1,800+ MCAT terms, defined.

A buffer is a solution containing a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists large pH changes when small amounts of strong acid or base are added. It works best when pH is within about 1 unit...

Race is a socially constructed category that groups people by perceived physical traits such as skin color. The MCAT stresses that its boundaries and meanings are set by society and change across time and place, rather than reflecting di...

Electronegativity is a relative measure of how strongly an atom pulls shared bonding electrons toward itself. It generally increases across a period and up a group, with fluorine highest, and a larger difference between bonded atoms make...

The representativeness heuristic is a mental shortcut that judges how likely something belongs to a category by how closely it resembles a typical member. It often leads people to neglect base rates, so a good match can seem likelier tha...

Torque is the rotational analog of force: a measure of how effectively a force twists an object about a pivot. Its magnitude is τ = rF sinθ, so it is largest when the force is perpendicular to the lever arm and zero when the force acts a...

An indicator is a weak acid or base whose protonated and deprotonated forms have different colors, so its color signals pH in a titration. It changes color near pH = its pKa, so the best choice has a pKa close to the pH at the equivalenc...

Phosphodiester bonds are the covalent links between nucleotides in DNA and RNA. A phosphate joins the 3′ carbon of one sugar to the 5′ carbon of the next, forming the sugar-phosphate backbone. Heating or denaturing breaks the hydrogen bo...

Cultural capital (Pierre Bourdieu) is the set of non-financial assets, such as education, knowledge, tastes, speech and manners, that signal and help maintain social class position. Because schools and employers often reward higher-class...

Reabsorption is the return of water and solutes from the nephron's filtrate to the blood through the peritubular capillaries. Most Na⁺ and water, and nearly all glucose, are reclaimed in the proximal tubule, while ADH and aldosterone fin...

Friction is a contact force parallel to the surfaces that resists their relative sliding (kinetic) or the tendency to slide (static). Static friction adjusts up to a maximum of μsN, while kinetic friction is μkN, and μk is usually smalle...

Intermolecular forces are attractions between separate molecules (or between ions and molecules), much weaker than the covalent bonds within a molecule. For molecules of similar size, strength usually rises from London dispersion to dipo...

A bond is the attraction holding atoms together; in organic chemistry it usually means a covalent bond, a shared electron pair. A double bond is one σ plus one π, a triple is one σ plus two π, and the weaker π bond is the usual site of r...

Coping is the set of thoughts and behaviors a person uses to manage a stressor and the emotions it causes. The MCAT usually sorts it into problem-focused coping (changing the stressor) and emotion-focused coping (managing the emotional r...

Denaturation is the loss of a protein's native 3D shape (secondary, tertiary and quaternary structure) when noncovalent forces or disulfide bonds are disrupted, usually abolishing function. Peptide bonds are not broken, so primary struct...

A standing wave is the pattern made when two waves of the same frequency travel in opposite directions, usually a wave and its reflection. It has fixed nodes and antinodes and ideally carries no net energy along the medium. Only waveleng...

The solvent is the component of a solution that dissolves the others (the solutes), usually the one present in the largest amount. By "like dissolves like," polar solvents such as water dissolve ionic and polar solutes, and nonpolar solv...

A leaving group is the atom or group that departs with the bonding electron pair from the substrate in substitution and elimination reactions. Good ones are generally weak bases, the conjugate bases of strong acids (I⁻ > Br⁻ > Cl⁻ > F⁻,...

Social facilitation is the change in a person's performance when others are present. The presence of others usually improves performance on simple or well-learned tasks and tends to worsen it on complex or unfamiliar ones, so task diffic...

Gastrulation is the embryonic stage in which the blastula's cells move and rearrange into three primary germ layers: ectoderm, mesoderm, and endoderm. It also forms the primitive gut (archenteron). The most testable point is which adult...

Social support is the emotional, esteem, informational, or tangible help a person receives, or believes is available, from their social network. It is linked to lower stress and better health, explained either by a direct effect on well-...

An inhibitor is a molecule that binds an enzyme and lowers its reaction rate. The MCAT sorts inhibitors by their effect on Km and Vmax: competitive raises Km with Vmax unchanged, noncompetitive lowers Vmax with Km unchanged.

Heat is energy transferred between systems because of a temperature difference, flowing spontaneously from hotter to colder. It is a transfer, not something an object contains. Adding heat usually raises temperature (Q = mcΔT), but not d...

Boiling point elevation is the rise in a solvent's boiling point when a nonvolatile solute dissolves in it. It is a colligative property, so it depends on the number of dissolved particles (i × molality), not their identity. At equal mol...

The availability heuristic is a mental shortcut in which people judge how likely or common something is by how easily examples come to mind. Vivid, recent or emotional events are easy to recall, so their frequency is often overestimated.

A hydrogen bond is an attraction between an H bonded to N, O, or F and a lone pair on another N, O, or F. It is an intermolecular force, so it raises boiling point and water solubility: an alcohol boils higher than an ether or alkane of...

