherited from a common ancestor, like the human arm, whale flipper, and bat wing. They are evidence of common ancestry.
Homozygous: Having two identical alleles for a gene. A homozygous recessive genotype lets a recessive trait show.
Horizontal gene transfer: The movement of genetic material between organisms outside of reproduction, common in prokaryotes. It increases genetic variation.
Hydrogen bond: A weak attraction between a slightly positive hydrogen and a slightly negative atom (like oxygen) on a neighboring molecule. Individually weak, but collectively they drive water's signature properties.
Hydrolysis: The reaction that breaks a polymer apart by adding water, splitting a covalent bond between monomers. The reverse of dehydration synthesis.
Hypertonic: A solution with higher solute concentration than the cell. Water moves out of the cell, which can cause it to shrink.
Hypotonic: A solution with lower solute concentration than the cell. Water moves into the cell, which can cause it to swell or burst.
I
Incomplete dominance: When the heterozygote's phenotype is an intermediate blend of the two homozygous phenotypes, like a pink flower from red and white parents.
Independent assortment: The random orientation of homologous chromosome pairs during metaphase I, producing many possible gamete combinations. One of Mendel's two laws.
Inducible operon: An operon usually turned off but switched on when a specific molecule is present, like the lac operon responding to lactose.
Interphase: The longest part of the cell cycle, made of G1, S, and G2, where the cell grows, copies its DNA, and prepares to divide. It is not a resting phase.
Intron: A non-coding segment of a primary mRNA transcript that is excised during processing. Only the remaining exons are kept in the mature mRNA.
Invasive species: A non-native species that exploits an empty niche free of natural predators or competitors, outcompeting natives and reducing biodiversity, like kudzu or zebra mussels.
Isotonic: A solution with equal solute concentration to the cell, so there is no net movement of water.
K
Keystone species: A species whose effect on an ecosystem is disproportionate to its abundance, like sea otters controlling urchins. Removing it often causes the ecosystem to collapse.
Kinesis: A change in the rate or randomness of movement in response to a stimulus, with no specific direction. The organism simply moves more or less until it reaches a better environment.
Krebs cycle: The reactions in the mitochondrial matrix that release carbon dioxide, synthesize some ATP, and reduce NAD+ and FAD to NADH and FADH2. Also called the citric acid cycle.
L
Lagging strand: The new DNA strand synthesized discontinuously in short fragments away from the replication fork, because polymerase only works 5' to 3'.
Law of independent assortment: Mendel's law that alleles of different genes on different chromosomes are inherited independently of one another. Explains dihybrid ratios.
Law of segregation: Mendel's law that the two alleles for a gene separate during gamete formation, so each gamete carries only one. Explains the 3:1 monohybrid ratio.
Leading strand: The new DNA strand synthesized continuously toward the replication fork, since its direction matches the unwinding.
Ligand: A signaling molecule (a peptide or small molecule) that binds to a receptor's binding domain. The ligand is the message; the receptor is the mailbox.
Ligand-gated channel: A membrane channel that opens or closes when a ligand binds, letting specific ions flow. Common in fast signaling like neurotransmission.
Ligase: The enzyme that joins the short fragments on the lagging strand into one continuous DNA strand.
Light reactions: The stage of photosynthesis in the thylakoid membranes (grana) that captures light energy to produce ATP and NADPH and splits water, releasing oxygen.
Lipid: A nonpolar, hydrophobic macromolecule. Includes fats, steroids, and phospholipids; used for energy storage, insulation, signaling, and membrane structure.
Logistic growth: Resource-limited growth modeled as dN/dt = r_max·N·(K−N)/K, producing an S-shaped curve. Growth slows and stops as the population approaches carrying capacity.
Lysosome: A membrane-enclosed sac of hydrolytic enzymes that digests material and participates in programmed cell death (apoptosis).
M
Meiosis: A two-division process that produces four haploid gamete cells from one diploid cell in sexually reproducing organisms. It halves the chromosome number and shuffles genetic material.
