1,800+ MCAT terms, defined.

A demyelinating disease damages the myelin sheath around axons, so action potentials conduct more slowly or fail to propagate. Multiple sclerosis (loss of CNS myelin made by oligodendrocytes) is the classic example. The testable point is...

In Erikson's psychosocial theory, industry vs. inferiority is the fourth stage, covering roughly ages 6 to 12. Children who master school and social tasks and are encouraged gain a sense of competence; repeated failure or discouragement...

A nonvolatile substance has negligible vapor pressure at the temperature of interest, so it stays in the liquid or solid rather than evaporating. The key point: a nonvolatile solute lowers the solvent's vapor pressure, which raises the b...

Kirchhoff's junction rule says the total current flowing into a circuit junction equals the total current flowing out, which is conservation of charge. On the MCAT, use it to find a missing branch current: parallel branch currents add to...

Initiative vs. guilt is Erikson's third psychosocial stage, in early childhood (about ages 3–6), when children start planning and beginning activities. Encouragement builds purpose; repeated criticism can leave guilt. It follows autonomy...

An axial bond is a bond on a chair cyclohexane carbon that points straight up or down, parallel to the ring's axis, alternating from carbon to carbon. Axial groups suffer 1,3-diaxial strain, so bulky groups usually prefer equatorial posi...

Integrity vs. despair is Erikson's eighth and final psychosocial stage, in late adulthood. People review their lives and either accept them with a sense of wholeness (integrity) or feel regret and fear of death (despair). Success yields...

pKw is −log Kw, where Kw is the ion-product constant for the autoionization of water. At 25 °C, Kw = 1.0 × 10⁻¹⁴ and pKw = 14, so pH + pOH = 14. Autoionization is endothermic, so Kw rises and pKw falls as temperature increases.

The lac repressor is the E. coli protein that binds the lac operator and blocks transcription of the lactose-use genes. Allolactose, made from lactose, binds it and releases it from DNA, so the operon is inducible. Mutants that cannot bi...

A dihybrid test cross mates an AaBb individual with a homozygous recessive (aabb) to reveal the gametes the AaBb parent makes. Independent assortment gives about 1:1:1:1 offspring; mostly parental types signal linked genes, and the recom...

Intimacy vs. isolation is Erikson's sixth psychosocial stage, in young adulthood (roughly 18 to 40). The task is forming close, committed relationships; success brings the virtue of love, while failure can leave a person lonely and isola...

Rate order is the exponent of a reactant's concentration in the experimentally determined rate law. It shows how strongly the rate depends on that concentration, and the overall order is the sum of the exponents. Orders come from data, n...

Kinesthetic sense (kinesthesia) is the awareness of body position and movement, from receptors in muscles, tendons, and joints. Many texts treat it as interchangeable with proprioception; some limit it to movement. It differs from the ve...

Non-superimposable means one structure cannot be laid over another so every atom matches, even after rotating it. A chiral molecule and its mirror image are non-superimposable and are called enantiomers, like D- and L-amino acids. Diaste...

Need theories explain motivation as behavior aimed at satisfying needs. Maslow's hierarchy ranks needs from physiological to self-actualization, with lower needs generally taking priority. Drive-reduction theory says a need creates a dri...

Dipole-dipole forces are attractions between the partial positive end of one polar molecule and the partial negative end of another. They are stronger than London dispersion forces for molecules of similar size, so they usually raise boi...

Newton's second law says the net force on an object equals its mass times its acceleration (Fnet = ma). Acceleration points along the net force and is smaller for larger mass. Testable point: zero net force means zero acceleration (const...

Early embryogenesis is the sequence from fertilization through cleavage, morula, blastocyst and gastrulation, ending in three germ layers (ectoderm, mesoderm, endoderm). Most testable: cleavage makes smaller cells without growth, and eac...

Out-group dynamics are the patterns in how people perceive and treat members of groups they do not belong to, compared with their own in-group. The most testable point: people tend to favor the in-group and see out-group members as more...

A benzyl group is C₆H₅CH₂−, a benzene ring attached through a CH₂ carbon (abbreviated Bn). That CH₂ is the benzylic position, where cations and radicals are resonance stabilized by the ring, so benzylic sites are unusually reactive.

The body's coordinated reaction to a perceived threat: a fast sympathetic–adrenal medulla surge (epinephrine, norepinephrine) and a slower HPA axis release of cortisol. Most testable: Selye's alarm, resistance, exhaustion stages, and chr...

A carboxyl group is –COOH: a carbon double-bonded to one O and single-bonded to an OH. In amino acids it is the acidic group at the C-terminus and in Asp and Glu side chains. Its pKa is well below 7.4, so at physiological pH it is deprot...

Snell's law relates the angles of a light ray on the two sides of a boundary between transparent media: n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal. Light entering a higher-index medium bends toward the normal; entering a...

A carbon center is a particular carbon atom identified by its role in a structure or reaction: electrophilic (electron poor, attacked by nucleophiles), nucleophilic (electron rich), or chiral (four different groups). Most testable: nucle...

The endomembrane system is the network of eukaryotic membrane compartments (nuclear envelope, ER, Golgi, lysosomes, vesicles, often the plasma membrane) linked by continuity or vesicle traffic. Most testable: secreted proteins move rough...

