1,800+ MCAT terms, defined.

Polar describes a bond or molecule with uneven electron sharing, giving partial positive and negative ends. A molecule is polar when its bond dipoles do not cancel, so geometry matters. Polar substances tend to dissolve in polar solvents.

Triacylglycerols (triglycerides) are glycerol joined to three fatty acids by ester bonds. They are the body's main long-term energy store, kept in adipocytes, and give more energy per gram than carbohydrate because their carbons are more...

A trait is a relatively stable characteristic that shapes how a person tends to think, feel, and act across situations and over time. Trait theories, such as the Big Five, describe personality as a profile of traits. A trait differs from...

Secretion is the export of a cell-made product, such as a hormone, enzyme, or mucus, usually by exocytosis. In the kidney, it means moving solutes from blood into the filtrate. Either way, material is added to a lumen or extracellular fl...

Potential energy is energy stored in a system by the position or arrangement of its parts, such as height in a gravitational field or a stretched spring. Only changes in U matter, and when only conservative forces act, a loss of potentia...

An orbital is a wavefunction describing an electron in an atom, whose square gives the probability of finding the electron at a location. Each orbital holds at most two electrons, and they must have opposite spins. A p subshell has 3 orb...

Operant conditioning is learning in which the consequences of a voluntary behavior change how likely it is to occur again. Reinforcement increases a behavior and punishment decreases it. Positive means a stimulus is added; negative means...

SN2 means substitution, nucleophilic, bimolecular: a nucleophile attacks the carbon bearing a leaving group from the back side in one concerted step, inverting its configuration. Rate = k[substrate][nucleophile], and the reaction is fast...

Social capital is the resources and advantages a person gains from their social networks and relationships, such as information, trust, and access to opportunities. Testable point: it is "who you know," a separate kind of capital from mo...

Sound is a longitudinal mechanical wave: a pressure disturbance of compressions and rarefactions that travels through a medium and cannot travel through a vacuum. Its speed is set by the medium (generally faster in solids than liquids th...

pKa is −log Ka, where Ka is the acid dissociation constant, so a lower pKa means a stronger acid. The most testable point: when pH = pKa, the acid and its conjugate base are present in equal concentrations, as at the half-equivalence poi...

An inducer is a small molecule that turns on transcription of an operon, usually by binding the repressor and causing it to release the operator. In the lac operon, allolactose (an isomer of lactose) is the natural inducer. A corepressor...

Cones are retinal photoreceptors for bright light, color and sharp detail, densest in the fovea. Most people have three types (short, medium, long wavelength), as trichromatic theory says; opponent-process theory describes later coding....

A virus is an acellular infectious particle: a DNA or RNA genome in a protein capsid, sometimes with a lipid envelope. It is an obligate intracellular parasite that typically needs host ribosomes and enzymes to replicate. Testable point:...

Displacement is a vector giving the straight-line change in position from start to finish, with both magnitude and direction. It depends only on the endpoints, not the path, so it can be zero on a round trip even though the distance trav...

Raoult's law says that in an ideal solution, the partial vapor pressure of a component equals its mole fraction in the liquid times its pure vapor pressure. A nonvolatile solute lowers the solvent's vapor pressure. Real solutions deviate...

Chromatography separates a mixture because each component partitions differently between a stationary phase and a moving mobile phase. In normal-phase systems (polar silica), less polar compounds stick less, travel farther and elute firs...

In operant conditioning, punishment is a consequence that makes a behavior less likely to recur. Positive punishment adds an aversive stimulus; negative punishment removes a desired one. The key trap: it is not negative reinforcement, wh...

A peptide bond is the covalent amide bond joining the carboxyl carbon of one amino acid to the amino nitrogen of the next, formed by a condensation (dehydration) reaction that releases H₂O. Resonance gives it partial double-bond characte...

Interference is forgetting that happens when other learned information competes with a memory and makes it harder to retrieve. Proactive interference is old material disrupting new; retroactive is new material disrupting old. It is one a...

Velocity is the rate of change of displacement with time. It is a vector with magnitude and direction, unlike speed, which is a scalar. The most testable point is that an object moving in a circle at constant speed still has changing vel...

The anode is the electrode where oxidation occurs, in every type of electrochemical cell. Its sign differs by cell type: negative in a galvanic cell, positive in an electrolytic cell. Electrons always leave the anode through the external...

In organic chemistry, oxidation raises the oxidation state of a carbon: it gains bonds to oxygen or another more electronegative atom, or loses bonds to hydrogen. The most testable point is alcohol oxidation: primary → aldehyde → carboxy...

Rods are retinal photoreceptors that work in dim light and detect brightness but not color. They are concentrated in the periphery of the retina and absent from the fovea, so dim objects are seen best slightly off-center. Cones handle co...

Na⁺ is the sodium ion, the main extracellular cation. The Na⁺/K⁺ ATPase keeps it high outside cells and low inside (3 Na⁺ out, 2 K⁺ in per cycle). Most testable: Na⁺ flowing in down this gradient through voltage-gated channels causes the...

