1,800+ MCAT terms, defined.

The rate-limiting step is the slowest step in a multistep reaction or pathway, so it sets the overall rate. In metabolism it is usually an early, essentially irreversible, tightly regulated enzyme step (PFK-1 in glycolysis); speeding up...

Electrolytic conduction is the flow of electric current through a solution or molten salt by the movement of ions, not electrons. Both cations and anions carry charge. More dissolved ions generally means higher conductivity, and heating...

Ethology is the scientific study of animal behavior in natural settings, with emphasis on its evolutionary and adaptive value. The MCAT links it to largely innate behaviors such as fixed action patterns triggered by sign stimuli, in cont...

Steric hindrance is the slowing of a reaction, or the destabilizing of a conformation, because bulky groups crowd the space atoms need. Most testable: SN2 rates fall as the electrophilic carbon becomes more substituted (methyl > 1° > 2°...

Exogenous means arising from outside the person or system. In psychology it usually describes attention pulled automatically by an external stimulus (bottom-up, stimulus-driven), unlike endogenous attention, which a person directs volunt...

In the Bohr model, orbits are fixed circular paths around the nucleus, each with a quantized energy set by n. An electron moving between orbits absorbs or emits a photon equal to the energy difference. Orbits are not orbitals, which are...

The electrostatic force vector is the force one charge exerts on another, directed along the line joining them: away for like charges (repulsion), toward for opposite charges (attraction). Its magnitude is k|q₁||q₂|/r², so doubling the d...

Generalization is when a response learned to one stimulus also occurs to similar stimuli. In conditioning, the more a new stimulus resembles the original CS or discriminative stimulus, the stronger the response tends to be. Its counterpa...

The plasma membrane is a fluid mosaic: a phospholipid bilayer with embedded and surface proteins, cholesterol, and carbohydrates on the outer face. Its hydrophobic core makes it selectively permeable, so small nonpolar molecules cross re...

Thymine dimers are covalent bonds between two adjacent thymine bases on the same DNA strand, formed by ultraviolet light. They distort the helix and block replication, and in humans they are mainly removed by nucleotide excision repair.

Transmittance is the fraction of incident light that passes through a sample, T = I/I₀. In IR spectroscopy it is plotted against wavenumber, so absorption appears as a downward dip: the lower the transmittance, the more strongly the samp...

A lexicon is the stored mental dictionary of all the words a person knows, including their meanings, sounds, and spellings. In MCAT language questions, it is the vocabulary component of language, separate from grammar (syntax), which gov...

Electric current is the rate at which net charge passes a point in a conductor, I = ΔQ/Δt, measured in amperes (C/s). Conventional current points the way positive charge would move, which in a metal wire is opposite to electron motion.

Neurotic describes someone high in neuroticism, the Big Five trait of emotional instability: a tendency toward anxiety, worry, moodiness and other negative emotions. The testable point is that this is one end of a continuous trait (oppos...

Radius of curvature (R) is the radius of the sphere of which a curved mirror or lens surface is a part. It is a physics optics term, not an organic chemistry one. Most testable: for a spherical mirror, focal length is half of R (f = R/2).

Amphoteric describes a species that can act as either an acid or a base. In biochemistry the standard example is an amino acid: its carboxyl group can donate a proton and its amino group can accept one. Its net charge therefore depends o...

Syntax is the set of grammar rules for arranging words into meaningful phrases and sentences in a language. The MCAT contrasts it with semantics (meaning): a sentence can be syntactically correct yet meaningless, or understandable despit...

Merkel cells are touch-sensing cells in the deepest layer of the epidermis (stratum basale) that contact sensory neurons. They detect sustained light touch and fine texture, and they are slowly adapting, unlike the rapidly adapting Meiss...

The kinetic theory of gases models a gas as particles in constant random motion that collide elastically and have negligible volume and no intermolecular forces. Its key result is that average kinetic energy is proportional to absolute t...

Plane polarized light is light whose electric field oscillates in a single plane, perpendicular to the direction of travel. Unpolarized light loses half its intensity at the first polarizer; a second polarizer at angle θ then transmits I...

A bonding orbital is a molecular orbital formed when atomic orbitals overlap in phase, concentrating electron density between the nuclei. It lies lower in energy than the atomic orbitals that combined, so electrons placed in it stabilize...

Spatial processing is the brain's analysis of where objects are, how they are oriented, and how to act on them. The MCAT ties it to the dorsal "where/how" visual stream and the parietal lobe, in contrast to the ventral "what" stream in t...

An amphoteric species can act as an acid or as a base, depending on what it reacts with. Water, HCO₃⁻, H₂PO₄⁻ and Al(OH)₃ are examples. The testable point is that the role depends on the partner: it donates H⁺ to a stronger base and acce...

In a right triangle, the square of the hypotenuse equals the sum of the squares of the two legs. In physics it gives the magnitude of a resultant from perpendicular vector components, R = √(Rx² + Ry²). It holds for right angles, so resol...

A hydroxyl group is an −OH unit covalently bonded to carbon. It is polar and forms hydrogen bonds, so it usually raises water solubility and boiling point. Most testable: hydroxyl (−OH, neutral, covalent) is not hydroxide (OH⁻, a free io...