On the MCAT, antisocial usually means antisocial personality disorder: a long-standing pattern of disregarding and violating others' rights, with deceit, impulsivity, and little remorse. It is a Cluster B disorder, and it does not mean s...

Chelation is the binding of one metal ion by a single ligand through two or more donor atoms, forming a ring. Chelates are usually more stable than complexes of comparable single-donor ligands, mainly because of a favorable entropy gain...

A thioester is an acyl group bonded to sulfur, R–C(=O)–S–R′, as in acetyl-CoA and succinyl-CoA. Its hydrolysis is strongly exergonic (about −31 kJ/mol for acetyl-CoA, similar to ATP), so it acts as an activated acyl carrier that can driv...

A dyad is a social group of exactly two people, the smallest possible group (a point from Georg Simmel). Dyads tend to be intense and intimate but fragile, because the group ends if either member leaves. Contrast this with a triad, which...

The carbonate ion, CO₃²⁻, is a planar, resonance-stabilized anion whose −2 charge is delocalized over three oxygens. The most testable point is that all three C–O bonds are identical, with bond order 4/3 and a length between a single and...

Homogeneous means uniform in composition and properties throughout, as in a solution, which is a single phase. The MCAT also applies it to equilibria and catalysts in which everything is in one phase. In a heterogeneous equilibrium, pure...

Binding energy is the energy needed to split a nucleus into separate protons and neutrons, equal to the mass defect times c². Higher binding energy per nucleon means a more stable nucleus. The curve peaks near iron-56, so fusion of light...

An achieved status is a social position a person earns or acquires through effort, choice, or merit, such as physician, spouse, or college graduate. It contrasts with ascribed status, which is assigned at birth or involuntarily. The key...

Evolutionary fitness is an individual's or genotype's reproductive success: the number of offspring that survive and reproduce, relative to others in the population. It is not strength or lifespan. Inclusive fitness also counts relatives...

Ascribed status is a social position a person is given by birth or by others, not one chosen or earned. Examples are sex, race, age and family of origin. The testable contrast is with achieved status, which comes from effort or choice, s...

Pyruvate decarboxylation is the step in which the pyruvate dehydrogenase complex converts pyruvate to acetyl-CoA in the mitochondrial matrix, releasing CO₂ and reducing NAD⁺ to NADH. It links glycolysis to the citric acid cycle and is es...

The coupling constant J is the spacing, in hertz, between adjacent peaks within an NMR multiplet. It measures spin–spin splitting between hydrogens on neighboring carbons. J is set by the molecule, not the spectrometer, so it stays the s...

Read as SO₃²⁻, the sulfite ion: sulfur bonded to three oxygens with one lone pair, giving a trigonal pyramidal shape and a sulfur oxidation state of +4. Testable point: it is pyramidal, unlike planar SO₃, and it can be oxidized to sulfat...

Equivalent capacitance is the capacitance of a single capacitor that could replace a network and store the same charge at the same voltage. Capacitors in parallel add directly, while in series the reciprocals add, so the series value is...

The episodic buffer is a component of Baddeley's working memory model. It is a temporary, limited-capacity store that binds information from the phonological loop, the visuospatial sketchpad and long-term memory into one unified episode....

A neural impulse is the action potential: a brief reversal of membrane potential that travels down an axon once a neuron reaches threshold. The neuron fires fully or not at all, so a stronger stimulus raises the firing rate, not the size...

A supercritical fluid is a substance above both its critical temperature and critical pressure, where it is no longer distinctly liquid or gas. The testable point: above the critical temperature, compression alone cannot liquefy a gas, b...

Vascular resistance is the opposition to blood flow in vessels, caused by viscous friction, and ΔP = QR. For idealized laminar flow, R = 8ηL/(πr⁴), so resistance varies as 1/r⁴: halving a vessel's radius raises its resistance 16-fold.

A cyclic diketone is a ring bearing two ketone C=O groups, such as 1,3-cyclohexanedione. Open-chain diketones also appear in intramolecular aldol closure: a 1,4-diketone typically closes to a five-membered ring and a 1,5-diketone to a si...

Uncouplers are molecules or proteins that let protons leak back across the inner mitochondrial membrane without passing through ATP synthase. The gradient's energy is released as heat instead of making ATP, so electron transport and O₂ u...

Gene flow is the transfer of alleles between populations when migrants (or gametes, such as pollen) breed in the new population. It changes allele frequencies and tends to make populations more alike. Reduced gene flow lets populations d...

Past-in-present discrimination is indirect institutional discrimination in which a current rule, neutral in intent, carries forward the effects of earlier discrimination. Testable point: unequal outcomes can persist without present preju...

Retrograde amnesia is the loss of memories formed before a brain injury or disease, while the ability to form new memories may be intact. Older memories are often better preserved than recent ones. It is the opposite of anterograde amnes...

Heat transfer is the movement of thermal energy between objects at different temperatures, spontaneously from hotter to colder, by conduction, convection, or radiation. Most testable: q = mcΔT, and in an isolated calorimeter the heat los...

The electrostatic force is the force between two electrically charged objects, given by Coulomb's law. Like charges repel and opposite charges attract, and the magnitude falls off as 1/r², so doubling the distance cuts the force to one-q...