1,800+ MCAT terms, defined.

Transesterification is the conversion of one ester into another by swapping the alkoxy group (OR′) for a different alcohol's OR″, under acid or base catalysis. It is a nucleophilic acyl substitution, and an excess of the new alcohol driv...

Fusion is melting, the solid-to-liquid phase change, which absorbs heat (ΔHfus > 0) at a constant temperature. The testable point is that heat added at the melting point breaks intermolecular attractions instead of raising temperature. (...

A quantity is exponential when its variable sits in the exponent, so its rate of change is proportional to its current value. Equal time steps multiply it by the same factor, as in radioactive decay and half-life. A straight line on a se...

Personality disorders are enduring, inflexible patterns of thinking, emotion, and behavior that deviate from cultural expectations and cause distress or impairment. The most testable point is the three DSM clusters: A (odd, eccentric), B...

Repression is a Freudian defense mechanism in which the ego pushes anxiety-provoking thoughts, memories, or urges out of conscious awareness into the unconscious. It is unconscious and involuntary, which separates it from suppression, wh...

Intercalated discs are the junctions between end-to-end cardiac muscle cells, containing desmosomes for mechanical adhesion and gap junctions for ion flow. The gap junctions let depolarization spread quickly, so the heart contracts as a...

The extracellular matrix (ECM) is the network of secreted proteins and polysaccharides between animal cells, especially in connective tissue. Collagen gives tensile strength, elastin gives recoil, and proteoglycans resist compression. In...

Acetic acid, CH₃COOH (ethanoic acid), is a two-carbon carboxylic acid and a weak acid (pKa about 4.76) that ionizes only partly in water. Its conjugate base, acetate, is resonance stabilized, and this explains the acidity and the titrati...

A subshell is the set of orbitals within a shell that share the same n and l, labeled s, p, d or f (such as 2p or 3d). A full s, p, d or f subshell holds 2, 6, 10 or 14 electrons, since it has 2l + 1 orbitals with two electrons each.

Newton's laws are three rules of classical mechanics: velocity stays constant unless a net force acts (first), Fnet = ma (second), and forces come in equal, opposite pairs on different objects (third). Most testable: acceleration depends...

Linguistic relativity (the Sapir-Whorf hypothesis) is the idea that the language a person speaks shapes how they perceive and think. The strong form, linguistic determinism, says language determines thought; the weak form, which has more...

Spreading activation is a model of semantic memory in which activating one concept in a network of linked nodes excites related nodes, making them easier to retrieve. It explains priming: related words are recognized faster after a prime.

The transition state is the highest-energy, unstable arrangement of atoms along a reaction pathway, with bonds partly broken and partly formed. Its energy above the reactants is the activation energy, and a catalyst speeds a reaction by...

A longitudinal wave is one in which the medium's particles oscillate parallel to the direction the wave travels, forming compressions and rarefactions. Sound in fluids is the classic example. Light and string waves are transverse, and on...

Unsaturated describes an organic compound with at least one pi bond (C=C or C≡C), so it has fewer hydrogens than the saturated alkane of the same carbon count. Each pi bond or ring lowers the hydrogen count by two, which the degrees-of-u...

Inhibition is a decrease in an enzyme's reaction rate caused by an inhibitor binding to it, reversibly or irreversibly. Most testable: a competitive inhibitor raises apparent Km and leaves Vmax unchanged, while a pure noncompetitive inhi...

Short-term memory is the temporary store in the Atkinson-Shiffrin model, holding about 7 ± 2 items for roughly 20–30 seconds unless rehearsed. Chunking raises how much information fits; newer accounts treat the active version as working...

White blood cells (leukocytes) are nucleated blood cells that defend the body and arise from stem cells in bone marrow. Neutrophils, macrophages and other phagocytes act mainly in innate immunity, while B and T lymphocytes carry out adap...

Humanistic theory is a personality perspective that stresses free will, innate goodness, and a drive toward self-actualization. The testable point: Rogers links a gap between real self and ideal self (incongruence) to distress, and uncon...

Electromotive force (emf) is the potential difference, in volts, that a galvanic cell can supply between its electrodes; despite the name, it is not a force. E°cell = E°cathode − E°anode, and a positive value means the cell reaction is s...

A current-carrying wire is a stream of moving charges that produces a circular magnetic field around it, with direction from the right-hand rule. Its strength is B = μ0I/(2πr), so it falls off as 1/r. Parallel wires with currents in the...

Zaitsev's rule says that in an elimination reaction (E1, or E2 with a small base) the major alkene is usually the more substituted, more stable one. A bulky base such as tert-butoxide often favors the less substituted (Hofmann) alkene in...

In Freud's psychoanalytic theory, the superego is the part of personality that holds internalized moral standards and ideals, learned from parents and society. It opposes the id's urges and pushes behavior toward what is morally right, n...

Actin filaments (microfilaments) are the thinnest cytoskeletal fibers, polymers of globular G-actin that form polar F-actin strands. They shape the cell, drive crawling and cytokinesis, and with myosin power muscle contraction. Testable:...

Boyle's law says that for a fixed amount of gas at constant temperature, pressure and volume are inversely proportional, so PV stays constant. Halving the volume doubles the pressure. It is an ideal gas result that real gases follow clos...