The absolute threshold is the minimum stimulus intensity needed for a person to detect it, conventionally defined as the intensity detected 50% of the time. It varies between people and conditions, and it is distinct from the difference...

Interphase is the part of the cell cycle between divisions, made of G1, S and G2, when the cell grows, carries out its normal functions and replicates its DNA. Most testable point: DNA is copied in S phase, so DNA content doubles but chr...

Atomic size is the radius of an atom, often taken as half the distance between the nuclei of two bonded identical atoms. It generally increases down a group (more shells) and decreases left to right across a period (greater effective nuc...

Quasi-experimental design is a research-methods term from psychology and sociology, not physics. It compares groups exposed to different conditions without random assignment, so the groups may differ beforehand. It supports weaker causal...

A methyl ketone is a ketone with a CH₃ group on the carbonyl carbon, R–C(=O)–CH₃; acetone is the simplest. The testable point is the iodoform test: I₂ with NaOH gives a yellow CHI₃ precipitate, which most other ketones do not give.

Body language is nonverbal communication through posture, gestures, facial expressions, eye contact, touch, and use of space. Facial expressions of basic emotions are widely recognized across cultures, but gestures and display rules vary...

A pentose is a monosaccharide with five carbons, general formula C₅H₁₀O₅. The key examples are ribose (in RNA) and 2-deoxyribose (in DNA). The most testable link is the pentose phosphate pathway, which makes NADPH and ribose-5-phosphate...

C fibers are slow, unmyelinated, small-diameter sensory neurons that carry dull, aching, burning, lingering pain, often called second pain. They contrast with thinly myelinated A-delta fibers, which carry fast, sharp first pain.

Spherical aberration is image blur from lenses or mirrors with spherical surfaces: rays far from the optical axis focus at a different point than rays near it, so a point source becomes a smeared spot. It can be reduced by limiting light...

Episodic memory is explicit (declarative) long-term memory for personal experiences tied to a particular time and place, such as your last birthday. The MCAT contrasts it with semantic memory, which stores general facts and meanings with...

A spermatid is a haploid (n), round, immature male germ cell produced when a secondary spermatocyte completes meiosis II. It has no tail yet and differentiates into a mature sperm cell through spermiogenesis, with no further division.

A mood disorder is a psychological disorder defined by a prolonged disturbance of emotional state that impairs functioning. The two families are depressive disorders and bipolar disorders. Most testable: bipolar disorders require a manic...

Phosphorylated means a phosphate group (PO₃²⁻) has been covalently attached to a molecule, usually by a kinase using ATP. In proteins it typically targets the OH of serine, threonine or tyrosine and can either activate or inhibit the pro...

Bromophenol blue is a weak-acid pH indicator that is yellow in its acid form and blue in its base form, changing color over roughly pH 3.0 to 4.6 (pKa near 4). It suits titrations whose equivalence point is acidic, such as a strong acid...

A Venturi tube is a pipe with a narrow throat used to relate fluid speed and pressure. By continuity the fluid moves faster in the throat, and by Bernoulli's principle the pressure there is lower (for a horizontal pipe and an ideal fluid...

Residential segregation is the physical separation of social groups, most often by race, ethnicity or income, into different neighborhoods. Testable point: it stems from institutional practices like redlining as well as individual choice...

NaOH is sodium hydroxide, a strong base that dissociates fully into Na⁺ and OH⁻. In organic reactions, OH⁻ acts as a nucleophile (SN2, ester hydrolysis) or as a base (E2, deprotonating acids), and the substrate and conditions decide whic...

Solomon Asch was a social psychologist whose line-judgment experiments showed conformity: people often gave an obviously wrong answer to match a unanimous group. The most testable point is that conformity drops sharply when even one othe...

The venae cavae are the superior and inferior vena cava, the large systemic veins that return deoxygenated blood to the right atrium. Veins are defined by direction (toward the heart), not oxygen content: the pulmonary veins, not the ven...

Resistivity (ρ) is a material's intrinsic opposition to current flow, independent of the sample's size or shape. It sets a wire's resistance through R = ρL/A, so a longer or thinner wire of the same material has more resistance while ρ i...

The amino acid sequence is the order of residues in a polypeptide, written from the N-terminus to the C-terminus; it is the protein's primary structure. It is set by the mRNA codons, and in most cases it determines how the protein folds,...

Amotivation is a state of having no intention to act: the person is neither intrinsically nor extrinsically motivated. In self-determination theory it sits at the low end of the motivation continuum. It is usually tied to feeling incompe...

An aldol condensation joins two aldehydes or ketones: an enolate's α-carbon attacks another carbonyl carbon to give a β-hydroxy carbonyl, which then loses H₂O to form a conjugated α,β-unsaturated carbonyl. The enolate partner must have a...

Loudness is the perceived strength of a sound. It is set mainly by intensity (proportional to amplitude squared) and is measured on the logarithmic decibel scale. Each 10 dB increase means a tenfold increase in intensity. Pitch tracks fr...

Fazio's model says an attitude can influence behavior without deliberate thought: it is activated from memory, shapes how you perceive the situation (along with social norms), and that perception guides action. The theory of planned beha...