1,800+ MCAT terms, defined.

An acetyl group is the two-carbon acyl group CH₃–C(=O)–, carried in cells on coenzyme A as acetyl-CoA, a high-energy thioester. Acetyl-CoA from pyruvate, fatty acids and ketogenic amino acids enters the citric acid cycle by combining wit...

An open system can exchange both energy (heat and work) and matter with its surroundings. The MCAT tests this by contrasting it with closed systems (energy only) and isolated systems (neither).

The heat of reaction is the heat absorbed or released when a reaction proceeds as written; at constant pressure it equals the enthalpy change, ΔH. Negative ΔH is exothermic, positive is endothermic, and ΔH can be found from formation ent...

Psychoticism is one of the three dimensions in Eysenck's trait theory, along with extraversion and neuroticism. High scorers tend to be aggressive, cold, impulsive, and nonconforming. It is a personality trait on a continuum, not psychos...

Stabilizing selection is natural selection that favors intermediate phenotypes and acts against both extremes of a trait. The population curve gets narrower (less variance) while the mean stays roughly the same. Classic example: human bi...

Formic acid (HCOOH, methanoic acid) is the simplest carboxylic acid, with an H where other carboxylic acids have an alkyl group. It is a weak acid, but stronger than acetic acid (pKa about 3.75 versus 4.76), because no electron-donating...

Basilar tuning is the frequency selectivity of the basilar membrane: each location along the cochlea vibrates most for a particular pitch, high near the base and low near the apex. Place theory explains this; frequency theory also contri...

Kw is the ion-product constant of water, Kw = [H₃O⁺][OH⁻], which equals 1.0 × 10⁻¹⁴ at 25 °C. It depends on temperature, not on added acid or base, so pH + pOH = 14 holds at 25 °C; when Kw rises with heating, neutral pH falls below 7.

A phosphatase is a hydrolase that removes a phosphate group from a substrate by adding water. It is not the reverse of a kinase reaction, because it does not make ATP. Dephosphorylation can activate or inhibit a protein, depending on whi...

Covert orienting is shifting attention to a location or object without moving the eyes or head, like aiming a mental spotlight. In cueing tasks, a valid cue speeds responses to a target at that location, and an invalid cue slows them.

Overt orienting is shifting attention by physically moving the eyes, head, or body toward a stimulus so its image falls on the fovea. Covert orienting shifts attention while gaze stays fixed. Testable point: overt versus covert depends o...

The tail sheath is the protein layer around the central tail tube of a contractile-tailed bacteriophage such as T4. When the phage binds a host, the sheath contracts and drives the tube through the bacterial envelope, so the DNA is injec...

Silica is silicon dioxide (SiO₂). As silica gel it is the polar stationary phase in TLC and column chromatography. With a less polar eluent, polar compounds stick to it more strongly and move more slowly (lower Rf) than nonpolar ones.

Nuclear fusion is the joining of two light nuclei into a heavier nucleus, releasing energy because the product has a higher binding energy per nucleon. It powers stars and needs extreme temperature to overcome the repulsion between posit...

Percent yield is the actual yield of a reaction (what is really isolated) divided by the theoretical yield (the maximum predicted from the limiting reagent), times 100. The testable point: find the limiting reagent first, since it sets t...

Spinal nerves are the 31 pairs of peripheral nerves that attach to the spinal cord and connect it to the body. Each is mixed: sensory axons enter through the dorsal root and motor axons leave through the ventral root. Testable point: spi...

An activator is a molecule that increases the activity of an enzyme (an allosteric activator binds away from the active site) or the transcription of a gene (a DNA-binding protein that helps recruit RNA polymerase). Most testable: an all...

Albert Bandura is the psychologist who developed social learning theory: people can learn by observing and imitating a model, without being directly reinforced themselves. His Bobo doll experiment is the classic example. He also proposed...

The molecular ion (M⁺•) is the radical cation formed when a molecule loses one electron in mass spectrometry. Its m/z equals the molecular mass, so it is usually the highest-m/z major peak apart from isotope peaks, though it can be weak...

A traveling wave is a disturbance that carries energy through a medium (or, for light, through space) while the medium's particles only oscillate about fixed positions. Its speed follows v = fλ and is set by the medium; the source sets t...

The triple point is the single temperature and pressure at which a pure substance's solid, liquid and gas phases coexist in equilibrium. On a phase diagram it is where the three boundary lines meet. For water it is 273.16 K (0.01 °C) at...

Echoic memory is the sensory register for sound. It briefly holds a large amount of raw auditory input, typically about 3–4 seconds, and the information decays unless attention moves it into short-term (working) memory. It lasts longer t...

Adaptive radiation is the relatively rapid diversification of one ancestral lineage into many species, each adapted to a different ecological niche. It is typically triggered by open niches, such as a new island chain or a mass extinctio...

Michaelis-Menten kinetics describes how the initial rate of many single-substrate enzymes rises hyperbolically with substrate concentration and levels off at Vmax. Km is the [S] at which the rate is ½Vmax, and a lower Km usually means ti...

Centripetal acceleration is the acceleration of an object moving on a circular path. It points toward the center and changes the direction of the velocity, not the speed. Its magnitude is v²/r, so doubling the speed at a fixed radius qua...

