1,800+ MCAT terms, defined.

An enolate anion is the resonance-stabilized conjugate base formed when a base removes an α hydrogen from a carbonyl compound; the negative charge is shared by the α carbon and the oxygen. It reacts as a carbon nucleophile, which is the...

Tip links are tiny filaments connecting the tip of each stereocilium on a hair cell to its taller neighbor. Bending toward the tallest stereocilia stretches them and opens mechanically gated ion channels, so the hair cell depolarizes; be...

A chiral carbon (stereocenter) is an sp³ carbon bonded to four different groups, so it has a nonsuperimposable mirror image. Most testable: every standard amino acid except glycine has a chiral α-carbon, and n chiral centers give at most...

Avogadro's law says that at constant temperature and pressure, the volume of a gas is directly proportional to its moles (V/n is constant). Equal volumes of different ideal gases at the same T and P hold equal numbers of particles, whate...

The Hardy-Weinberg principle describes a population that is not evolving: with a large population, random mating, and no mutation, migration or selection, allele and genotype frequencies stay constant. Its equations are p + q = 1 and p²...

In Vygotsky's sociocultural theory, the zone of proximal development is the gap between what a learner can do alone and what they can do with help from a more knowledgeable person. Learning is most effective on tasks in this zone, suppor...

An equatorial bond on a chair cyclohexane points outward around the ring's "equator", roughly in the ring's average plane, rather than straight up or down like an axial bond. Substituents are usually more stable equatorial because that a...

Ambivalent (resistant) attachment is an insecure infant attachment style in which the child is very distressed when the caregiver leaves and is not easily soothed on return, seeking contact yet resisting it. It is linked to inconsistent...

A chelate is a coordination complex in which one polydentate ligand binds a central metal ion through two or more donor atoms, forming ring structures. Chelates are typically more stable than similar complexes of monodentate ligands (the...

The incident angle is the angle between an incoming ray and the normal, the line perpendicular to the surface at the point where the ray hits. It is measured from the normal, not from the surface. It equals the reflected angle and, throu...

Regulation by covalent modification changes an enzyme's activity by attaching or removing a chemical group, most often a phosphate on a serine, threonine or tyrosine side chain. Phosphorylation can either activate or inhibit, depending o...

A critical period is a limited window in development when a particular experience, such as language exposure, is needed for an ability to develop normally. If the input is missing, the deficit often lasts. A sensitive period is the softe...

Host range is the set of species, and often the cell types, that a particular virus can infect. It depends mainly on whether viral surface proteins can bind receptors on the host cell, so a change in either one can widen or narrow the ra...

Deep sleep is slow-wave sleep, the non-REM stage (stages 3 and 4 in older systems, N3 in the newer one) marked by high-amplitude, low-frequency delta waves. It is the hardest stage to wake from, and sleepwalking and night terrors tend to...

A formyl group is –CHO, a carbonyl carbon bonded to one hydrogen; it is the functional group of an aldehyde. Testable point: it is easily oxidized to a carboxylic acid and reduced to a primary alcohol, and it is named with the prefix for...

Dismutation (disproportionation) is a redox reaction in which a single species is both oxidized and reduced, so one element ends up in a higher and a lower oxidation state, as in 2H₂O₂ → 2H₂O + O₂. Under standard conditions it is spontan...

Partial interference happens when two overlapping waves are neither exactly in phase nor exactly out of phase, so the resultant amplitude falls between the fully destructive and fully constructive values. Full cases need a path differenc...

Emotional support is a type of social support in which others give empathy, comfort, love, trust, and caring, so a person feels valued and understood. The testable point is telling it apart from esteem, informational, and tangible (instr...

Fructose 1,6-bisphosphate is the six-carbon glycolytic intermediate formed when PFK-1 uses ATP to phosphorylate fructose 6-phosphate, the committed step. Aldolase then splits it into DHAP and glyceraldehyde 3-phosphate. Gluconeogenesis b...

An informal sanction is an unofficial reward or punishment (praise, a stare, ridicule, gossip, shunning) that peers, family or community give in response to following or breaking a norm. The testable point is that it is not codified or a...

Fractional crystallization separates the components of a solid mixture by their different solubilities: as a solution cools or evaporates, the least soluble component generally crystallizes first and is filtered off. Diastereomers can be...

A randomized controlled trial randomly assigns participants to a treatment group or a control group, then compares outcomes. Random assignment spreads confounders evenly, so it best supports a causal claim. This is a research-design idea...

Hypothalamic-pituitary axes are hormone cascades: the hypothalamus releases hormones that control the anterior pituitary, which releases tropic hormones that act on a target gland. The most testable point is that the target gland's hormo...

Manifest functions are the intended, openly recognized consequences of a social institution or practice, a concept from functionalism tied to Robert Merton. They contrast with latent functions, which are unintended and unrecognized. To c...

Isoelectric focusing separates proteins in a gel with a pH gradient and an electric field. Each protein migrates until the local pH equals its pI, where its net charge is zero and it stops. It separates by pI (charge), not by size.

