1,800+ MCAT terms, defined.

A basal body is a microtubule structure at the base of a eukaryotic cilium or flagellum. It anchors the appendage and templates its growth. It has the same 9 triplet microtubule arrangement as a centriole (9+0), unlike the 9+2 doublet pa...

Cosine (cos) is the trigonometric ratio adjacent/hypotenuse. In physics it gives a vector's component along the axis the angle is measured from, such as Fx = F cos θ. Most testable: cos is 1 at 0° and 0 at 90°, so that component shrinks...

Chirality is the property of a molecule that cannot be superimposed on its mirror image. It usually arises from a stereocenter, such as an sp³ carbon with four different groups. Testable point: a chiral molecule has an enantiomer, but a...

Nitrogen nucleophiles, such as ammonia and amines, attack electrophiles with the lone pair on nitrogen. Neutral amines form amides with acyl derivatives, imines or enamines with carbonyls, and alkylated amines by SN2; amide N and ammoniu...

A shadowing task is a dichotic listening method in which a person repeats aloud the message in one ear while a different message plays in the other. It studies selective attention: people usually notice physical features of the ignored m...

Refracted light is light that bends as it crosses a boundary into a different medium, because its speed changes. Entering a medium with a larger index of refraction, it bends toward the normal. Light hitting along the normal is not bent.

Electrochemical refers to redox chemistry linked to electrical energy. A galvanic (voltaic) cell uses a spontaneous reaction to make current; an electrolytic cell uses an external source to force a nonspontaneous one. In both, oxidation...

A glycolipid is a membrane lipid with a carbohydrate attached by a glycosidic bond, most often on a ceramide (sphingolipid) backbone. In animal cells it sits in the outer leaflet of the plasma membrane with its sugar facing the extracell...

The SI unit of electric charge is the coulomb (C), the charge delivered by 1 ampere in 1 second. The smallest free charge is the elementary charge, e = 1.60 × 10⁻¹⁹ C, so the net charge on an ordinary object is a whole-number multiple of e.

Intermolecular attractions are forces between separate molecules, not the bonds inside them. Roughly ranked, London dispersion < dipole–dipole < hydrogen bonding, and stronger attractions generally mean a higher boiling point and lower v...

Nucleophilic acyl substitution is a reaction where a nucleophile adds to a carbonyl carbon, then the leaving group departs, swapping the group on an acyl compound. Reactivity follows leaving-group ability: acyl chloride > anhydride > est...

The tetris effect is the carryover of a long, repetitive, pattern-based activity into later thoughts, mental images and dreams, often at sleep onset. The testable point is that it can occur in amnesic patients who cannot recall the activ...

Native PAGE is polyacrylamide gel electrophoresis run without SDS or reducing agents, so proteins stay folded and keep their subunit associations. They separate by net charge, size, and shape together, unlike SDS-PAGE, which separates ma...

Trichromatic theory (Young-Helmholtz) says color vision comes from three cone types, most sensitive to short (blue), medium (green), and long (red) wavelengths, with each color coded by their relative activity. It explains the receptor s...

The coulomb (C) is the SI unit of electric charge, equal to one ampere-second (1 C = 1 A·s). The elementary charge is e = 1.60 × 10⁻¹⁹ C. The most testable uses are Q = IΔt and the charge unit in Coulomb's law.

Specific heat (c) is the heat needed to raise the temperature of one unit mass of a substance by one degree. A higher c means a smaller temperature change for the same heat added. Water's is unusually high, about 4.18 J/(g·K).

Clonus is a series of rhythmic, involuntary muscle contractions set off by a sudden, sustained stretch, such as a quick upward push on the foot. It typically reflects an overactive stretch reflex after loss of descending inhibition. It i...

Aerotolerant anaerobes do not use oxygen for metabolism and rely on fermentation for ATP, but they survive in the presence of O₂ and grow equally well with or without it. Unlike obligate anaerobes, they are not killed by O₂.

Peptide chemistry covers how amino acids link through amide (peptide) bonds, formed when a carboxyl group and an amine condense and release water. Most testable: resonance gives the peptide bond partial double-bond character, so it is pl...

A K-complex is a brief, large EEG wave, a sharp negative peak followed by a slower positive swing, that is a hallmark of stage 2 (N2) non-REM sleep along with sleep spindles. On the MCAT, spindles plus K-complexes on a theta background p...

A chromatid is one of the two essentially identical DNA copies of a replicated chromosome. The pair, called sister chromatids, is joined at the centromere. Replication in S phase doubles the chromatids and DNA content but not the chromos...

Radiation is heat transfer by electromagnetic waves, which needs no medium and so works across a vacuum. The power an object radiates scales with T⁴ (kelvin), so doubling absolute temperature raises it sixteenfold. In nuclear physics, ra...

A weighted average counts each value in proportion to its share of the sample, not equally. In general chemistry it gives atomic mass from isotope masses and fractional abundances, and the result lies nearer the mass of the most abundant...

Rationalization is a defense mechanism in which a person explains a behavior or outcome with logical, socially acceptable reasons that hide the real, anxiety-provoking motive. Testable point: the facts are admitted but reinterpreted, unl...

