1,800+ MCAT terms, defined.

Motor proteins (myosin, kinesin, dynein) use energy from ATP hydrolysis to move along cytoskeletal filaments. Myosin works on actin; kinesin and dynein work on microtubules, with most kinesins moving toward the plus end and dynein toward...

Skeletal muscle is striated, voluntary, multinucleated muscle attached to bones and controlled by somatic motor neurons. It contracts by the sliding filament mechanism: Ca²⁺ binds troponin, and myosin pulls actin, shortening the sarcomer...

Attraction (interpersonal attraction) is the tendency to like and want to approach or form a relationship with another person. The most testable point is that proximity, repeated exposure, and similarity usually increase liking, and simi...

Voltage is the electric potential difference between two points: the change in electric potential energy per unit charge, measured in volts (1 V = 1 J/C). A positive charge tends to move toward lower potential, and across a resistor the...

A titration curve plots the pH of the analyte against the volume of titrant added. The steep rise marks the equivalence point, which is at pH 7 only for a strong acid with a strong base. For a weak acid, pH = pKa at the half-equivalence...

The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure. In organic chemistry it mostly reflects intermolecular forces: stronger forces and larger molecules usually give a higher boiling point.

The limbic system is a set of interconnected structures beneath the cortex, centrally the amygdala, hippocampus, and hypothalamus, that link emotion, motivation, and memory. Most testable: the amygdala handles fear and aggression, and th...

Gel electrophoresis separates charged macromolecules, such as DNA or proteins, by driving them through a porous gel in an electric field. With uniform charge per mass, as for DNA fragments or proteins in SDS-PAGE, smaller molecules trave...

Acceleration is the rate at which velocity changes with time, a vector measured in m/s². It is nonzero whenever speed or direction changes, so an object moving in a circle at constant speed is still accelerating.

A quantum number is one of four values (n, l, ml, ms) that together specify an electron's state in an atom: its shell, subshell shape, orbital orientation, and spin. The most testable point is that no two electrons in an atom can have th...

Arousal is the level of physiological and psychological activation, from drowsy to highly alert, driven by the reticular formation and sympathetic nervous system. Most testable: under the Yerkes-Dodson law, performance generally peaks at...

A G-protein coupled receptor is a seven-transmembrane receptor that, when its ligand binds, activates an associated G protein by exchanging GDP for GTP on the α subunit. Testable point: Gα-GTP regulates an effector (adenylyl cyclase or p...

Resonance is the delocalization of π electrons or lone pairs over several atoms, shown by drawing Lewis structures that differ only in electron placement. The real molecule is a hybrid that is more stable than any single contributor, and...

Classical conditioning is associative learning in which a neutral stimulus, repeatedly paired with a stimulus that already triggers a reflexive response, comes to trigger a similar response by itself. The most testable point is labeling...

A wave is a disturbance that carries energy from place to place without net transport of the medium itself. Mechanical waves need a medium; electromagnetic waves do not. Most testable: v = fλ, and at a boundary the frequency stays fixed...

The Aufbau principle says that in an atom's ground state, electrons generally fill orbitals from lowest to highest energy (1s, 2s, 2p, 3s, 3p, 4s, 3d...). The most testable point is that 4s fills before 3d. A few elements, such as Cr and...

Telomeres are repeated, noncoding DNA sequences (TTAGGG in humans) at the ends of linear eukaryotic chromosomes. They shorten with each round of replication because DNA polymerase cannot fully copy the lagging-strand end, unless telomera...

General adaptation syndrome is Hans Selye's three-stage model of the body's response to prolonged stress: alarm, resistance, exhaustion. The most testable point is the order. Stress that continues past resistance leads to exhaustion and...

Distillation separates liquids by boiling point: heating vaporizes mostly the more volatile component, which is condensed and collected. The lower-boiling component distills first; fractional distillation handles close boiling points, an...

Taste (gustation) is the chemical sense in which dissolved molecules bind receptor cells in taste buds, mainly on the tongue. The five basic tastes are sweet, sour, salty, bitter, and umami. Everyday "flavor" also depends heavily on smell.

An antigen is a molecule, often a protein or polysaccharide on a pathogen or foreign cell, that is specifically bound by an antibody or a lymphocyte receptor and can trigger an adaptive immune response. Testable point: the antibody binds...

Refraction is the change in direction of a wave when it crosses a boundary into a medium where its speed differs. Light entering a medium with a higher index of refraction slows and bends toward the normal. Frequency stays the same; spee...

Enthalpy (H) is the heat content of a system, H = U + PV. At constant pressure, the heat absorbed or released equals ΔH: negative is exothermic, positive is endothermic. It is a state function, so ΔH depends only on initial and final sta...

The fundamental attribution error is the tendency, when explaining other people's behavior, to overweight dispositional (internal) causes like personality and underweight situational (external) ones. The most testable point is the contra...

Alternative splicing joins the exons of one eukaryotic pre-mRNA in different combinations, producing different mature mRNAs and protein isoforms from a single gene. It is a main reason a cell can make more distinct proteins than it has g...

