1,800+ MCAT terms, defined.

An elimination reaction removes H and a leaving group from adjacent carbons to form a C=C double bond. E2 is one concerted step that needs a strong base; E1 goes through a carbocation. The more substituted alkene is usually the major pro...

Isoelectric focusing is a gel electrophoresis method that separates proteins by their isoelectric point (pI) along a pH gradient. Each protein migrates until the surrounding pH equals its pI, where its net charge is zero and it stops. Se...

An unmyelinated axon lacks an insulating myelin sheath, so the action potential regenerates at each adjacent patch of membrane (continuous conduction) instead of jumping between nodes of Ranvier. Testable point: it conducts more slowly t...

Compliance is changing your behavior because someone directly asked you to, often without changing your private beliefs. The MCAT contrasts it with conformity (matching group norms, often unspoken) and obedience (following an order from...

Nuclear binding energy is the energy needed to separate a nucleus into its protons and neutrons, and equally the energy released when they combine. It corresponds to the mass defect by E = Δm c². Greater binding energy per nucleon means...

pH calculations use pH = −log[H⁺] to turn an acid or base concentration into a pH. A strong acid gives [H⁺] equal to its concentration; a weak acid needs Ka, often with the approximation [H⁺] ≈ √(Ka·C). Each pH unit is a tenfold change i...

An electronegative atom strongly attracts the shared electrons in a covalent bond. Electronegativity increases across a period and up a group (F is highest), and unequal sharing makes a polar bond with δ− on the more electronegative atom.

A baby boomer is a member of the large birth cohort born after World War II, usually about 1946 to 1964 in the United States. Most testable: as this cohort ages and retires, the share of older adults and the old-age dependency ratio typi...

Debranching enzyme removes the α-1,6 branches of glycogen during glycogenolysis. It transfers a few glucose residues from a branch to another chain, then hydrolyzes the last α-1,6-linked glucose, releasing free glucose rather than glucos...

A particle is an idealized object treated as a single point with mass (and sometimes charge) but no size or rotation. Most testable: a charged particle moving perpendicular to a magnetic field follows a circle, r = mv/(qB), at constant s...

R/S configuration is the Cahn–Ingold–Prelog label for the absolute arrangement of four different groups around a stereocenter. Rank the groups by priority, point the lowest away from you, and trace 1 → 2 → 3: clockwise is R, counterclock...

Confirmation bias is the tendency to seek, notice, interpret, and remember information in ways that support what one already believes, while discounting evidence that contradicts it. It is a cognitive bias in judgment, and it often helps...

Disruptive (diversifying) selection is natural selection that favors both extreme phenotypes over the intermediate one, so a single-peaked trait distribution becomes bimodal. It raises variance, and it can contribute to speciation but do...

Kin selection is the evolutionary idea that a behavior costly to the individual can spread if it helps genetic relatives survive and reproduce, raising inclusive fitness. Testable point: help is predicted to be greater toward closer kin,...

The Reynolds number is a dimensionless ratio of inertial to viscous forces in a flowing fluid, Re = ρvD/η. Low values mean smooth laminar flow and high values mean turbulent flow. Higher speed, density or diameter, or lower viscosity, ra...

VSEPR (valence shell electron pair repulsion) theory predicts molecular shape by placing electron domains, meaning bonds and lone pairs, around a central atom as far apart as possible. Testable point: the molecular geometry names only th...

Phosphorylation is the covalent addition of a phosphate group, usually from ATP, to a molecule. In proteins, a kinase typically adds it to the OH of serine, threonine, or tyrosine, and a phosphatase removes it. It can activate or inhibit...

A behaviorist explains behavior as learned from the environment and treats only observable behavior as the proper subject of psychology. The most testable point: behaviorist explanations rely on classical and operant conditioning (Pavlov...

Piaget's stages are four steps of cognitive development, taken in a fixed order: sensorimotor, preoperational, concrete operational, formal operational. Ages are approximate. The testable skill is matching a behavior to a stage, such as...

MHC (major histocompatibility complex) proteins are cell-surface proteins that present peptide fragments to T cells. MHC I is on nearly all nucleated cells and presents intracellular peptides to CD8⁺ T cells; MHC II is on antigen-present...

Carbonic acid (H₂CO₃) is a weak diprotic acid: a carbonyl carbon bearing two OH groups, formed when CO₂ dissolves in water. It loses a proton to give bicarbonate. Most testable: it is the acid member of the CO₂/bicarbonate buffer, so mor...

A thin lens is an idealized lens whose thickness is negligible, so light bends at a single plane. Image position follows 1/f = 1/o + 1/i, with f positive for converging and negative for diverging lenses. A converging lens gives a real or...

A ligand is an ion or molecule that donates a lone pair to a central metal ion, forming a coordinate covalent bond in a complex ion. The ligand is the Lewis base and the metal is the Lewis acid. Adding a ligand that forms a stable comple...

Sigmund Freud founded psychoanalysis, the view that much behavior comes from unconscious conflict among the id, ego, and superego, shaped by childhood psychosexual stages. Most testable: match each structure and each defense mechanism to...

A pKa is the negative log of an acid's Ka, and it equals the pH at which a group is half protonated and half deprotonated. Below the pKa the protonated form predominates, and above it the deprotonated form does. A lower pKa means a stron...