Meiosis I: The first meiotic division, where homologous chromosomes pair, exchange material, and separate, reducing the chromosome number from diploid to haploid.
Meiosis II: The second meiotic division, where sister chromatids separate, similar to mitosis. It yields four haploid cells, each with unduplicated chromosomes.
Messenger RNA (mRNA): The RNA that carries genetic information from DNA in the nucleus to the ribosome, where it is translated into protein.
Metabolic pathway: A sequence of enzyme-catalyzed reactions where each product becomes the reactant for the next step, allowing controlled transfer of energy.
Metaphase: The mitotic stage where spindle fibers align chromosomes along the cell's equator, setting up an even split.
Miller-Urey experiment: Showed that organic monomers like amino acids can form abiotically from a simulated early-Earth atmosphere and energy. It supports the possibility of life's chemistry, not the full mechanism.
Mitochondrion: The double-membrane organelle of aerobic cellular respiration. Its highly folded inner membrane boosts surface area so more ATP can be synthesized.
Mitosis: Division of the nucleus that produces two genetically identical daughter cells, ensuring each gets a complete genome. Drives growth, tissue repair, and asexual reproduction.
Mitotic spindle: The apparatus of microtubules that attaches to and moves chromosomes during mitosis. Used by both mitosis and meiosis.
Molecular clock: A method using mutation accumulation rates to estimate when two lineages diverged. It can date splits even where the fossil record is absent.
Monohybrid cross: A cross tracking the inheritance of a single gene with two alleles. Yields the classic 3:1 phenotypic ratio in heterozygote offspring.
Monomer: A small molecule that serves as a building block. Monomers bond together to form polymers.
Monosaccharide: A single simple sugar, such as glucose. The monomer for larger carbohydrates.
Mutation: A random change in DNA that adds new genetic variation to a population. It is the ultimate source of all new alleles, but it is not directed by environmental need.
Mutualism: A symbiotic interaction (+/+) where both species benefit, like clownfish and sea anemones.
N
NADH: An electron carrier that delivers high-energy electrons from glycolysis and the Krebs cycle to the electron transport chain.
NADPH: An electron carrier produced in the light reactions (at photosystem I) that supplies energy and electrons to power the Calvin cycle.
Natural selection: The differential survival and reproduction of individuals based on heritable phenotypic variation. It acts on individuals, but populations are what evolve.
Negative feedback: A loop that reduces the initial stimulus to return a system to its set point, like blood-sugar control by insulin and glucagon. It is the main way organisms maintain homeostasis.
Niche partitioning: The division of resources or habitat that lets competing species coexist by using slightly different niches. It is how species avoid competitive exclusion.
Nitrogen cycle: The cycling of nitrogen via fixation, assimilation, ammonification, nitrification, and denitrification, driven largely by soil microorganisms. Plants need ammonia or nitrate because they can't use atmospheric N2 directly.
Nitrogen fixation: The conversion of atmospheric N2 gas into ammonia by specialized bacteria. It's the bottleneck step that makes nitrogen biologically available to plants.
Nitrogenous base: The information-carrying part of a nucleotide. The five bases are adenine, thymine, guanine, cytosine (DNA), and uracil (RNA).
Non-nuclear inheritance: Inheritance of traits encoded by mitochondrial or chloroplast DNA, which is typically passed down maternally and does not follow Mendelian ratios.
Non-random mating: Mating based on genotype or phenotype rather than chance. It shifts genotype frequencies (more homozygotes) without changing allele frequencies, so it alone doesn't cause evolution.
Noncompetitive inhibitor: A molecule that binds an allosteric site (away from the active site), changing the enzyme's shape and activity.
Nondisjunction: The failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate properly. It produces gametes with the wrong chromosome number.
Nonsense mutation: A point mutation that creates a premature stop codon, truncating the protein early and usually disrupting its function.
