1,800+ MCAT terms, defined.

A zwitterion is a molecule carrying both a positive and a negative charge, so its net charge is zero. An amino acid near neutral pH is the standard case (−NH₃⁺ and −COO⁻). Testable point: the zwitterion predominates at the isoelectric po...

Electronic geometry is the arrangement of all electron domains (bonding groups and lone pairs) around a central atom, predicted by VSEPR. It counts lone pairs, unlike molecular geometry, which describes only the positions of the atoms.

An expectation is a belief about how a person will act or how an event will turn out, drawn from schemas, experience, or norms. It shapes perception and behavior, so in a self-fulfilling prophecy the belief leads to actions that help mak...

The lagging strand is the new DNA strand made discontinuously during replication, as short Okazaki fragments. DNA polymerase extends only a 3′ end, so on this strand it works away from the fork. Each fragment needs its own RNA primer, an...

Linguistic determinism is the strong form of the Sapir-Whorf hypothesis: the language a person speaks determines, or limits, what they can think. The weaker form, linguistic relativity, says language only influences thought, and most evi...

Paramagnetic describes an atom, ion, or molecule with one or more unpaired electrons, which is weakly attracted into an external magnetic field. Diamagnetic species have their electrons paired and are weakly repelled. To classify one, wr...

Angular frequency (ω) is the rate at which the phase of an oscillation or rotation advances, in radians per second. One cycle is 2π radians, so ω = 2πf = 2π/T. The testable point is converting between ω (rad/s), f (Hz) and T (s).

Lipase is a hydrolase that cleaves the ester bonds of triglycerides, releasing free fatty acids and monoglycerides or glycerol. Most testable: pancreatic lipase acts in the small intestine on fat droplets emulsified by bile salts, so blo...

A thiol is an R–SH group, the sulfur analog of an alcohol; in proteins it is the cysteine side chain. Oxidation joins two thiols into a disulfide bond (R–S–S–R) that stabilizes tertiary and quaternary structure, and reducing agents break...

An oxidizing agent is the species that takes electrons from another species, so it is itself reduced. In organic chemistry, it converts alcohols to carbonyl compounds or acids. Testable point: the oxidant is the one reduced, and its stre...

Broca's aphasia is a language disorder from damage to Broca's area, in the left frontal lobe in most people. It causes slow, effortful, nonfluent speech with relatively preserved comprehension. Key contrast: Wernicke's aphasia gives flue...

A nonmetal is an element on the upper right of the periodic table (plus hydrogen) with high electronegativity and ionization energy. Nonmetals tend to gain electrons to form anions or share electrons in covalent bonds. Metal + nonmetal u...

Elastic force is the restoring force a stretched or compressed elastic object, such as a spring, exerts to return toward its equilibrium length. Within the elastic limit it is proportional to the displacement and opposite in direction (F...

Projection bias is the tendency to assume that other people, or our own future selves, share our current thoughts, feelings, and preferences. Testable point: it is a cognitive bias about assumed similarity, not Freud's projection defense...

A second messenger is a small intracellular molecule or ion (cAMP, IP₃, DAG, Ca²⁺) released after an extracellular first messenger binds a surface receptor; it relays and amplifies the signal. Testable: hydrophilic hormones usually signa...

Histone acetylation is the transfer of an acetyl group from acetyl-CoA to lysine residues on histone tails, done by histone acetyltransferases (HATs). It neutralizes the lysine's positive charge, loosens DNA-histone binding, and generall...

Social interaction is how people act toward and respond to one another, shaped by status, roles, norms, and setting. The MCAT tests the frameworks that explain it, such as symbolic interactionism and Goffman's dramaturgical approach, and...

The immune response is the body's coordinated defense against pathogens and other foreign antigens. It combines a fast, broad innate arm with a slower, antigen-specific adaptive arm. Most testable: memory cells make a second exposure to...

Orbital hybridization is a valence bond model in which an atom's s and p orbitals mix into equivalent hybrid orbitals, one per electron domain (bonded atom or lone pair). Two, three or four domains give sp, sp² or sp³, which sets geometr...

Nuclear fission is the splitting of a heavy nucleus, such as uranium-235, into two smaller nuclei, usually after it absorbs a neutron. It releases neutrons and a large amount of energy because the products are more tightly bound per nucl...

Sleep spindles are brief bursts of fast, rhythmic EEG activity (roughly 12–14 Hz) seen in stage 2 non-REM sleep, usually alongside K-complexes. The most testable point: spindles and K-complexes are the EEG signature that marks stage 2 sl...

An alkyl group (R) is a saturated hydrocarbon substituent, CnH2n+1, made by removing one hydrogen from an alkane (methyl, ethyl, propyl). Alkyl groups donate electron density weakly, so they stabilize carbocations: 3° > 2° > 1° > methyl.

The Yerkes-Dodson law says performance is best at a moderate level of arousal and drops when arousal is too low or too high, giving an inverted U. The best level depends on the task: easy or well-practiced tasks peak at higher arousal, d...

Bond dissociation energy is the enthalpy needed to break one mole of a specific bond in the gas phase, with each atom keeping one electron. It is positive. Stronger, shorter, higher-order bonds generally have larger values, and ΔH ≈ Σ BD...

