1,800+ MCAT terms, defined.

Thermal equilibrium is the state in which objects in thermal contact have reached the same temperature, so there is no net heat flow between them. The most testable point: for an isolated pair, the final temperature comes from setting he...

Sleep stages are phases of the ~90-minute sleep cycle, identified by EEG: non-REM stages N1, N2, N3 (deepest, slow-wave) and REM. Most testable: from N1 to N3 EEG frequency falls and amplitude rises, while REM looks like waking brain act...

Inorganic phosphate (Pi) is free phosphate, PO₄³⁻ and its protonated forms such as HPO₄²⁻ and H₂PO₄⁻, not bonded to carbon. It is released when ATP or a phosphate ester is hydrolyzed, and its stability relative to the reactant helps make...

Cooperativity is when ligand binding at one site on a multisubunit protein changes the affinity of its other sites for that ligand. In positive cooperativity, as with hemoglobin and O₂, this gives a sigmoidal binding curve instead of the...

A cochlear implant is an electronic device that bypasses damaged hair cells and electrically stimulates the auditory nerve, so the brain still receives sound signals. It is used for many cases of sensorineural hearing loss, whereas a hea...

Senescence is the permanent loss of a living cell's ability to divide, usually after a limited number of divisions as telomeres shorten (the Hayflick limit), or after DNA damage. Testable point: telomerase can delay it, and most cancer c...

A sigma (σ) bond is a covalent bond formed by head-on overlap of orbitals along the line between the two nuclei. Every single bond is one sigma bond, and double and triple bonds contain exactly one sigma bond plus pi bonds. Atoms can usu...

Corrective lenses are lenses placed in front of the eye so the image lands on the retina. Myopia (image in front of the retina) is corrected with a diverging lens of negative power; hyperopia (image behind it) with a converging lens of p...

Life course theory is a sociological view that lives unfold from birth to death within social and historical contexts, shaped by cohort, timing of events, linked lives and personal agency. Unlike stage theories, it does not assume one un...

Inversion (Walden inversion) is the flip in three-dimensional arrangement at a stereocenter during an SN2 reaction, because the nucleophile attacks from the side opposite the leaving group. The most testable point: a chiral substrate giv...

Long-term potentiation (LTP) is a lasting increase in the strength of a synapse after repeated or intense stimulation. It is the leading cellular model of how learning and memory are stored, notably in the hippocampus. Testable point: th...

A complex ion forms when a metal cation bonds to ligands (Lewis bases) through coordinate covalent bonds, as in Ag(NH₃)₂⁺. Its formation constant Kf is usually large, so complexing removes free metal ion and raises the solubility of a sp...

DC circuits are closed loops in which a constant emf source, such as a battery, drives current in one direction through resistors and other elements. The most testable point: series resistors share the same current, while parallel resist...

Motion parallax is a monocular depth cue: when you move, nearby objects appear to sweep across your visual field faster than distant ones, and the brain uses that difference to judge distance. It works with one eye, unlike binocular cues...

The parathyroid glands are small endocrine glands, usually four, on the back of the thyroid. They release parathyroid hormone (PTH) when blood Ca²⁺ is low, and PTH raises blood calcium through bone, kidney, and (via vitamin D) intestine....

The ion product is the product of dissolved ion concentrations, each raised to its coefficient. For water it is Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C. For a sparingly soluble salt it is Q, compared with Ksp to predict whether a precipita...

The decibel scale is a base-10 logarithmic scale for sound intensity level, β = 10 log(I/I₀), where I₀ = 10⁻¹² W/m² is the threshold of hearing (0 dB). Most testable: every 10 dB increase multiplies intensity by 10, and 3 dB roughly doub...

The suffix -oic acid names a carboxylic acid (R–COOH) in IUPAC nomenclature: the alkane name drops its final -e and takes -oic acid, as in ethanoic acid (acetic acid). The carboxyl carbon is C-1 of the chain, and the group outranks other...

Place theory says pitch is coded by where along the basilar membrane in the cochlea the vibration peaks and bends hair cells: high frequencies near the base, low frequencies near the apex. It explains high pitches well; frequency theory...

DNA sequencing determines the order of nucleotides in a DNA molecule. The classic MCAT method is Sanger (chain-termination) sequencing, where dideoxynucleotides lacking a 3′-OH stop synthesis, so the sizes of the resulting fragments reve...

A precipitate is an insoluble solid that comes out of solution when dissolved ions combine. The testable point is to compare the ion product Q with Ksp: if Q > Ksp, a precipitate forms; if Q < Ksp, the solution is unsaturated and no soli...

Electric power is the rate at which a circuit element transfers or converts electrical energy, measured in watts (J/s). The key relation is P = IV = I²R = V²/R; which form to use depends on whether current or voltage is the same across t...

The theory of planned behavior (Ajzen) says the best direct predictor of a behavior is the intention to do it. Intention is shaped by attitude toward the behavior, subjective norms, and perceived behavioral control. Perceived control is...

The capsid is the protein coat that encloses a virus's DNA or RNA genome, built from repeating viral-encoded subunits called capsomeres. Capsid plus genome is the nucleocapsid. Some viruses add a lipid envelope outside it; naked viruses...

Configuration is the fixed three-dimensional arrangement of groups around a stereocenter (R or S) or a double bond (E or Z, cis or trans). It changes only if a bond is broken and re-formed, unlike conformation, which changes by rotation...