Nucleic acid: A polymer of nucleotides that stores and transmits biological information. DNA and RNA are the two types.
Nucleotide: The building block of DNA and RNA, made of a sugar, a phosphate group, and a nitrogenous base. Their sequence stores genetic information.
Null hypothesis: The default assumption that observed results match expected results with no significant difference. Students commonly state an alternate hypothesis by mistake.
O
Operon: A cluster of genes in prokaryotes transcribed together and controlled as a unit, such as the lac operon. It can be inducible or repressible.
Optimal range: The temperature and pH conditions at which an enzyme works most efficiently. Conditions outside this range disrupt structure and reduce activity.
Organelle: A specialized subcellular structure that carries out a specific function, often membrane-bound, allowing the cell to compartmentalize its work.
Osmoregulation: The mechanisms organisms use to control internal solute concentration and water balance, maintaining homeostasis and survival.
Osmosis: The diffusion of water across a selectively permeable membrane, from high water potential (low solute) to low water potential (high solute).
Outgroup: The lineage least closely related to the organisms being compared, used to root a tree and identify which traits are ancestral versus derived.
Oxidation-reduction reaction: A reaction involving the transfer of electrons, where one molecule loses electrons (oxidation) and another gains them (reduction). These power electron transport chains.
Oxidative phosphorylation: ATP production in aerobic cellular respiration driven by the proton gradient and chemiosmosis across the inner mitochondrial membrane, with oxygen as the terminal electron acceptor.
P
Parasitism: A symbiotic interaction (+/−) where one organism benefits at the host's expense. Selection can favor reduced virulence if it improves the parasite's transmission.
Passive transport: Net movement of molecules down their concentration gradient (high to low) without any input of cellular energy.
Peptide bond: The covalent bond linking amino acids, formed between the carboxyl group of one and the amine group of the next, building a polypeptide chain.
Phenotype: The observable expression of an organism's traits, resulting from genotype and environment. What you see versus the underlying alleles.
Phenotypic plasticity: The ability of a single genotype to produce different phenotypes under different environmental conditions, like fur color that changes with season.
Phenotypic variation: Differences in observable traits within a population. Natural selection can only act on this variation, so without it, no adaptive evolution occurs.
Phospholipid: A lipid with a hydrophilic phosphate head and two hydrophobic fatty acid tails. This dual nature lets phospholipids form the bilayers of cell membranes.
Phospholipid bilayer: The double layer of phospholipids forming the membrane core: hydrophilic phosphate heads face the watery surroundings while hydrophobic tails point inward.
Phosphorus cycle: The cycling of phosphate from weathered rock through producers, consumers, and decomposition back to soil. Unlike carbon and nitrogen, it has no significant atmospheric reservoir.
Phosphorylation cascade: A relay where one activated protein adds phosphate groups to the next, switching each on in sequence. It amplifies the signal so a few ligand molecules can trigger a large response.
Photophosphorylation: ATP production in photosynthesis driven by the proton gradient across the thylakoid membrane via chemiosmosis through ATP synthase.
Photosynthesis: The process that uses carbon dioxide, water, and light energy to make carbohydrates and oxygen. It captures energy that fuels much of life.
Photosystem: A complex of chlorophyll and proteins embedded in the thylakoid membrane. Photosystems I and II capture light energy and are connected by an electron transport chain.
Phylogenetic tree: A branching diagram showing hypothesized evolutionary relationships, calibrated to time via fossils or a molecular clock. Nodes represent the most recent common ancestor of the lineages above them.
Plasma membrane: The selectively permeable boundary separating a cell's interior from its environment, built mainly from a phospholipid bilayer with embedded proteins.
Plasmid: A small, circular, extra-chromosomal DNA molecule found in prokaryotes and some eukaryotes. Widely used as a vector in biotechnology.
Pleiotropy: When a single gene affects multiple, seemingly unrelated traits. Because the traits come from one gene, they do not segregate independently.