Unpaired electrons are electrons alone in an orbital, so their spin magnetic moments are not canceled by a partner. Atoms with them are attracted to magnetic fields (paramagnetic, or ferromagnetic in domain-forming metals like iron); ful...

Integrins are transmembrane receptors, each made of an α and a β subunit, that bind extracellular matrix proteins such as fibronectin, collagen and laminin and connect to the cytoskeleton inside. Testable point: integrins link cell to ma...

The fovea is a small pit at the center of the retina that is densely packed with cones and has essentially no rods. It gives the sharpest, most color-sensitive vision in bright light, so acuity and color fall off in the periphery, where...

Venules are the smallest veins: thin-walled vessels that collect blood from capillaries and merge into larger veins. Pressure in them is low and they are very compliant, so venules and veins act as a blood reservoir. They are named by di...

A phase diagram plots pressure against temperature and shows which phase of a substance (solid, liquid or gas) is stable in each region. Lines are two-phase equilibria and the triple point has all three. Testable: the solid–liquid line t...

Faraday's law says a changing magnetic flux through a loop of wire induces an emf equal to the number of turns times the rate of change of flux. The most testable point is that only a change in flux induces an emf. If the flux stays cons...

Ammonia (NH₃) is a small neutral molecule with a lone pair on nitrogen, so it acts as a weak base and as a nucleophile. Its most testable organic use is reacting with electrophiles such as alkyl halides to make amines, a reaction that of...

The medulla oblongata is the lowest part of the brainstem, in the hindbrain, just above the spinal cord. It controls automatic, life-sustaining functions such as breathing, heart rate, blood pressure, and reflexes like swallowing and vom...

Urea, (NH₂)₂C=O, is the main nitrogen waste from amino acid breakdown in humans. The liver makes it from toxic ammonia in the urea cycle, and the kidneys excrete it. Testable point: the liver makes urea, and it is not uric acid, which co...

Social behaviorism is George Herbert Mead's theory that the self and mind develop through social interaction, as people use symbols and language to take on others' perspectives. The most testable point is his stages of self-development:...

Noble gases are the Group 18 elements (He through Rn). Their filled valence shells give them the highest ionization energies in their periods and make them generally unreactive, though Xe forms compounds such as XeF₄. Boiling points rise...

The farad (F) is the SI unit of capacitance: a capacitor has a capacitance of 1 F if it stores 1 coulomb of charge per volt across it (1 F = 1 C/V). Capacitance is set by geometry and the dielectric, not by the charge or voltage applied.

Mirror neurons are neurons that fire both when an individual performs an action and when that individual watches another perform a similar action. They are proposed to support observational learning, imitation, and empathy, though their...

A pathogen is an infectious agent, such as a bacterium, virus, fungus, protozoan, or parasitic worm, that causes disease in a host. The MCAT tests whether you can identify the type of pathogen and predict which immune response or treatme...

Phosphoric acid (H₃PO₄) is a weak triprotic acid whose conjugate bases, the phosphates, form esters and anhydrides in ATP and the DNA backbone. Testable point: its three pKa values (about 2, 7 and 12) mean H₂PO₄⁻ and HPO₄²⁻ are the main...

Sigmoidal means S-shaped. In biochemistry it marks cooperative binding, as in hemoglobin's O₂ curve or the rate curve of many allosteric enzymes. Non-cooperative binding, as in myoglobin or Michaelis-Menten enzymes, gives a hyperbola ins...

Gay-Lussac's law says that for a fixed amount of gas at constant volume, pressure is directly proportional to absolute temperature: P/T = constant. Temperature must be in kelvin, so doubling T in kelvin doubles P.

The watt (W) is the SI unit of power, the rate at which energy is transferred or work is done. One watt is one joule per second. The testable point is that a watt measures a rate, not an amount of energy.

The otolithic organs, the utricle and saccule in the inner ear's vestibule, sense linear acceleration and head position relative to gravity. The usual MCAT contrast is that the semicircular canals sense rotation instead.

An enamine is a compound with a nitrogen bonded directly to a C=C double bond (N–C=C). It forms from an aldehyde or ketone and a secondary amine with loss of water. Its β carbon is nucleophilic, like an enol or enolate; primary amines us...

Hypnosis is a technique in which a guide uses relaxation, focused attention, and suggestions to produce heightened suggestibility. Whether it is a distinct altered state of consciousness (state view) or the result of expectation and role...

Ruffini endings are slowly adapting mechanoreceptors in the deep dermis that respond to skin stretch and sustained pressure. The key point is that they keep firing during a steady stimulus, unlike rapidly adapting Meissner and Pacinian c...

Reaction order is the exponent on a reactant's concentration in the experimentally determined rate law. The overall order is the sum of the exponents. Most testable: find the order from initial-rate data by seeing how the rate changes wh...

The newton (N) is the SI unit of force: the net force that gives a 1 kg mass an acceleration of 1 m/s², so 1 N = 1 kg·m/s². The most testable point is that newtons measure force, including weight, not mass.

Structural proteins give cells and tissues shape, support and strength. Examples are collagen, elastin, keratin, and the cytoskeletal proteins actin and tubulin. Collagen and keratin are fibrous, but actin and tubulin are globular subuni...

A neutral stimulus is one that does not trigger the response of interest before learning. In classical conditioning it is paired with an unconditioned stimulus and becomes a conditioned stimulus. It is the same stimulus before and after,...