Destructive interference is the superposition of overlapping waves so their displacements partly or fully cancel, giving a smaller net amplitude. For two in-phase sources, it occurs where the path difference is a half-integer multiple of...

A fixed-action pattern is an innate, stereotyped sequence of behavior set off by a specific cue (a sign stimulus) and usually carried through to completion once begun. It is species-typical and unlearned, so the MCAT contrasts it with le...

Pacinian corpuscles are encapsulated mechanoreceptors deep in the skin that detect vibration and deep pressure. They are rapidly adapting, so they fire at the start and end of a stimulus but very little during steady pressure.

Tetrahedral describes an atom with four groups or electron domains arranged at about 109.5°, as in an sp³ carbon. Most testable: a planar sp² carbonyl carbon becomes tetrahedral when a nucleophile adds, forming the tetrahedral intermediate.

The mere exposure effect is the tendency to like a stimulus more after repeated contact with it, even without reward or awareness of the earlier exposures. It underlies familiarity-based attraction (proximity) and tends to hold for neutr...

A Lewis structure draws a molecule or ion's valence electrons as bonds (shared pairs) and lone pairs. Most testable: the best structure or resonance contributor usually has formal charges near zero, with any negative charge on the more e...

Cohesive forces are attractions between molecules of the same substance, such as hydrogen bonds between water molecules. They produce surface tension. Testable point: if adhesion to the wall exceeds cohesion the meniscus is concave; if c...

A dehydrogenase is an oxidoreductase that oxidizes its substrate by removing hydrogen (as hydride plus a proton) and passing the electrons to a coenzyme such as NAD⁺ or FAD. The coenzyme is reduced, to NADH or FADH₂, and that NADH or FAD...

The forebrain is the most anterior of the three embryonic brain divisions (forebrain, midbrain, hindbrain). It includes the cerebral cortex, thalamus, hypothalamus, and limbic structures such as the amygdala and hippocampus. The MCAT tes...

Optimal arousal is the moderate level of physiological and mental activation at which performance tends to be best, as described by arousal theory and the Yerkes-Dodson law. Performance forms an inverted U with arousal, and the peak is l...

Intermediate filaments are cytoskeletal fibers made of proteins such as keratin, vimentin, neurofilaments and lamins. They give cells mechanical strength and anchor structures like the nucleus. Most testable: they are structural and comp...

In ¹H NMR, a proton (or set of equivalent protons) with n equivalent neighboring protons on the adjacent atom is usually split into n + 1 peaks. Testable point: an ethyl group gives a triplet (CH₃) and a quartet (CH₂). Equivalent protons...

London dispersion forces are weak attractions between atoms or molecules, caused by temporary dipoles that induce dipoles in neighbors. They act in every substance and grow with polarizability (more electrons, larger size), so for simila...

Gravitational potential energy is energy stored by a mass because of its position in a gravitational field; near Earth's surface U = mgh. It rises as the object moves higher, and only the change in U matters because the zero level is cho...

Selectins are cell-surface adhesion proteins that bind specific carbohydrate groups on other cells' glycoproteins in a Ca²⁺-dependent way. They mediate the weak, transient binding that makes leukocytes roll along blood vessel walls befor...

Regression is a Freudian defense mechanism in which a person under stress or anxiety reverts to behavior typical of an earlier, less mature developmental stage, such as an adult throwing a tantrum. The exam tests spotting it in a vignett...

A phosphodiester is a phosphate group esterified to two alcohol groups, R–O–P(O)(O⁻)–O–R′. In DNA and RNA, phosphodiester bonds join the 3′ OH of one nucleotide to the 5′ phosphate of the next. They form the covalent, negatively charged...

An acceptor is the species that receives something in a reaction: a proton (Brønsted-Lowry base), an electron pair (Lewis acid), or electrons (oxidizing agent). Testable point: identify what is accepted, since a Lewis acid and a Brønsted...

A longitudinal wave is one in which particles of the medium oscillate parallel to the direction the wave travels, forming compressions and rarefactions. Sound is the key example, and it needs a medium, so it cannot travel through a vacuum.

The Hawthorne effect is a change in participants' behavior caused by knowing they are being observed or given attention, rather than by the variable the researcher manipulated. It threatens a study's validity, and a control group that ge...

A genome is the complete set of genetic material of a cell, organism or virus: genes plus noncoding DNA (RNA in some viruses). Most somatic cells of one individual share the same genome, so cell types differ mainly through differential g...

A zymogen is an inactive enzyme precursor that is activated when a protease cuts specific peptide bonds, as in trypsinogen → trypsin. Because the cleavage is covalent, activation is irreversible, and it protects the cells that make prote...

A donor is the species that supplies something to a partner. Mainly it is an electron-pair donor (a Lewis base, such as a ligand) that forms a coordinate covalent bond with a Lewis acid. A Brønsted-Lowry acid is a proton donor, and in a...

Conduction is heat transfer through a material, or between materials in contact, as colliding particles pass kinetic energy from hotter to cooler regions with no bulk motion. Testable point: the rate rises with area and temperature diffe...

Learned helplessness is a passive, resigned state that develops after repeated exposure to unavoidable aversive events, so an individual stops trying to escape even when escape later becomes possible. The most testable point is that the...