The inner ear is the fluid-filled part of the ear that holds the cochlea, where hair cells transduce sound vibrations into neural signals, and the vestibular system, which senses balance and head position. Testable point: transduction ha...

An acyl-CoA is a fatty acid joined to coenzyme A by a high-energy thioester bond, the activated form that cells oxidize or build into lipids. Most testable: activating a fatty acid costs 2 ATP equivalents (ATP → AMP + PPi), and long-chai...

Solvation is the stabilizing attraction between solvent molecules and dissolved solute particles, which surround each particle. In water it is called hydration. Dissolving is favored when solute-solvent attractions are comparable to or s...

Fluorescence spectroscopy excites a molecule with light and measures the light it re-emits. The emitted photon typically has a longer wavelength and lower energy than the absorbed one, because some energy is lost as heat first. This diff...

Proximity is physical nearness between people, and it is a major factor in interpersonal attraction: people tend to form friendships and relationships with those who live, work or sit close to them. It works largely through repeated cont...

A hydrocarbon is a compound made only of carbon and hydrogen, such as an alkane, alkene, alkyne or arene. It is nonpolar and held together by London dispersion forces, so boiling point rises with chain length and falls with branching.

Selective attention is focusing on one stimulus while filtering out others. The exam often uses dichotic listening and the cocktail party effect, and models differ on where filtering happens: early (Broadbent, Treisman's attenuation) or...

Metaphase is the stage of mitosis, and of meiosis I and II, when chromosomes line up at the cell's equator (the metaphase plate), attached to spindle microtubules from opposite poles. Sister chromatids separate next, in anaphase. In meio...

An acid-base reaction is usually treated as proton transfer from a Brønsted-Lowry acid to a base, giving a conjugate base and conjugate acid. The Lewis view instead uses electron-pair acceptor and donor. Testable: equilibrium favors the...

Cadherins are Ca²⁺-dependent transmembrane proteins that bind one cell to another, usually by binding the same cadherin type on the neighboring cell. They anchor adherens junctions (via actin) and desmosomes (via intermediate filaments)....

Internalization is taking in a group's norms, values, or beliefs until they become one's own, held privately and not just shown in public. In Kelman's three responses to social influence, it is the deepest and most lasting, unlike compli...

NMR (nuclear magnetic resonance) spectroscopy uses a magnetic field and radio waves to probe the environments of nuclei such as ¹H and ¹³C. Most testable: chemical shift, integration and splitting reveal the type, number and neighbors of...

Acid-base equilibria are the reversible proton-transfer reactions of acids and bases in water, quantified by Ka, Kb, and pH. The most testable point: a larger Ka means a stronger acid and a weaker conjugate base, and a buffer (HA with A⁻...

Image formation is how a mirror or lens redirects light from an object so the rays either meet (real image) or only appear to diverge from a point (virtual image). Most testable: 1/f = 1/o + 1/i gives image distance, and the sign of i te...

The Cannon-Bard theory says the feeling of an emotion and the body's physiological arousal occur at the same time and independently, both triggered by thalamic signals after a stimulus. The key test point is timing: James-Lange puts arou...

The cocktail party effect is selective attention: you can focus on one conversation in a noisy room, yet often notice your own name in an ignored one. It shows unattended input gets some processing, as Treisman's attenuation model and la...

The mitral valve (bicuspid, or left atrioventricular, valve) is the one-way valve between the left atrium and left ventricle. It opens when atrial pressure exceeds ventricular pressure and closes as the ventricle contracts, preventing ba...

The Michaelis-Menten model describes how the initial rate of a simple, non-allosteric enzyme rises with substrate concentration, giving a hyperbola that levels off at Vmax. Km is the [S] at which v = ½Vmax, and a lower Km often means tig...

Nucleophilicity is how fast a species donates an electron pair to an electrophilic atom, usually carbon; it is kinetic, unlike basicity. It often tracks basicity, but not always: in polar protic solvents I⁻ > F⁻, while in polar aprotic s...

Bond enthalpy is the average energy needed to break one mole of a given bond in gaseous molecules. Breaking is endothermic; forming releases that energy. Testable point: ΔH°rxn ≈ Σ(bonds broken) − Σ(bonds formed), an estimate that is neg...

Ionisation energy is the minimum energy needed to remove an electron from an atom or ion, leaving it at rest far away. In the Bohr model of hydrogen it is 13.6 eV from the ground state (n = 1) and smaller from higher levels, because it e...

Ethnicity is a social category based on shared culture, such as ancestry, language, religion, and traditions. Race, by contrast, is a socially constructed category based on perceived physical traits. The most testable point: ethnicity is...

Intuition is a fast, automatic judgment reached without conscious step-by-step reasoning, the "gut feeling" of dual-process models (System 1). It can work well with expertise but is more prone to heuristic-driven bias than slow, delibera...

E/Z isomerism is geometric isomerism about a C=C bond, named by ranking the two groups on each sp2 carbon by Cahn-Ingold-Prelog priority. Higher-priority groups on the same side give Z; on opposite sides give E. E and Z isomers are diast...

Mechanical efficiency is the fraction of the work or energy put into a machine that comes out as useful work, e = Wout/Win. Real machines lose some input as thermal energy through friction, so efficiency is below 100% and cannot exceed it.