Phenolphthalein is an acid-base indicator, a weak organic acid that is colorless in acidic solution and pink in basic solution, changing color over roughly pH 8 to 10. It suits titrations whose equivalence point is above pH 7, such as a...

Complementary colors are two colors of light that add together to make white, such as red and cyan, green and magenta, or blue and yellow. Testable point: a pigment or filter that absorbs one color from white light typically appears as t...

A methyl carbon has no other carbons attached; a primary (1°) carbon has one. Methyl and primary alkyl halides are usually the best SN2 substrates because backside attack is unhindered, and they generally avoid SN1 because their carbocat...

In Piaget's theory, the preoperational stage (roughly ages 2 to 7) is when children use symbols such as words and pretend play but cannot yet perform logical mental operations. The most testable points are egocentrism and failure of cons...

The buffer region is the flat stretch of a weak acid or weak base titration curve where pH changes little because both the weak acid and its conjugate base are present. It is centered on the half-equivalence point, where pH = pKa, and sp...

Drag is the resisting force a fluid (liquid or gas) exerts on an object moving through it, pointing opposite the object's velocity relative to the fluid. It increases with speed, so a falling object reaches terminal velocity when drag pl...

A kinase cascade is a signaling pathway in which each kinase phosphorylates and activates the next, so one signal is amplified into a large response. Testable point: blocking one kinase stops the steps downstream of it, while a constitut...

Random mating means individuals pair without regard to the genotype or phenotype in question, so pairings depend only on how common each type is. It is one condition for Hardy-Weinberg equilibrium. Nonrandom mating, such as assortative m...

Allosteric describes regulation in which a molecule (effector) binds a site on a protein other than the active site and changes the protein's shape and activity. Effectors can be activators or inhibitors, and allosteric enzymes typically...

The retardation factor (Rf) in thin-layer or paper chromatography is the distance a compound travels divided by the distance the solvent front travels. It is between 0 and 1. On normal-phase silica, a more polar compound sticks to the pl...

An excited state is an arrangement of an atom's or molecule's electrons with more energy than the ground state, usually because an electron absorbed energy and moved to a higher level. Testable point: a photon is absorbed when its energy...

The work-energy principle says the net work done on an object by all the forces acting on it equals its change in kinetic energy. Positive net work speeds the object up, and negative net work slows it down. The work of one force alone is...

The actual self is the set of attributes you believe you currently have, one part of self-concept. The MCAT contrasts it with the ideal self (who you wish to be) and the ought self (who you feel you should be). Gaps between them are link...

Ultraviolet spectroscopy measures how strongly a sample absorbs UV light (about 200–400 nm) as electrons jump from π or nonbonding orbitals to π* orbitals. Most testable: the more extended the conjugation, the smaller the energy gap and...

Guanidinium is the protonated guanidino group on arginine's side chain, with its positive charge shared by resonance over three nitrogens. Its pKa is about 12.5, so it is positively charged at physiological pH, which makes arginine a bas...

Mutations disable the checkpoints and growth signals that normally limit division, so cells divide unregulated, as in cancer. Oncogenes arise by gain of function (one mutant allele often suffices); tumor suppressors like p53 and Rb typic...

Non-associative learning is a change in response to a repeated stimulus without linking it to another stimulus or a consequence. The two forms tested are habituation (response decreases) and sensitization (response increases).

The solubility product constant, Ksp, is the equilibrium constant for a slightly soluble ionic solid dissolving into its ions; the solid is left out of the expression. Comparing the ion product Q with Ksp predicts precipitation: a solid...

A decibel (dB) is a logarithmic unit for sound intensity level, comparing an intensity to the hearing threshold I₀ = 1.0 × 10⁻¹² W/m². The key point: every +10 dB means intensity is multiplied by 10, and +3 dB roughly doubles it.

Aversive control is changing behavior with unpleasant stimuli, either by punishing a behavior to make it less likely or by negatively reinforcing one (escape or avoidance) to make it more likely. Negative reinforcement is not punishment:...

An α-hydrogen is a hydrogen on the carbon next to a carbonyl group (the α-carbon). It is unusually acidic for a C–H because the enolate left after deprotonation is resonance-stabilized, so a carbonyl compound needs an α-H to form an enol...

Awareness is knowing about your own thoughts, feelings and sensations and about events around you. It is the core of consciousness. The MCAT treats it as a matter of degree, from alert wakefulness through drowsiness and sleep, with alter...

The water dissociation constant, Kw, is the equilibrium constant for water's autoionization, 2 H₂O ⇌ H₃O⁺ + OH⁻. Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C, so pH + pOH = 14 at that temperature. Kw changes with temperature, not with added a...

Electric force is the push or pull between charged objects: like charges repel and opposite charges attract. Its size follows Coulomb's law, proportional to the product of the charges and inversely proportional to the square of their sep...

The citrate shuttle moves acetyl-CoA from the mitochondrial matrix to the cytosol as citrate, because acetyl-CoA itself cannot cross the inner membrane. Cytosolic ATP-citrate lyase regenerates acetyl-CoA for fatty acid synthesis, so the...