Appraisal is the cognitive evaluation of what an event means for you and whether you can cope with it. In Lazarus's model, primary appraisal judges whether an event is irrelevant, benign-positive, or stressful, and secondary appraisal ju...

The Rf (retention factor) is the distance a compound travels divided by the distance the solvent front travels in thin-layer or paper chromatography. It is between 0 and 1. On a normal-phase silica plate, less polar compounds usually hav...

Ultrasound imaging builds a picture from echoes of high-frequency sound pulses (above 20,000 Hz) reflected at boundaries between tissues of different acoustic impedance. Echo delay gives depth, d = vt/2; higher frequency gives finer reso...

A rate constant, k, is the proportionality constant in a rate law, linking reaction rate to reactant concentrations. It does not change as concentrations change, but it increases with temperature or with a catalyst that lowers activation...

The central executive is the component of Baddeley's working memory model that directs attention and coordinates the other parts. It allocates limited attentional resources between the phonological loop, visuospatial sketchpad and episod...

A tropic hormone acts on another endocrine gland and stimulates it to make or release its own hormone. Classic examples are the anterior pituitary's TSH, ACTH, FSH and LH. Most testable: the end hormone usually feeds back negatively, so...

tRNA charging is the ATP-driven attachment of a specific amino acid to its matching tRNA by an aminoacyl-tRNA synthetase, forming aminoacyl-tRNA. Most testable: the synthetase, not the ribosome, sets which amino acid is used, so a mischa...

An isobaric process is a change that occurs at constant pressure. For a gas, volume and temperature change together, and the work done by the gas is W = PΔV, the area under a horizontal line on a PV diagram.

The atm (atmosphere) is a unit of pressure, defined from average air pressure at sea level: 1 atm = 760 mmHg = 760 torr = 1.01325 × 10⁵ Pa. The most testable point is matching units: use pressure in atm when R = 0.0821 L·atm/(mol·K).

Psychosocial refers to the interaction of psychological factors (thoughts, emotions) and social factors (relationships, culture, income) in shaping behavior and health. Testable: Erikson's psychosocial stages each center on a conflict, a...

Protein electrophoresis separates proteins by their movement through a gel in an electric field. In SDS-PAGE, SDS gives proteins a similar charge per mass, so they separate by size and smaller ones travel farther toward the positive elec...

Competence is the sense of being capable of handling tasks and reaching goals effectively. It appears as a basic psychological need in self-determination theory and as the virtue gained from Erikson's industry versus inferiority stage. S...

Radioactive decay is the spontaneous change of an unstable nucleus into a more stable one, emitting alpha, beta or gamma radiation. Most testable: it is first-order, so each half-life halves the number of undecayed nuclei, whatever the s...

A complex ion is a central metal cation bonded to ligands (Lewis bases that donate lone pairs) through coordinate covalent bonds, such as Cu(NH₃)₄²⁺. Its most testable effect: forming a complex pulls the metal out of solution equilibria...

Cranial nerves are 12 pairs of peripheral nerves that connect directly to the brain instead of the spinal cord; each is sensory, motor, or mixed. Testable points: they belong to the PNS, not the CNS, and the vagus nerve (X) carries paras...

Independent assortment is Mendel's second law: alleles of different genes are sorted into gametes independently, because homologous pairs orient randomly at metaphase I. It holds for genes on different chromosomes (and nearly for genes f...

The 3' poly-A tail is a string of roughly 100–250 adenine nucleotides added to the 3' end of eukaryotic pre-mRNA after transcription. It protects the mRNA from degradation and aids export and translation. It is added by poly-A polymerase...

Keto-enol tautomerism is the rapid equilibrium between a carbonyl compound (keto form, C=O) and an enol (C=C–OH), which differ in the position of a hydrogen and a double bond. For simple aldehydes and ketones the keto form is usually str...

Material culture is the physical objects, technologies, and spaces a society creates and uses, such as tools, clothing, buildings, and art. It is contrasted with symbolic (nonmaterial) culture, which covers beliefs, values, language, and...

The thin lens equation, 1/f = 1/o + 1/i, links a lens's focal length to the object distance and image distance. The testable point is the sign of i: positive means a real image on the opposite side of the lens from the object, negative m...

A covalent bond forms when two atoms share one or more pairs of electrons, usually between nonmetals. The most testable point: going from single to double to triple bonds between the same atoms, the bond gets shorter and stronger (higher...

Hypomania is a milder elevated or irritable mood state with high energy and reduced need for sleep, lasting at least four days in DSM-5 criteria. Unlike mania, it causes no marked impairment and no psychosis. Hypomania plus major depress...

LiAlH₄ is lithium aluminum hydride, a strong hydride (H⁻) donor that reduces carbonyl compounds. It reduces aldehydes, ketones, esters, and carboxylic acids to alcohols, and amides and nitriles to amines. Unlike NaBH₄, it reduces esters...

RNA primers are short RNA strands made by primase that pair with the DNA template and give DNA polymerase a free 3′-OH to extend, since it cannot start a strand on its own. The lagging strand needs a new primer for each Okazaki fragment.

A hypnic jerk (sleep start) is a brief, involuntary muscle twitch, often with a sensation of falling, that happens as a person drifts into sleep. The testable point: it occurs in stage 1 non-REM sleep, when theta waves replace alpha wave...