1,800+ MCAT terms, defined.

The Strecker synthesis makes an α-amino acid from an aldehyde, ammonia and cyanide: an α-aminonitrile forms first, then acid hydrolysis turns the nitrile into COOH. With achiral reagents the product is a racemic mixture, a frequent MCAT...

In demography, fecundity is the biological capacity of a woman, man, or population to reproduce, the potential number of children. Fertility is the number of births that actually occur, so fecundity is usually higher than fertility.

Phosphoglycerate kinase catalyzes step 7 of glycolysis: it moves a phosphoryl group from 1,3-bisphosphoglycerate to ADP, making ATP and 3-phosphoglycerate. This is substrate-level phosphorylation, and the reaction is reversible, so it al...

Chlorine (Cl, Z = 17) is a period 3 halogen with seven valence electrons. It readily gains one to form Cl⁻, so Cl₂ is a strong oxidizing agent. Testable point: Cl has a more negative electron affinity than F, even though F is more electr...

Personality theories are frameworks that explain why people show stable patterns of thought, feeling, and behavior. The MCAT's main families are psychoanalytic, humanistic, trait, social cognitive, behaviorist, and biological, and the us...

An acyl group is R–C(=O)–, a carbonyl carbon bonded to an R group and with one open bond to attach elsewhere. It is the unit transferred in nucleophilic acyl substitution, and reactivity of acyl derivatives tracks how good the leaving gr...

Subliminal stimuli are stimuli below the absolute threshold, so they are not consciously perceived, yet they can still be processed and can bias later responses through priming. Testable point: lab effects are real but small and brief, a...

The adrenal glands sit atop the kidneys. The cortex makes steroid hormones (cortisol, aldosterone); the medulla makes the catecholamines epinephrine and norepinephrine. Testable point: the cortex is driven by hormones (ACTH, angiotensin...

Constructive interference is the superposition of two or more waves whose displacements add to give a larger amplitude. It occurs when waves meet in phase, so path difference is a whole number of wavelengths (0, λ, 2λ, …).

Dipole-dipole interactions are attractions between the partial positive end of one polar molecule and the partial negative end of another. For molecules of similar size and mass, they typically make boiling points higher than dispersion...

Allosteric regulation is control of a protein's activity by a molecule binding at a site other than the active site, which shifts the protein's conformation. Allosteric enzymes often show sigmoidal rate versus [S] curves rather than hype...

Vygotsky's sociocultural theory holds that cognitive development is driven largely by social interaction and cultural tools, especially language. The most testable idea is the zone of proximal development: tasks a child cannot yet do alo...

A resonance structure is one of two or more valid Lewis structures for the same species that differ only in where π electrons and lone pairs are placed, not in atom positions. The real molecule is a resonance hybrid, and delocalization m...

Two quantities are inversely proportional when their product stays constant, so multiplying one by a factor divides the other by that same factor. The MCAT tests this by scaling one variable and asking for the other, such as doubling pre...

Ionization is the formation of ions from a neutral atom or molecule. For atoms, ionization energy is the energy needed to remove an electron from a gaseous atom; it generally rises across a period and falls down a group. In solution, mol...

In Marx's conflict theory, the proletariat is the working class: people who do not own the means of production and must sell their labor for wages to the bourgeoisie, the owners. Class position depends on relation to production, not on i...

Tension is not a formally defined MCAT term in psychology or sociology. It is a descriptive word for an unpleasant state of arousal, discomfort or strain that tends to motivate a change. The most testable case is cognitive dissonance, wh...

The M-line is the dark line at the center of the sarcomere, in the middle of the H-zone. It anchors the thick (myosin) filaments, holding them in register. During contraction the sarcomere shortens, but the M-line stays in the middle and...

Amino acid catabolism is the breakdown of amino acids: the amino group is removed by transamination and deamination and ends up as urea, while the carbon skeleton enters energy metabolism. Most testable: glucogenic skeletons feed glucone...

Formaldehyde (methanal, H₂C=O) is the simplest aldehyde: its carbonyl carbon carries two hydrogens and no alkyl group. With little steric hindrance and no electron-donating alkyl groups, it is a highly reactive electrophile toward nucleo...

An open pipe is a tube open at both ends, so a standing sound wave has a displacement antinode at each end. Its resonant frequencies are fn = nv/2L for every integer n (all harmonics), unlike a pipe closed at one end, which gives only od...

Planck's constant, h = 6.63 × 10⁻³⁴ J·s, is the proportionality constant between a photon's energy and its frequency (E = hf). It shows that light and atomic energy levels are quantized. Photon energy depends on frequency, not on intensity.

An electroencephalogram (EEG) records the brain's electrical activity through scalp electrodes, mainly from summed postsynaptic potentials in the cortex. Most testable: as sleep deepens, waves slow and grow taller (beta, alpha, theta, de...

Opponent-process theory says color is coded by paired channels (red–green, blue–yellow, black–white) in which the two members oppose each other in the same cells. It explains negative afterimages. It works alongside trichromatic theory:...

The harmonic number n is the positive integer that labels a standing-wave mode, with n = 1 the fundamental. The nth harmonic has a frequency n times the fundamental. Strings and pipes open at both ends allow every integer n; a pipe close...