Point mutation: A mutation where one nucleotide is substituted for another. It changes at most one codon and does not shift the reading frame.
Polarity: The uneven distribution of charge across a molecule. Water is polar because oxygen pulls electrons harder than hydrogen, giving it a partial negative end and partial positive ends.
Poly-A tail: A string of adenine nucleotides added to the 3' end of eukaryotic mRNA that makes the transcript more stable.
Polymer: A large molecule built from repeating monomer subunits joined by covalent bonds. Carbohydrates, proteins, and nucleic acids are all polymers.
Polymerase chain reaction (PCR): A technique that amplifies DNA by repeatedly denaturing it, annealing primers, and extending new strands. Makes many copies from a tiny sample.
Polysaccharide: A complex carbohydrate made of many monosaccharides linked together, either linear or branched. Examples include starch, glycogen, and cellulose.
Population genetics: The study of allele and genotype frequencies in populations and how they change over time. Evolution is fundamentally a change in allele frequencies.
Population growth (dN/dt = B − D): The rate of change of population size equals births minus deaths. It's the foundation of both exponential and logistic growth models.
Positive feedback: A loop that amplifies the initial stimulus, pushing the variable further from the set point. Drives processes that need to run to completion, such as labor contractions during childbirth.
Post-zygotic isolation: Reproductive barriers acting after fertilization, such as hybrid inviability or hybrid sterility. More energetically costly than pre-zygotic barriers because reproduction is wasted.
Pre-zygotic isolation: Reproductive barriers that prevent mating or fertilization (habitat, temporal, behavioral, mechanical, or gametic). Selection favors these because they avoid wasting energy on unfit offspring.
Predation: An interaction (+/−) where one organism consumes another. Predators can stabilize prey populations by preventing them from overshooting carrying capacity.
Primary structure: A protein's specific linear sequence of amino acids. This sequence dictates all higher levels of folding and the protein's overall shape.
Primary succession: Succession beginning where no soil exists (bare rock, lava, glacial retreat). Pioneer species like lichens build soil from scratch, so it takes centuries.
Producer: An autotroph that captures energy from sunlight (photosynthesis) or inorganic chemicals (chemosynthesis) to make organic molecules. Producers are the energy entry point for ecosystems.
Promoter: A DNA sequence where RNA polymerase and transcription factors bind to start transcription. Its location relative to a gene helps control expression.
Prophase: The first stage of mitosis where sister chromatids condense, the spindle begins to form, and centrosomes move to opposite poles.
Protein: A polymer of amino acids whose folded shape determines its function. Roles include catalysis, transport, structure, and signaling.
Proton gradient: A difference in hydrogen ion concentration across a membrane, built by the electron transport chain. Its stored energy drives ATP synthesis.
Punctuated equilibrium: A model where evolution occurs in rapid bursts after long periods of stasis, leaving few transitional fossils.
Punnett square: A grid used to predict the genotypes and phenotypes of offspring from a cross. A core tool for working out inheritance probabilities.
Purine: A nitrogenous base with a double-ring structure: adenine and guanine. Purines always pair with single-ring pyrimidines.
Pyrimidine: A nitrogenous base with a single-ring structure: cytosine, thymine, and uracil. Pyrimidines pair with double-ring purines.
Pyruvate: The end product of glycolysis. It is transported into the mitochondrion where it is oxidized in the Krebs cycle.
Q
Quaternary structure: The structure formed when multiple polypeptide chains assemble into one functional protein.
R
R group: The variable side chain that distinguishes one amino acid from another. Its chemistry (nonpolar, polar, or ionic) shapes how that region of a protein folds and behaves.
Receptor protein: A protein that recognizes a specific ligand and starts a cellular response. May sit on the cell surface or inside the cytoplasm/nucleus depending on whether the signal can cross the membrane.
Recessive allele: An allele whose trait appears only when two copies are present, since it is masked by a dominant allele in heterozygotes.