Self-efficacy is a person's belief in their ability to succeed at a specific task or goal, a concept from Bandura's social cognitive theory. Higher self-efficacy tends to predict more effort and persistence, and it is task-specific, unli...

Bile is a fluid made by the liver, stored in the gallbladder, and released into the duodenum, where its bile salts emulsify fats. It increases the surface area for lipase but is not an enzyme and does not hydrolyze fats itself.

Light is electromagnetic radiation: a transverse wave of oscillating electric and magnetic fields that travels at c = 3.00 × 10⁸ m/s in vacuum. The most testable point: entering a new medium, its frequency stays the same while speed and...

Ionization energy is the energy required to remove an electron from a gaseous atom or ion in its ground state. The first ionization energy generally increases across a period and decreases down a group, with exceptions such as B below Be...

A substrate is the molecule an enzyme binds at its active site and converts to product. Raising substrate concentration speeds the reaction until the enzyme saturates at Vmax, and Km is the [S] at which the rate is half of Vmax.

B.F. Skinner was the behaviorist who developed operant conditioning: behavior is shaped by its consequences. Reinforcement increases a behavior and punishment decreases it. The most testable point is telling positive from negative reinfo...

An imine has a C=N double bond (R₂C=NR′) and is also called a Schiff base. It forms when an aldehyde or ketone condenses with ammonia or a primary amine, losing water. A secondary amine gives an enamine instead.

Pascal's principle says a pressure change applied to an enclosed, incompressible fluid at rest is transmitted undiminished to every point in the fluid. In a hydraulic lift, F₁/A₁ = F₂/A₂, so a small input force can balance a large load,...

A strong acid dissociates essentially completely in water, so Ka is much greater than 1 and, for a monoprotic acid, [H₃O⁺] equals the acid concentration. Examples are HCl, HBr, HI, HNO₃, HClO₄ and H₂SO₄ (first proton). Strong describes d...

Cognitive dissonance is the uncomfortable tension felt when a person's attitudes conflict with each other or with their behavior. People are typically motivated to reduce it, often by changing an attitude to fit the behavior. Less extern...

The elaboration likelihood model (Petty and Cacioppo) says persuasion works through two routes: the central route, based on careful thought about argument quality, and the peripheral route, based on surface cues. Which route dominates de...

Cleavage is the series of rapid mitotic divisions of the zygote after fertilization, with little or no growth between divisions. Cell number rises while individual cells shrink, so the embryo's overall size stays roughly constant as it b...

In biochemistry, synthesis is anabolism: building larger molecules from smaller precursors, as in protein, glycogen, fatty acid and DNA synthesis. It is usually endergonic, so cells typically couple it to ATP hydrolysis or other exergoni...

A hemiacetal is a carbon bonded to one −OH and one −OR group (plus two other substituents), formed when an alcohol adds to an aldehyde. Open-chain hemiacetals are usually unfavorable, but cyclic five- or six-membered ones, such as the ri...

Surface tension is the force per unit length (or energy per unit area) that makes a liquid surface act like a stretched sheet, because cohesive forces pull surface molecules inward. It generally falls with higher temperature and with sur...

The ideal gas law, PV = nRT, links the pressure, volume, amount and absolute temperature of a gas whose particles have negligible volume and no attractions. Most testable: T must be in kelvin. Real gases deviate most at high pressure and...

A reference group is a group a person uses as a standard for judging their own behavior, attitudes, or social standing. It can be a group the person belongs to or one they only hope to join. The testable point: comparison with the refere...

A stimulus is any event, object, or energy that can be detected and can evoke a response. The most testable point: in classical conditioning, a neutral stimulus repeatedly paired with an unconditioned stimulus becomes a conditioned stimu...

A vector is a quantity with both magnitude and direction, such as displacement, velocity, acceleration, force and momentum. Vectors add by components or head-to-tail, so direction changes the result; magnitudes alone do not simply add.

The Arrhenius equation, k = Ae^(−Ea/RT), gives a reaction's rate constant in terms of temperature and activation energy. Raising T or lowering Ea increases k, and k depends exponentially on T, not linearly.

An amine is an organic compound with a nitrogen atom bonded to one, two or three carbon groups (RNH₂, R₂NH, R₃N). The nitrogen lone pair makes amines weak bases and nucleophiles, so they are protonated to ammonium ions in acid. This driv...

Ion channels are integral membrane proteins with a water-filled pore that lets specific ions cross the bilayer by facilitated diffusion, down their electrochemical gradient and without ATP. Many are gated (voltage, ligand or mechanical),...

Encoding is the first stage of memory: converting incoming information into a form the brain can store. The most testable point is that deeper, meaning-based (semantic) encoding usually gives better recall than shallow encoding by sound...

The cell membrane (plasma membrane) is a selectively permeable phospholipid bilayer with embedded proteins that separates the cell from its surroundings. Small nonpolar molecules cross it by simple diffusion, while ions and large polar m...

Altruism is helping another person at a cost to oneself when the motive is the other's welfare, not a reward. The MCAT tests whether a case fits genuine empathy (empathy-altruism hypothesis) or self-interest (social exchange, reciprocity...