AP Biology Glossary: 20 Key Terms and Concepts
AP Biology uses precise scientific vocabulary. On the FRQ section, using a term correctly earns credit. Using it vaguely — or confusing related terms — costs credit. These 20 definitions cover the concepts that appear most frequently on the exam and where imprecision most commonly costs students points.
Contents
- 1. Active Transport
- 2. Allosteric Regulation
- 3. ATP (Adenosine Triphosphate)
- 4. Calvin Cycle
- 5. Cell Cycle Checkpoint
- 6. Chemiosmosis
- 7. Codominance
- 8. Competitive Inhibition
- 9. Crossing Over (Recombination)
- 10. Epistasis
- 11. Facilitated Diffusion
- 12. Founder Effect
- 13. Gene Expression
- 14. Hardy-Weinberg Equilibrium
- 15. Lac Operon
- 16. Natural Selection
- 17. Negative Feedback
- 18. Osmoregulation
- 19. Signal Transduction
- 20. Water Potential (Ψ)
1. Active Transport
The movement of molecules or ions across a cell membrane against their concentration gradient (from low to high concentration), requiring the input of cellular energy (ATP). Because active transport moves substances "uphill," it requires energy from ATP hydrolysis and specific carrier proteins embedded in the membrane.
FRQ note
When a question asks you to distinguish active transport from passive transport, the key distinction is energy requirement and direction relative to gradient — not the presence of a protein. Facilitated diffusion also uses proteins but is passive.
2. Allosteric Regulation
The binding of a molecule (allosteric effector) to a site on an enzyme other than the active site, causing a conformational change that alters the enzyme's activity. An allosteric activator increases enzyme activity; an allosteric inhibitor decreases it. Noncompetitive inhibition is a form of allosteric regulation.
FRQ note
"Allosteric" means the regulatory site is separate from the active site. Students who confuse the allosteric site with the active site lose the mechanism points.
3. ATP (Adenosine Triphosphate)
The primary energy currency of the cell. ATP stores chemical energy in the phosphate bonds between its three phosphate groups. When the terminal phosphate is hydrolyzed (ATP → ADP + Pᵢ), the released energy powers cellular work: active transport, muscle contraction, biosynthesis, and more. ATP is regenerated continuously from ADP during cellular respiration and photosynthesis.
FRQ note
Writing "ATP provides energy" earns partial credit at best. Writing "the hydrolysis of ATP to ADP and inorganic phosphate releases energy that drives [specific process]" earns full credit.
4. Calvin Cycle
The light-independent reactions of photosynthesis occurring in the stroma of the chloroplast. Carbon dioxide is fixed by RuBisCO to ribulose bisphosphate (RuBP), producing 3-carbon intermediates (3-phosphoglycerate) that are reduced using ATP and NADPH from the light reactions to produce glyceraldehyde-3-phosphate (G3P). G3P is the precursor for glucose and other organic molecules. RuBP is regenerated to continue the cycle.
FRQ note
The Calvin cycle requires ATP and NADPH from the light reactions — it does not directly use light. Questions that ask what would happen if ATP or NADPH were unavailable are testing whether you understand this dependency.
5. Cell Cycle Checkpoint
A regulatory control point in the cell cycle at which the cell assesses whether conditions are suitable to proceed to the next phase. The three major checkpoints are the G1 checkpoint (cell size, nutrients, DNA integrity), the G2 checkpoint (DNA replication completeness, DNA damage), and the spindle assembly checkpoint (M phase — all chromosomes must be attached to spindle fibers before anaphase begins). Checkpoint failure can lead to uncontrolled cell division and cancer.
FRQ note
Questions often ask what happens when checkpoints fail. The answer requires naming which protein is involved (tumor suppressors like p53 at G1) and the consequence (progression with damaged DNA → mutation → potential tumor).
6. Chemiosmosis
The process by which a proton (H⁺) gradient across a membrane is used to drive ATP synthesis. In cellular respiration, the electron transport chain pumps H⁺ from the mitochondrial matrix into the intermembrane space. The resulting electrochemical gradient drives H⁺ back through ATP synthase into the matrix, releasing energy that phosphorylates ADP to ATP. An analogous process occurs in thylakoid membranes during the light reactions of photosynthesis.
FRQ note
Chemiosmosis is one of the most frequently tested mechanisms in AP Biology. Students must distinguish where the protons are pumped (matrix → intermembrane space), where they flow back (through ATP synthase), and what that flow accomplishes (ATP synthesis). Vague answers about "the gradient making energy" do not earn credit.