Regulatory sequence: A stretch of DNA that binds regulatory proteins to control whether and how much a gene is transcribed.
Reverse transcriptase: An enzyme in retroviruses that copies viral RNA into DNA, reversing the usual DNA-to-RNA flow. The resulting DNA integrates into the host genome.
Ribosomal RNA (rRNA): RNA that forms the functional building blocks of ribosomes and helps catalyze peptide bond formation during translation.
Ribosome: A non-membrane structure of rRNA and protein that synthesizes proteins by reading mRNA. Found in all forms of life, evidence of common ancestry.
RNA polymerase: The enzyme that synthesizes RNA from a DNA template, reading the template 3' to 5' and building RNA 5' to 3'. It binds promoter regions to start transcription.
RNA primer: A short RNA segment that DNA polymerase needs as a starting point to begin synthesizing a new DNA strand.
RNA processing: The enzyme-mediated modification of a eukaryotic mRNA transcript before translation, including capping, poly-A tail addition, and splicing.
RNA world hypothesis: Proposes that RNA was the earliest genetic material because it can both store information (like DNA) and catalyze reactions (like proteins, as ribozymes), solving the chicken-and-egg problem.
S
S phase: The synthesis phase where DNA replicates, turning each chromosome into two identical sister chromatids joined at a centromere.
Saturated fatty acid: A fatty acid with only single bonds between its carbons, so the chain is straight. Packs tightly and tends to be solid at room temperature.
Second messenger: A small intracellular molecule, such as cyclic AMP (cAMP), that spreads and amplifies a signal after the receptor is activated. It carries the message from the membrane into the cell's interior.
Secondary structure: Local folding of the polypeptide backbone into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.
Secondary succession: Succession on disturbed land where soil and often a seed bank remain, as after a fire or farm abandonment. It is much faster because it skips the slow pioneer stages.
Selective permeability: The membrane property of allowing some substances through while blocking others, a consequence of its hydrophobic interior.
Semiconservative replication: Replication in which each new DNA molecule keeps one original strand and one newly made strand. Confirmed by the Meselson-Stahl experiment.
Set point: The target value a feedback system works to maintain, like a thermostat setting. Negative feedback corrects deviations back toward it.
Sex-linked trait: A trait controlled by a gene on a sex chromosome (X or Y). X-linked recessive traits appear more often in XY individuals, who have only one X.
Sexual selection: A form of natural selection where traits evolve because they boost mating success, even at a survival cost, like the peacock's tail. Reproductive success is part of fitness.
Shared derived character: A trait (synapomorphy) inherited from a common ancestor and used to group lineages on a tree. Relatedness is based on these, not overall appearance.
Signal transduction pathway: The chain of events that links a receptor binding its ligand to the final cellular response. Each step can relay, amplify, or modify the original signal.
Silent mutation: A nucleotide change that does not alter the amino acid sequence, because of redundancy in the genetic code. It has no effect on the protein.
Sister chromatids: The two identical copies of a chromosome produced during S phase, joined at the centromere. They separate during anaphase of mitosis.
Sodium-potassium pump: An ATP-driven membrane protein (Na+/K+ ATPase) that actively moves sodium out and potassium in, helping establish and maintain the membrane potential.
Speciation: The formation of new species when two populations become reproductively isolated and can no longer exchange genes effectively.
Species diversity: A measure of community structure combining the number of species and their relative abundance, often quantified with Simpson's Diversity Index.
Specific heat capacity: The energy needed to raise a substance's temperature. Water's is high, so it resists temperature swings and helps organisms maintain stable internal temperatures.
Stabilizing selection: Selection that favors intermediate phenotypes and removes both extremes, narrowing variation. Human birth weight is the classic example.
Start codon: The AUG codon that begins translation and codes for methionine. It sets the reading frame for the rest of the mRNA.
Steroid: A lipid that acts as a hormone, supporting functions like growth, development, energy metabolism, and homeostasis.
Stop codon: A codon that signals the end of translation, causing the finished polypeptide to be released. No amino acid is added.