The central cation is the positively charged metal ion at the center of a coordination complex, such as Cu²⁺ in [Cu(NH₃)₄]²⁺. It is a Lewis acid that accepts electron pairs from ligands. Testable point: the complex's net charge equals th...

A reagent is a substance added to a starting material (the substrate) to cause a specific reaction, and it is usually consumed. The testable point: a reagent is used up, unlike a catalyst, and its identity (such as NaBH₄ versus LiAlH₄) d...

Antiparallel strands run in opposite directions. In a DNA double helix, one strand runs 5'→3' while its partner runs 3'→5', which lets the base pairs line up; β-pleated sheets can also be antiparallel. Most testable: a new strand is read...

Polygamy is marriage involving more than two spouses at the same time. It has two forms: polygyny (one husband, several wives), the more common form in societies that permit polygamy, and polyandry (one wife, several husbands). It contra...

Phosphodiesterase (PDE) is an enzyme that hydrolyzes the second messengers cAMP and cGMP to AMP and GMP, ending the signal. Testable point: inhibiting PDE keeps cAMP high and prolongs the response, while in rods light-activated PDE lower...

In demography, fertility is the actual number of children born in a population, usually given as a rate. It is not the same as fecundity, the biological capacity to reproduce. Fertility typically falls as a society industrializes and wom...

Vector addition combines quantities that have magnitude and direction (displacement, velocity, force) into one resultant, either head-to-tail graphically or by adding x and y components. Most testable: the resultant's magnitude is usuall...

A Lewis base is a species that donates a lone pair of electrons to form a bond with an electron-pair acceptor (a Lewis acid). The testable point: the definition does not require H⁺, so it is broader than the Brønsted definition, and ever...

PBr₃ is phosphorus tribromide, a reagent that converts primary and secondary alcohols into alkyl bromides. It works by SN2, so the carbon bearing the OH is inverted if it is a stereocenter, and carbocation rearrangements are generally av...

Maslow's hierarchy is a humanistic theory of motivation that ranks needs in a pyramid: physiological, safety, love and belonging, esteem, self-actualization. Lower needs generally must be reasonably met before higher ones drive behavior,...

Cell adhesion molecules (CAMs) are membrane proteins that attach a cell to neighboring cells or to the extracellular matrix. The key split: cadherins (Ca²⁺-dependent) mainly join cell to cell, while integrins join cell to matrix. Selecti...

Adhesive forces are attractive intermolecular forces between unlike substances, such as a liquid and its container wall. If adhesion exceeds the liquid's cohesion, the meniscus is concave and the liquid rises in a narrow tube; if cohesio...

A neutralization reaction is an acid reacting with a base to form a salt, and water in the Arrhenius case. It means the acid and base have consumed each other in stoichiometric amounts, not that the pH is 7. The pH at equivalence depends...

Anthropomorphism is attributing human traits, emotions, or intentions to nonhuman animals, objects, or natural events. The testable point is that it is an interpretive bias in studying animal behavior, so researchers describe observable...

Smooth muscle is nonstriated muscle in the walls of hollow organs and vessels, made of spindle-shaped cells that usually have one nucleus. It is mostly involuntary, under autonomic control. Testable point: it has no troponin; Ca²⁺ binds...

An NMR spectrum plots the radio-frequency absorption of nuclei (usually ¹H) in a magnetic field against chemical shift in ppm. Signal count shows distinct hydrogen environments, position shows deshielding, area shows number of H, and spl...

Dynamic pressure is the kinetic energy per unit volume of a moving fluid, ½ρv², measured in pascals. In Bernoulli's equation for ideal flow, faster fluid has higher dynamic pressure and, at constant height, lower static pressure.

Dynamic equilibrium is the state in which the forward and reverse processes continue at equal rates, so the net concentrations of reactants and products stay constant. The most testable point: nothing stops at equilibrium, and concentrat...

Cholesterol metabolism is how cells make, transport, use, and remove cholesterol. It is built from acetyl-CoA, and the key testable point is that HMG-CoA reductase is the rate-limiting enzyme, lowered by high cholesterol and favored by i...

In Freud's psychosexual theory, the Electra complex is a girl's unconscious desire for her father and rivalry with her mother during the phallic stage (about ages 3–6). The name comes from Jung, as a counterpart to the Oedipus complex. I...

The hindbrain is the posterior brain division next to the spinal cord, made up of the medulla, pons and cerebellum. It runs basic survival functions, balance and movement coordination. Most testable: cerebellum for coordination, medulla...

A virion is a complete, infectious virus particle outside a host cell: a DNA or RNA genome inside a protein capsid, sometimes with a lipid envelope. It has no metabolism of its own, so new virions are made only when a host cell's machine...

Chemoselectivity is a reagent's preference for reacting with one functional group over another in the same molecule. The most testable case: NaBH₄ typically reduces aldehydes and ketones but not esters, while the stronger LiAlH₄ reduces...

An equipotential line (a surface in three dimensions) joins points at the same electric potential, so the field does no net work on a charge moved along it. It is perpendicular to the electric field lines, which point toward lower potent...

Room temperature is the ambient lab temperature, roughly 20–25 °C. When no number is given, the MCAT usually means 25 °C (about 298 K), the temperature at which standard data are commonly tabulated. Convert to kelvin before any gas-law o...