7. Codominance
A pattern of inheritance in which both alleles in a heterozygote are fully expressed simultaneously, producing a phenotype that shows both traits distinctly rather than a blend. Classic example: ABO blood type. Type AB individuals carry both Iᴬ and Iᴮ alleles; both antigens are present on red blood cells.
FRQ note
Codominance is not the same as incomplete dominance. In codominance, both phenotypes appear distinctly in the heterozygote. In incomplete dominance, the phenotype is an intermediate blend. This distinction is commonly tested in genetics FRQs.
8. Competitive Inhibition
A type of enzyme inhibition in which an inhibitor molecule with structural similarity to the substrate binds the active site, preventing substrate binding. Competitive inhibition is reversible — increasing substrate concentration can overcome inhibition by outcompeting the inhibitor for active sites. Vmax is unchanged; Km (apparent) is increased.
FRQ note
The effect on Vmax is the distinguishing point. Competitive inhibitors reduce efficiency at lower substrate concentrations but don't reduce maximum rate if enough substrate is present. Noncompetitive inhibitors reduce Vmax regardless of substrate concentration. Exams test this distinction directly.
9. Crossing Over (Recombination)
The exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis. Crossing over at points called chiasmata produces recombinant chromosomes with new combinations of alleles, increasing genetic diversity in offspring. The frequency of crossing over between two gene loci is proportional to the physical distance between them on the chromosome — this is the basis for genetic mapping.
FRQ note
Crossing over occurs between non-sister chromatids of homologous chromosomes — not sister chromatids of the same chromosome. This distinction matters for inheritance questions.
10. Epistasis
A gene interaction in which one gene (the epistatic gene) masks or modifies the expression of a different gene (the hypostatic gene) at a different locus. Epistasis results in modified Mendelian ratios in F₂ generations (e.g., 9:3:3:1 becomes 9:3:4 or 12:3:1 depending on the type). Epistasis differs from dominance, which involves allele interactions at the same locus.
FRQ note
When a genetics problem shows a ratio that differs from standard Mendelian expectations, epistasis is a likely cause. Identifying epistasis requires showing which gene masks which and why.
11. Facilitated Diffusion
The passive movement of molecules across a cell membrane through specific protein channels or carrier proteins, down their concentration gradient (from high to low). No energy is required. Examples include glucose transport via GLUT proteins and water movement through aquaporins. Facilitated diffusion is selective — each protein facilitates a specific molecule.
FRQ note
Students often incorrectly say facilitated diffusion requires energy because it uses proteins. It does not. The protein provides a path but the concentration gradient provides the driving force.
12. Founder Effect
A form of genetic drift that occurs when a new population is established by a small number of individuals from a larger source population. The new population has reduced genetic diversity and allele frequencies that may differ substantially from the source population, simply due to the random sampling of the small founding group. This can lead to unusually high prevalence of otherwise rare alleles in the new population.
FRQ note
The key distinction between the founder effect and bottleneck effect: both are forms of genetic drift involving small population size, but the founder effect describes establishing a new population; the bottleneck effect describes a surviving remnant of an existing population after a catastrophic reduction.
13. Gene Expression
The process by which information encoded in a gene is used to produce a functional product — typically a protein. Gene expression includes transcription (DNA → mRNA) and translation (mRNA → polypeptide), along with post-translational modifications. Gene expression is regulated at multiple levels: transcriptional, post-transcriptional, translational, and post-translational.
FRQ note
A common exam question asks students to identify a point at which gene expression could be regulated and explain the mechanism. Know at least three distinct levels: transcription factor binding, mRNA splicing, translation initiation.
14. Hardy-Weinberg Equilibrium
A theoretical state in which allele and genotype frequencies in a population remain constant across generations, assuming no evolution is occurring. Five conditions must be met: large population size, random mating, no mutation, no gene flow, no natural selection. The equilibrium is described by the equations p + q = 1 and p² + 2pq + q² = 1, where p and q are allele frequencies and p², 2pq, and q² are homozygous dominant, heterozygous, and homozygous recessive genotype frequencies.
FRQ note
The most common Hardy-Weinberg FRQ asks students to calculate allele or genotype frequencies, then identify which condition is being violated based on a described scenario. Know the five conditions and be able to calculate in both directions (from genotype frequency to allele frequency, and vice versa).
