1,800+ MCAT terms, defined.

A category is a mental grouping of objects, people, or events judged to share features, so new items can be classified and their properties inferred. The MCAT tests how categories are represented: by a prototype (the best or average exam...

A functional group is a specific atom or group of atoms that gives an organic molecule its characteristic reactivity and properties, largely regardless of the rest of the molecule. Most testable: identify the group (carbonyl, hydroxyl, a...

Freud's psychosexual stages are five phases of development (oral, anal, phallic, latency, genital) in which libido centers on a different body zone. Unresolved conflict at a stage, from frustration or overindulgence, can cause a fixation...

A node is a point in a standing wave where the medium has zero displacement at all times, because the two oppositely traveling waves always interfere destructively there. Adjacent nodes are λ/2 apart; fixed string ends are nodes, but ope...

The common ion effect is the decrease in solubility of a sparingly soluble salt, or in ionization of a weak acid or base, when the solution already contains an ion the species produces. Le Châtelier's principle explains it: the added ion...

The double helix is the structure of double-stranded DNA: two antiparallel strands coiled together, with sugar-phosphate backbones outside and bases paired inside by hydrogen bonds (A–T, G–C). Most testable: pairing gives Chargaff's rule...

Impression management is the deliberate effort to control how others see us by shaping our behavior, appearance, and speech. The MCAT ties it to Goffman's dramaturgical approach, where people act like performers managing front-stage beha...

Contraction is force generation in muscle by the sliding filament mechanism: myosin heads bind actin, perform a power stroke, and pull thin filaments toward the sarcomere center. Ca²⁺ turns it on and ATP is needed to detach myosin. Filam...

Archimedes' principle says the buoyant force on an object that is fully or partly submerged in a fluid equals the weight of the fluid it displaces. Most testable: the force depends on the fluid's density and the submerged volume, not on...

Colligative properties are solution properties that depend on the number of dissolved solute particles, not their identity: vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. Most testable:...

sp³ describes an atom whose one s and three p orbitals mix into four equivalent hybrid orbitals pointing to the corners of a tetrahedron (about 109.5°). Carbon with four single bonds is sp³. The testable point: count electron domains; fo...

Pheromones are chemical signals released by one individual that influence the behavior or physiology of another member of the same species. They are detected by smell, and in many animals by the vomeronasal organ. Whether humans respond...

Hydrophobic means "water-fearing": a nonpolar molecule or group that cannot hydrogen bond with water and tends to cluster away from it. The key testable point is that this clustering is driven mainly by a gain in the entropy of water, no...

Obedience is changing behavior because an authority figure gives a direct command. In Milgram's shock experiments, most participants in the baseline version went to the maximum voltage, and obedience tended to fall when the authority was...

An image is the point where light rays from an object meet (real) or appear to come from (virtual) after reflection or refraction. Real images form where rays actually converge and can be projected on a screen; virtual images cannot. A n...

Henry's law says that at constant temperature, the concentration of a gas dissolved in a liquid is proportional to that gas's partial pressure above the liquid. Doubling the partial pressure doubles the dissolved amount, in dilute soluti...

Actor-observer bias is the tendency to explain our own behavior with situational (external) causes but to explain other people's behavior with dispositional (internal) causes. It is an attribution bias, so the testable contrast is self →...

The plasma membrane is the phospholipid bilayer, with embedded proteins and cholesterol, that surrounds a cell and is selectively permeable. Small nonpolar molecules cross it by simple diffusion, while ions and large polar molecules usua...

Chemical shift (δ) is the position of an NMR signal, in ppm relative to TMS at 0. Nuclei deshielded by nearby electronegative atoms or pi systems resonate at higher δ (downfield, to the left), and more shielded nuclei appear upfield.

Divided attention is attending to more than one stimulus or task at the same time. Because attentional capacity is limited, performance on one or both tasks usually declines, especially when the tasks are novel, effortful, or compete for...

The mitochondrial matrix is the compartment enclosed by the inner mitochondrial membrane. It houses pyruvate dehydrogenase, most citric acid cycle enzymes and β-oxidation. The electron transport chain pumps H⁺ out of it, so the matrix is...

A resistor is a circuit element that opposes current, producing a potential drop across it and dissipating electrical energy as heat. The most testable point is V = IR combined with the series and parallel rules: adding a resistor in ser...

A mole is the SI amount of substance containing 6.022 × 10²³ elementary entities (atoms, molecules, ions). It links the atomic scale to the lab scale: the molar mass in g/mol is numerically equal to the mass of one atom or molecule in amu.

Locus of control is a person's belief about what determines outcomes in their life. An internal locus attributes outcomes to one's own actions; an external locus attributes them to luck, fate, or other people. An internal locus tends to...

A residue is the unit of an amino acid that remains in a peptide or protein after the elements of water are lost in forming peptide bonds. The most testable point is that a residue's side chain (R group) usually determines its role in fo...