A circadian rhythm is a roughly 24-hour cycle in physiology and behavior, such as sleep and wakefulness, body temperature, and hormone release. It is driven by the suprachiasmatic nucleus (SCN) of the hypothalamus and reset by light, whi...

A nucleophile is an electron-rich species that donates a lone pair or pi-bond pair to an electron-poor atom, usually carbon, forming a new bond. The testable point: nucleophilicity is a rate (kinetic) property, whereas basicity is thermo...

The Nernst equation gives a cell's potential under nonstandard conditions: E = E° − (RT/nF) ln Q. A higher product-to-reactant ratio Q lowers E, and at equilibrium Q = K and E = 0. In a concentration cell E° = 0.

Natural selection is the differential survival and reproduction of individuals with heritable traits, so alleles that improve fitness become more common in a population over generations. Individuals do not evolve; populations do. Selecti...

Lipid rafts are small, dynamic regions of the plasma membrane enriched in cholesterol and sphingolipids. They are thicker and more ordered (less fluid) than the surrounding bilayer and are thought to cluster certain signaling proteins. C...

Capacitance is a capacitor's ability to store charge per unit potential difference, C = Q/V, measured in farads. For a parallel-plate capacitor, C = κε₀A/d, so it rises with plate area and dielectric constant and falls with plate separat...

A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy, and it is regenerated rather than consumed in the net reaction. It does not change ΔG, ΔH, or Keq, so it speeds the forward and reverse r...

A stereotype is a generalized belief about the traits of a social group, applied to individual members of that group. It is the cognitive piece of the trio: stereotype (belief), prejudice (attitude or feeling), discrimination (behavior).

A carbonyl is a carbon double-bonded to oxygen (C=O). Oxygen's electronegativity makes the carbon partially positive and electrophilic, so nucleophiles attack it. Most testable: aldehydes, ketones, acids and their derivatives contain it,...

Somatosensation is the body's sense of touch, pressure, vibration, temperature, pain, and body position, detected by receptors in the skin, muscles, and joints. Its signals reach the somatosensory cortex in the parietal lobe, where corti...

Density is mass per unit volume of a substance, ρ = m/V, in kg/m³. The most testable point is that an object's average density compared with the fluid's density usually decides whether it floats or sinks.

A phase change is a transition of a substance between solid, liquid and gas, such as melting or boiling. For a pure substance at constant pressure, temperature stays constant during the change because the heat goes into intermolecular at...

An in-group is a social group a person belongs to and identifies with, defined in contrast to an out-group the person does not belong to. The most testable point is in-group bias: people tend to favor their own group and judge out-group...

Recombination frequency is the percentage of offspring with a new allele combination, not seen in either parent, produced by crossing over between two linked genes. The farther apart the genes, the higher the frequency, and it does not e...

Anomers are two cyclic sugar stereoisomers that differ in configuration at the anomeric carbon, the former carbonyl carbon made a new stereocenter by ring closure. In a D-sugar, α has the anomeric OH trans to the CH₂OH group and β has it...

An acetal is a carbon bonded to two alkoxy (OR) groups, made from an aldehyde or ketone plus two alcohols with acid catalysis. Most testable: acetals are stable to base, nucleophiles and hydrides but hydrolyze in aqueous acid, so they se...

Psychosocial theory, in the MCAT sense, is Erikson's theory that personality develops through eight lifespan stages, each built around a social crisis such as trust vs mistrust or identity vs role confusion. The most testable point is ma...

Electric charge is a property of matter that produces and responds to electric force. It is positive or negative, measured in coulombs, with like charges repelling and opposite charges attracting. Net charge is conserved and comes in who...

An electrolytic cell uses an external power source to drive a nonspontaneous redox reaction (E°cell < 0, ΔG > 0). Oxidation is still at the anode and reduction at the cathode, but the anode is positive and the cathode negative, the rever...

Habituation is a decrease in response to a harmless stimulus that is presented repeatedly. It is a simple form of non-associative learning, since no stimuli are paired. The most testable point: a new or different stimulus can restore the...

NMR (nuclear magnetic resonance) spectroscopy detects ¹H or ¹³C nuclei absorbing radio-frequency energy in a magnetic field, revealing the kinds of hydrogens or carbons in a molecule. Most testable: chemical shift gives the environment,...

Facilitated diffusion is passive movement of polar or charged solutes across a membrane through channel or carrier proteins, down their gradient with no ATP spent. Carrier-mediated transport saturates at a maximum rate, while simple diff...

Consciousness is awareness of yourself and your surroundings, and it varies across states such as alertness, drowsiness, sleep, hypnosis, meditation and drug-induced states. Most testable: waking and sleep states have characteristic EEG...

A reflex arc is the neural pathway of a reflex: receptor, sensory (afferent) neuron, integration center, motor (efferent) neuron, effector. Spinal reflexes are integrated in the spinal cord, so the response can occur without input from t...

A proton is a positively charged particle in the nucleus, with charge +1.60 × 10⁻¹⁹ C and mass about 1.67 × 10⁻²⁷ kg. Its count is the atomic number Z. Most testable: it has the same charge size as an electron but about 1836 times the ma...