15. Lac Operon
A prokaryotic gene regulation system in E. coli that controls genes for lactose metabolism. When lactose is absent, a repressor protein binds the operator, blocking transcription of the structural genes. When lactose is present, allolactose (a lactose derivative) binds the repressor, removing it from the operator and allowing transcription. The lac operon is an example of negative inducible regulation — the default is "off," and the inducer turns it on.
FRQ note
Students commonly confuse the inducer (allolactose) with lactose. The actual inducer is allolactose. Exams also distinguish the lac operon (inducible, catabolic) from the trp operon (repressible, anabolic) — know both and why they differ in their default state.
16. Natural Selection
The mechanism of evolution by which individuals with heritable traits better suited to the environment survive and reproduce at higher rates than those without those traits, causing the frequency of advantageous alleles to increase in the population over generations. Natural selection acts on phenotype; evolution occurs at the level of allele frequency in the population.
FRQ note
Natural selection does not act on genotype directly and does not cause mutations. It selects among existing variation. A common FRQ error is stating that organisms "develop" a trait in response to selection — this is Lamarckian and incorrect.
17. Negative Feedback
A regulatory mechanism in which a system's output inhibits further output of that system, returning the system toward equilibrium. Negative feedback maintains homeostasis. Examples: body temperature regulation (high temperature triggers cooling mechanisms), blood glucose regulation (high blood glucose triggers insulin release from the pancreas, promoting glucose uptake and storage), and the hypothalamic-pituitary-gonadal axis.
FRQ note
Negative feedback is stabilizing — the response opposes the stimulus. Positive feedback amplifies the signal (e.g., oxytocin during labor). Questions often ask students to distinguish them and give examples.
18. Osmoregulation
The regulation of solute concentrations and water balance in body fluids, maintaining homeostasis. Organisms use a variety of osmoregulatory strategies depending on their environment: freshwater animals (hyperosmotic to surroundings) excrete dilute urine and actively transport ions in; marine fish (hyposmotic to seawater) drink water and excrete concentrated, small-volume urine. The kidney's loop of Henle and countercurrent exchange mechanism is central to water conservation in terrestrial vertebrates.
FRQ note
Osmoregulation questions frequently ask students to predict the direction of water movement under different osmotic conditions, or to describe what happens to an organism placed in a hypertonic or hypotonic environment. Always anchor the answer in water potential or concentration gradient direction.
19. Signal Transduction
The process by which a chemical signal (ligand) received at the cell surface is converted into a cellular response through a cascade of molecular events inside the cell. The three stages are reception (ligand binds receptor), transduction (signal is relayed and amplified through second messengers and protein kinases), and response (changes in gene expression, enzyme activity, or cell behavior). Signal transduction allows cells to respond to signals without the signal molecule entering the cell.
FRQ note
Students must be able to trace a complete signal from receptor to response, naming specific components. "The signal is passed to the nucleus and genes turn on" does not earn mechanism points. Name the receptor type, second messenger (if applicable), the kinase, and the downstream effect.
20. Water Potential (Ψ)
A measure of the tendency of water to move from one region to another, expressed in pressure units (MPa). Water always moves from regions of higher water potential to regions of lower water potential (more negative). Water potential is determined by two components: solute potential (Ψs, always negative — solutes reduce water potential) and pressure potential (Ψp, usually positive in plant cells — pressure increases water potential). The formula: Ψ = Ψs + Ψp.
FRQ note
Water potential is provided on the AP Biology formula sheet, but students must understand how to apply it. The key prediction: water moves from high (less negative) to low (more negative) water potential — toward higher solute concentration and lower pressure, all else equal. Be prepared to calculate water potential given solute concentration and predict the direction of water movement between two cells or compartments.
Quick reference: terms by unit
| Unit | Terms covered |
|---|---|
| Unit 1: Chemistry of Life | ATP, Water Potential |
| Unit 2: Cell Structure and Function | Active Transport, Facilitated Diffusion, Osmoregulation |
| Unit 3: Cellular Energetics | Chemiosmosis, Calvin Cycle |
| Unit 4: Cell Communication and Cell Cycle | Allosteric Regulation, Cell Cycle Checkpoint, Negative Feedback, Signal Transduction |
| Unit 5: Heredity | Codominance, Crossing Over, Epistasis |
| Unit 6: Gene Expression and Regulation | Gene Expression, Lac Operon |
| Unit 7: Natural Selection | Founder Effect, Hardy-Weinberg Equilibrium, Natural Selection |
| Unit 8: Ecology | (see Energy Flow and Biogeochemical Cycles on the Unit 8 page) |
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