Aggression is behavior intended to harm another individual, whether physical or verbal. The MCAT ties it to biological factors (amygdala, testosterone, lower serotonin) and to learning, as in Bandura's Bobo doll study, where children imi...

Km is the Michaelis constant: the substrate concentration at which an enzyme works at half of Vmax. A lower Km means the enzyme reaches half-maximal rate at lower substrate, which usually indicates higher apparent affinity for the substr...

The Doppler effect is the change in observed wave frequency when a source and an observer move relative to each other. The observed frequency is higher when they approach and lower when they separate, while the wave speed in the medium s...

The cathode is the electrode where reduction occurs, so species gain electrons there. This is true in every electrochemical cell. Its sign is what changes: positive in a galvanic cell, negative in an electrolytic cell.

Groupthink is a flawed group decision process in which the drive for harmony and unanimity overrides realistic evaluation of alternatives, so members silence doubts. It is most likely in cohesive, insulated groups with a directive leader...

Enantiomers are stereoisomers that are non-superimposable mirror images, with every stereocenter inverted. They have identical physical properties in achiral settings, but rotate plane-polarized light equally in opposite directions and r...

A micro-level sociological theory holding that society is built from everyday interactions in which people share and negotiate meanings through symbols such as language and gestures. Testable point: it explains small-scale behavior throu...

O₂ is molecular oxygen, the final electron acceptor of the electron transport chain, where it is reduced to H₂O. Without it the chain stalls, ATP yield drops, and cells shift toward glycolysis and fermentation. Blood carries it on hemogl...

A force is a push or pull, a vector measured in newtons, that can change an object's velocity. The net force (vector sum of all forces on the object) sets acceleration through Fnet = ma. Zero net force means constant velocity, which may...

Concentration is the amount of solute per unit of solution (or solvent), most often molarity: moles of solute per liter of solution. Diluting a solution lowers its concentration while the moles of solute stay the same, which is the idea...

A cofactor is a nonprotein helper, either a metal ion or a small organic molecule (a coenzyme, often vitamin-derived), that an enzyme needs to be active. The enzyme without it is an apoenzyme; with it, a holoenzyme. Testable point: losin...

Self-esteem is a person's overall evaluation of their own worth, the evaluative part of the self-concept. The most testable point is the contrast with self-efficacy, which is belief in one's ability to succeed at a specific task, not a g...

Hydrogen bonding is an attraction between an H bonded to N, O, or F and a lone pair on another N, O, or F atom. It is usually the strongest intermolecular force between neutral molecules, so it raises boiling point, melting point and wat...

A lens is a transparent curved object that refracts light to form an image. In air, a convex lens converges rays (f > 0) and a concave lens diverges them (f < 0). Most testable: use 1/o + 1/i = 1/f, where the sign of i tells real from vi...

Solubility is the maximum amount of solute that dissolves in a given amount of solvent at a set temperature, giving a saturated solution at equilibrium. Most testable: for a salt, solubility is linked to Ksp, and a common ion lowers it.

Conformity is changing your behavior, beliefs, or perceptions to match a group's norms because of real or imagined group pressure, usually without any direct order. The classic study is Asch's line-judgment experiment, where a unanimous...

Genetic drift is a random change in allele frequencies from one generation to the next due to chance sampling of gametes and survivors. Its effects are strongest in small populations, and it tends to reduce genetic variation, potentially...

A repressor is a regulatory protein that binds DNA and lowers transcription of a gene, in bacteria usually by binding the operator and blocking RNA polymerase. Testable point: an inducer (allolactose in lac) pulls the repressor off DNA,...

Diastereomers are stereoisomers that are not mirror images of each other. Unlike enantiomers, they generally differ in physical properties such as melting point, boiling point and solubility, so ordinary methods can separate them.

The Heisenberg uncertainty principle says a particle's position and momentum cannot both be known with unlimited precision at the same time. The product of their uncertainties has a minimum value of h/4π. This is why electrons are descri...

Prejudice is a preconceived, usually negative attitude toward people because of their group membership. It is the feeling component of an attitude; stereotypes are beliefs and discrimination is behavior, and prejudice can occur without d...

Social constructionism is the sociological view that many things we treat as natural or objective, such as race, gender, illness, deviance, or money, get their meaning through shared interaction and agreement. The testable point: these m...

Kinetic energy is the energy an object has because of its motion, K = ½mv². It depends on the square of speed, so doubling speed quadruples kinetic energy. The work-energy theorem says net work on an object equals its change in kinetic e...

Gibbs free energy (G = H − TS) is the energy available to do useful work at constant temperature and pressure. Its change, ΔG = ΔH − TΔS, predicts direction: ΔG < 0 is spontaneous (exergonic), ΔG > 0 is nonspontaneous, and ΔG = 0 is equi...

An anhydride (acid anhydride) is a carboxylic acid derivative with two acyl groups joined through one oxygen, RC(=O)–O–C(=O)R, formally two acids minus water. It is a reactive acylating agent: more reactive than esters and amides, less t...