The halo effect is a social perception bias in which one positive trait, such as attractiveness, shapes overall judgments of a person and leads us to assume other good traits. The most testable point is that a single impression spreads t...

A synapse is the junction where a neuron passes a signal to another neuron, a muscle cell, or a gland cell. At a chemical synapse, an action potential triggers Ca²⁺ entry, vesicle release of neurotransmitter, and receptor binding on the...

Focal length is the distance from a mirror or thin lens to its focal point, where rays parallel to the axis converge or appear to diverge from. It is positive for converging elements and negative for diverging ones. Key relation: 1/f = 1...

A solute is the substance dissolved in a solvent to form a solution, and it is usually the component present in the smaller amount. Most testable: colligative properties depend on the number of dissolved solute particles, so a dissociati...

Mismatch repair is a post-replication pathway that removes wrongly paired but chemically normal bases from the newly made DNA strand and refills the gap. The testable point is strand choice: bacteria mark the parent strand by methylation...

Social identity is the part of a person's self-concept that comes from belonging to social groups, such as nationality, religion, gender, or team. Social identity theory says people boost self-esteem by favoring their ingroup over outgro...

Diffraction is the spreading of a wave as it passes through an opening or around an edge. It is most noticeable when the opening is comparable to or smaller than the wavelength, so longer wavelengths spread more.

Activation energy (Ea) is the minimum energy reactants must have to reach the transition state and react. A catalyst lowers Ea without changing ΔG, ΔH, or Keq, so it speeds up the forward and reverse reactions.

The cortex is the folded outer layer of the cerebrum, where much higher-order processing happens: perception, language, planning, and voluntary movement. Most testable: link each lobe to its main function, such as occipital for vision an...

In operant conditioning, reinforcement is a consequence that makes the preceding behavior more likely to recur. Positive reinforcement adds a stimulus and negative reinforcement removes an aversive one; both increase behavior, whereas pu...

Gap junctions are protein channels that join the cytoplasm of adjacent animal cells, letting ions and small molecules pass directly between them. The testable point is electrical coupling: they let depolarization spread cell to cell, as...

REDOX is short for reduction-oxidation: reactions in which electrons move from one species to another. The species that loses electrons is oxidized and the one that gains them is reduced. In metabolism, fuels are oxidized as NAD⁺ and FAD...

A lone pair is two valence electrons held on one atom and not shared in a bond. It counts in VSEPR geometry and makes an atom a Lewis base or nucleophile. Most testable: lone pairs repel more than bonding pairs, so bond angles shrink (NH...

Half-life is the time for half of the nuclei in a radioactive sample to decay. It is constant for a given isotope and does not depend on sample size, so after n half-lives the fraction remaining is (1/2)ⁿ.

Valence electrons are the electrons in an atom's outermost occupied shell, and they do the bonding. For main-group elements the count equals the group's ones digit (group 1 has 1, group 17 has 7; helium has 2). Counting them correctly is...

Peer pressure is social influence from peers of similar age or status that leads a person to change behavior or attitudes to fit the group. The MCAT treats it as conformity, usually driven by normative social influence (the wish for acce...

A subculture is a group within a larger society whose members share distinctive values, norms, or practices that set them apart from the mainstream, while they usually still take part in it. The testable contrast is with counterculture,...

Electron affinity is the energy released when a neutral gaseous atom gains one electron to form an anion. It generally increases across a period and decreases down a group, with exceptions. Halogens have the largest values, so they readi...

Polarity is an uneven sharing of electron density, giving partial charges (δ+ and δ−). A bond is polar when electronegativities differ, and a molecule is polar when its bond dipoles do not cancel. Polar molecules mix with polar solvents...

A kinase is an enzyme that transfers a phosphate group from ATP to a substrate. Protein kinases phosphorylate serine, threonine, or tyrosine side chains to change a protein's activity. Phosphorylation can activate or inhibit, depending o...

A self-fulfilling prophecy is a belief or expectation, often false at first, that changes how people behave so that the expectation comes true. The key test point is that the belief produces the outcome through behavior, as when teacher...

The axon is a neuron's long projection that conducts action potentials away from the cell body toward the axon terminals. Myelin speeds conduction because the signal regenerates at the nodes of Ranvier (saltatory conduction), and a wider...

Zimbardo's 1971 study randomly assigned college students to be guards or prisoners in a mock prison; the guards became harsh and the study ended early. The MCAT uses it to show how situations and assigned social roles can strongly shape...

Cell potential (E°cell, or emf) is the voltage an electrochemical cell can produce, found from the cathode's and anode's reduction potentials. A positive E°cell usually means a spontaneous galvanic cell; a negative value means the reacti...

A quinone is a conjugated six-membered ring with two C=O groups, usually at the 1,4 (para) positions, made by oxidizing a hydroquinone. It is reversibly reduced by 2 e⁻ and 2 H⁺ to a hydroquinone, which is why ubiquinone (coenzyme Q) can...