Hans Eysenck was a trait theorist who described personality with a few broad, largely heritable dimensions: extraversion–introversion and neuroticism, later adding psychoticism. Testable point: in his arousal theory, extraverts have lowe...

In organic chemistry, ppm (parts per million) is the unit of chemical shift (δ) in NMR spectroscopy. It reports how far a signal lies from the reference (TMS, 0 ppm) as a fraction of the spectrometer frequency. Higher ppm means downfield...

The ideal self is the person you want to be, built from your aspirations and goals, as opposed to the real (actual) self. A large gap between them tends to bring distress and lower self-esteem. Rogers calls this incongruence. Higgins tie...

Kinematics describes motion using displacement, velocity, acceleration and time, without considering the forces that cause it. Most testable: the standard equations, such as v = v₀ + at, apply when acceleration is constant, as in free fa...

A salt bridge links the two half-cells of an electrochemical cell with an inert electrolyte, letting ions move to keep each solution neutral and complete the circuit. Anions migrate toward the anode and cations toward the cathode; electr...

Imitation is copying the behavior of a model, the core of observational learning in Bandura's social learning theory. Learning can occur without direct reinforcement, and seeing a model rewarded or punished (vicarious reinforcement) chan...

The founder effect is a kind of genetic drift in which a small group splits off from a larger population and starts a new one. Allele frequencies in the new population can differ sharply from the source, so rare alleles may become common...

The carboxyl group is –COOH: a carbonyl carbon (C=O) also bonded to an OH. It defines carboxylic acids and is a weak acid because the carboxylate left after losing H⁺ is resonance stabilized. Most testable: pKa is typically near 4–5, so...

The incus (anvil) is the middle of the three middle-ear ossicles. It relays eardrum vibrations from the malleus to the stapes, which pushes on the cochlea's oval window. Testable point: the order malleus → incus → stapes transmits and am...

When object A exerts a force on object B, B exerts a force on A that is equal in magnitude and opposite in direction. The two forces act on different objects, so they never cancel each other. This is the key point the MCAT tests.

A dipole is a separation of partial positive and partial negative charge, which arises when bonded atoms differ in electronegativity. A molecule is polar only if its bond dipoles do not cancel, so molecular shape matters as much as the b...

Lesion studies infer what a brain region does by observing behavior or cognition after that region is damaged, whether by injury, stroke, surgery, or deliberate damage in animals. The testable point: a lost function suggests the region c...

An antibonding orbital is a molecular orbital formed by out-of-phase overlap of atomic orbitals. It has a node between the nuclei and lies higher in energy than the parent orbitals, so electrons in it weaken the bond and lower the bond o...

Receptor tyrosine kinases (RTKs) are membrane receptors whose intracellular domain is an enzyme that phosphorylates tyrosine residues. Ligand binding (growth factors, insulin) typically causes dimerization and autophosphorylation, which...

The malleus (hammer) is the middle-ear bone attached to the eardrum. It passes vibrations to the incus and then the stapes, which drives the oval window of the cochlea. Testable point: the order of the sound path, and that the ossicles a...

Genetic leakage is the movement of alleles from one population or species into another, typically when reproductive isolation is incomplete and hybrids backcross with a parent group. It is a form of gene flow, and it tends to make gene p...

Absolute pressure is the total pressure at a point, measured from a perfect vacuum as zero. It equals gauge pressure plus atmospheric pressure, and at depth h in a static liquid open to the air, P = P₀ + ρgh. Gas laws need absolute press...

The melting point is the temperature at which a pure solid and its liquid are in equilibrium at a given pressure; it equals the freezing point. Stronger intermolecular forces or bonding generally mean a higher value, and impurities lower...

A phobia is an anxiety disorder of intense, persistent fear of a specific object or situation, out of proportion to the real danger, with avoidance. The MCAT explains it by classical conditioning (acquisition) and negative reinforcement...

Vacuum (suction) filtration separates an undissolved solid from a liquid by drawing the mixture through filter paper in a Büchner funnel, with lower pressure in the flask below. It is faster than gravity filtration, and it is the usual w...

Polyandry is a form of marriage in which one woman has two or more husbands at the same time. It is a type of polygamy (plural marriage) and is documented much less often than its counterpart, polygyny, in which one man has several wives.

Buoyancy is the net upward force a fluid exerts on an object placed in it, equal to the weight of the fluid the object displaces (Archimedes' principle). Most testable: it depends on the fluid's density and the submerged volume, not on t...

Avogadro's number, about 6.02 × 10²³ mol⁻¹, is the number of particles (atoms, molecules, ions) in one mole of a substance. It links the atomic scale to the lab scale: a substance's molar mass in g/mol is the mass of 6.02 × 10²³ of its p...

Base pairing is the hydrogen bonding between complementary nitrogenous bases: A with T (or U in RNA) by two H-bonds, and G with C by three. Because G–C pairs are stronger, DNA with more G–C content melts at a higher temperature.

The anterior chamber is the fluid-filled space at the front of the eye, between the cornea and the iris. It holds aqueous humor, which nourishes the cornea and lens and helps maintain eye pressure. Light passes through it on its way to t...