Stroma: The fluid inside the chloroplast's inner membrane and surrounding the thylakoids, where the Calvin cycle takes place.
Substrate: The specific reactant an enzyme acts on. Its shape and charge must be compatible with the enzyme's active site.
Surface area-to-volume ratio: The ratio of a cell's surface to its volume. Smaller cells have a higher ratio and exchange materials more efficiently, which limits how large a cell can grow.
Surface tension: The taut 'skin' at a water surface created by cohesion among water molecules. Strong enough to support small insects.
Symbiosis: A close, long-term interaction between two species, including mutualism, commensalism, and parasitism. The categories differ by the fitness effect on each partner.
Sympatric speciation: Speciation that occurs without geographic separation, in populations with overlapping ranges. It is rare and usually requires strong disruptive selection or polyploidy.
Synapsis: The pairing of homologous chromosomes during prophase I, bringing non-sister chromatids close enough to exchange material. Sets the stage for crossing over.
T
Taxis: Directed movement toward or away from a stimulus, like a moth flying toward light. The movement has a clear direction relative to the cue.
Telophase: The final mitotic stage where the spindle breaks down and a new nuclear envelope forms around each set of chromosomes.
Terminal electron acceptor: The final molecule that receives electrons at the end of the electron transport chain. In aerobic respiration this is oxygen.
Tertiary structure: The overall 3D shape of a single polypeptide, held together by hydrogen bonds, hydrophobic interactions, ionic interactions, and disulfide bridges.
Test cross: A cross between an organism of unknown genotype and a homozygous recessive individual. Reveals whether the unknown carries a hidden recessive allele.
Thylakoid: A flattened membrane sac inside the chloroplast where the light reactions occur. Thylakoids contain chlorophyll and electron transport proteins.
Tonicity: A description of a solution's solute concentration relative to a cell: hypotonic, hypertonic, or isotonic. It determines which way water moves.
Topoisomerase: The enzyme that relaxes the supercoiling and tension ahead of the replication fork, preventing the DNA from tangling.
Transcription: The process where RNA polymerase uses one DNA template strand to build a complementary RNA molecule in the 5' to 3' direction. The first step of gene expression.
Transcription factor: A protein that binds promoter or enhancer DNA to turn transcription on or off. In eukaryotes, shared factors coordinate groups of genes.
Transduction: The transfer of genetic material between cells by a virus. A source of genetic variation in prokaryotes.
Transfer RNA (tRNA): An RNA molecule that carries a specific amino acid and has an anticodon that base-pairs with an mRNA codon. It delivers amino acids during translation.
Transformation: A form of horizontal gene transfer where a cell takes up free DNA from its environment. Also a key step in bacterial biotechnology.
Translation: The process where ribosomes read mRNA codons and assemble the matching amino acids into a polypeptide. It turns the genetic message into protein.
Trophic cascade: A chain of indirect effects when a change at one trophic level ripples through others, as when reintroduced wolves alter elk behavior and let riverside vegetation recover.
Trophic level: A feeding position in a food chain: producers, primary/secondary/tertiary consumers, and decomposers. Only about 10% of energy passes to the next level.
U
Unsaturated fatty acid: A fatty acid with one or more carbon-carbon double bonds, which kink the chain. The kinks prevent tight packing, so it tends to be liquid at room temperature.
V
Vacuole: A membrane-bound storage sac. In plant cells, a large central vacuole maintains turgor pressure; in animal cells, vacuoles are smaller and more numerous.
Vestigial structures: Reduced, non-functional remnants of structures that were functional in ancestors, like a whale's pelvic bones. They make sense only as evolutionary leftovers.
W
Water potential: The measure that predicts the direction of water movement; water flows from high to low water potential. It combines pressure potential and solute potential.

Where these terms show up

Every AP Biology term below belongs to one of the units on the exam. Each unit page has sample questions that put the vocabulary to work:

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