Spectroscopy is the use of how molecules absorb electromagnetic radiation at specific energies to work out their structure. The MCAT's main types are IR (functional groups), ¹H NMR (hydrogen environments) and UV-vis (conjugation). Photon...

Relative poverty means having much less income or resources than most others in one's society, so a person falls below the standard of living typical there. It is defined by comparison, unlike absolute poverty, which is lack of basic nec...

An isolated system exchanges neither matter nor energy (heat or work) with its surroundings, so its total energy stays constant. A closed system differs: it can exchange energy but not matter. In mechanics it also means no net external f...

A complete ionic equation writes every strong electrolyte in a reaction as separate ions in aqueous solution, while solids, liquids, gases, and weak electrolytes stay as whole formulas. Cancelling the spectator ions, which appear unchang...

Inducible describes a gene, enzyme, or operon that is expressed at a low level until a specific molecule, the inducer, turns it on. The classic MCAT example is the lac operon, where allolactose, a lactose derivative, inactivates the repr...

Endolymph is the fluid inside the membranous labyrinth of the inner ear, which includes the cochlear duct and the semicircular canals. Its movement bends hair-ority stereocilia, turning sound vibration (cochlea) or head motion (vestibula...

A Newman projection views a molecule straight down one C–C bond: the front carbon is a point where three bonds meet, and the back carbon is a circle. It is used to compare conformations. Staggered is lower in energy than eclipsed, and in...

A closed system can exchange energy (heat and work) with its surroundings but not matter. Its internal energy changes according to ΔU = Q − W. Closed is not the same as isolated, which exchanges neither matter nor energy.

Molar solubility is the number of moles of a sparingly soluble salt that dissolve per liter of saturated solution (mol/L). It is calculated from Ksp, and a common ion lowers it. Ksp values can be compared directly only for salts that giv...

Semantic means relating to meaning. Semantic memory is explicit long-term memory for general facts, concepts, and word meanings, not tied to a personal time or place. Semantic encoding (processing meaning) generally gives better recall t...

Ependymal cells are ciliated glial cells that line the brain's ventricles and the spinal cord's central canal. Those in the choroid plexus help make cerebrospinal fluid (CSF), and cilia help circulate it. Testable point: they handle CSF,...

1/Km is the reciprocal of the Michaelis constant, the substrate concentration giving half of Vmax. A larger 1/Km usually signals higher apparent substrate affinity, though this is approximate. On a Lineweaver-Burk plot, the x-intercept i...

An internal plane of symmetry is an imaginary mirror plane that cuts a molecule into two halves that are reflections of each other. A molecule with one is achiral, so a compound with stereocenters and an internal plane is meso and optica...

Electric potential (V) is the electric potential energy per unit charge at a point, measured in volts (J/C). Positive charges tend to move toward lower potential and negative charges toward higher. Potential is a scalar, so contributions...

Stigma is a socially defined attribute or label that is deeply discrediting, so others devalue the person carrying it (Goffman). The testable point is that stigma arises from social reaction in a given context, not from the trait itself,...

Absolute poverty is a lack of the resources needed for basic survival, such as food, clean water, shelter, and clothing, judged against a fixed standard. It does not depend on how others in the society live. The testable contrast is with...

The junction rule (Kirchhoff's first rule) says the total current flowing into a junction equals the total current flowing out. It expresses conservation of charge. Most testable point: currents in parallel branches split but always add...

Molecular weight is the sum of the average atomic masses of all atoms in a molecular formula, in amu. It is numerically equal to molar mass in g/mol, so it converts grams to moles: n = m/M. Heavier gas molecules also effuse more slowly.

1/Vmax is the reciprocal of an enzyme's maximum velocity. It is the y-intercept of a Lineweaver-Burk plot (1/v₀ versus 1/[S]). A higher intercept means a lower Vmax, so competitive inhibition leaves the intercept unchanged and other inhi...

Impulse control is the ability to resist an urge or delay a reward instead of acting on it right away. It depends heavily on the prefrontal cortex, and in Freud's model on the ego restraining the id. Frontal lobe damage, as in Phineas Ga...

Leak channels are ion channels that stay open at rest, letting ions diffuse down their electrochemical gradients with no ATP. K⁺ leak channels are the main reason the resting potential is negative and close to the K⁺ equilibrium potentia...

The just noticeable difference (difference threshold) is the smallest change in a stimulus that a person detects half the time. By Weber's law, it is roughly a constant fraction of the starting intensity, so heavier or brighter baselines...

The loop rule (Kirchhoff's second rule) says the potential changes around any closed loop in a circuit sum to zero, which expresses conservation of energy. On the MCAT, you write one such equation per loop, with signs set by the directio...

Normality (N) is concentration in equivalents per liter of solution, where one equivalent is one mole of H⁺ or OH⁻ (acid-base) or of electrons (redox). It equals molarity × equivalents per mole, so it depends on the reaction: 1 M H₂SO₄ i...

UV spectroscopy measures how much ultraviolet light (about 200–400 nm in a standard UV-Vis instrument) a sample absorbs as electrons jump from filled to empty orbitals. Most testable: more extended conjugation means a smaller energy gap...

Exhaustion is the third and last stage of Selye's general adaptation syndrome (alarm, resistance, exhaustion). When a stressor lasts long enough, the body's coping resources tend to run down, and vulnerability to illness rises. Chronic s...