1,800+ MCAT terms, defined.

An aprotic solvent has no O–H or N–H bond, so it cannot donate hydrogen bonds. Polar aprotic solvents (DMSO, acetone, DMF, acetonitrile) generally favor SN2 because they leave anionic nucleophiles poorly solvated and more reactive.

An electron configuration lists the subshells an atom's electrons occupy and the number in each, such as 1s² 2s² 2p⁴ for oxygen. Electrons fill by Aufbau order, Pauli exclusion and Hund's rule; transition-metal cations lose ns electrons...

Specific gravity is the ratio of a substance's density to the density of a reference substance, usually water (1000 kg/m³ at 4 °C). It has no units. The testable point: if it is below 1, an object floats in water; above 1, it sinks.

Social stratification is a society's ranking of people into unequal layers by wealth, income, power, prestige and education. Functionalists see it as rewarding important roles; conflict theorists see it as preserving elite advantage. Cas...

Anaphase is the stage of mitosis and meiosis II in which sister chromatids separate and are pulled to opposite poles of the cell. The key point is that in meiosis I, anaphase I separates homologous chromosomes while sister chromatids sta...

A disulfide bond is a covalent S–S linkage formed when the thiol (–SH) groups of two cysteine side chains are oxidized. It stabilizes tertiary and quaternary structure, especially in secreted and extracellular proteins, and is broken by...

Molality (m) is moles of solute per kilogram of solvent. Because it uses mass rather than volume, it does not change with temperature, so colligative property equations such as ΔTf = i Kf m use it instead of molarity.

Potential difference (voltage) is the change in electric potential energy per unit charge between two points, measured in volts (J/C). Positive charges tend to move toward lower potential and negative charges toward higher, and ΔU = qΔV.

Adoption studies compare adopted children with their biological relatives and their adoptive relatives to separate genetic from environmental influence on a trait. Greater resemblance to biological parents suggests heredity; greater rese...

Fischer esterification is the acid-catalyzed reaction of a carboxylic acid with an alcohol to form an ester and water, by nucleophilic acyl substitution. It is reversible, so excess alcohol or removal of water pushes it toward the ester...

An instinct is an innate, unlearned, species-typical behavior pattern triggered by a specific stimulus. In the instinct theory of motivation, behavior comes from inherited programming rather than learning, but the theory mostly labels be...

Oxidation state (oxidation number) is the hypothetical charge an atom would have if every bond's shared electrons went to the more electronegative atom. Oxidation is a rise in oxidation state and reduction is a fall, so it shows what is...

A state function is a property set by the system's current state, not by the path taken to reach it. P, V, T, internal energy U, enthalpy H, entropy S and Gibbs free energy G are state functions; work and heat are path-dependent, so they...

Broca's area is a region of the frontal lobe, in the left hemisphere for most people, that supports speech production and grammar. Damage causes Broca's (expressive, nonfluent) aphasia: effortful, halting speech with comprehension largel...

Leukocytes are white blood cells, nucleated cells made from stem cells in bone marrow that defend the body. The most testable point is sorting them: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monoc...

Hydrophilic means water-loving: a polar or charged molecule or region that interacts favorably with water by hydrogen bonding or ion–dipole forces. Testable point: in folded soluble proteins, hydrophilic side chains tend to face the aque...

Non-polar describes a bond or molecule with no net charge separation, because electrons are shared about equally or polar bond dipoles cancel by symmetry. Most testable: non-polar substances dissolve best in non-polar solvents (like diss...

Drive-reduction theory says a physiological need (like hunger) creates an unpleasant drive that pushes behavior aimed at reducing it and restoring homeostasis. Its testable weakness is that it does not easily explain behaviors that raise...

Transition metals are the d-block elements (groups 3–12), whose atoms or common ions typically have a partially filled d subshell. They usually show several oxidation states, form colored complex ions, and lose their ns electrons before...

The work function (φ) is the minimum energy needed to free an electron from a metal's surface. In the photoelectric effect, electrons are ejected only if photon energy hf is at least φ, so the threshold frequency is f₀ = φ/h.

Identification is changing your attitudes or behavior to resemble a person or group you admire or want to belong to. It is one of Kelman's three processes of social influence. The testable point: it tends to last only as long as the bond...

Acidity is how readily a compound donates a proton (H⁺), measured by Ka or pKa. A lower pKa means a stronger acid. The most testable point is that acidity is judged by the stability of the conjugate base: the more stable A⁻ is, the stron...

A transferase is an enzyme that moves a functional group, such as a phosphate or amino group, from a donor molecule to an acceptor molecule. Kinases and aminotransferases are examples. A testable point: a phosphatase is a hydrolase, not...

The olfactory bulb is the brain structure that receives smell signals from receptor neurons in the nasal lining and sends them on to olfactory cortex and limbic areas. Unlike most other senses, smell does not need a thalamic relay first.

Blood cells are the cellular part of blood: red blood cells (erythrocytes) carry O₂, white blood cells (leukocytes) defend against pathogens, and platelets (cell fragments) aid clotting. A top testable point: mature mammalian red cells l...

Atomic radius is the size of an atom, usually estimated as half the distance between the nuclei of two identical bonded atoms. It typically decreases across a period (higher effective nuclear charge) and increases down a group (added ele...

Magnetic force is the force a magnetic field exerts on a moving charge or a current-carrying wire. It points perpendicular to both the velocity and the field, so it does no work and changes a charge's direction but not its speed.

World-systems theory (Immanuel Wallerstein) treats the world as one capitalist economy with core, semi-periphery, and periphery nations. Core nations tend to draw cheap labor and raw materials from the periphery, which helps keep global...

A racemic mixture contains equal amounts of two enantiomers, written (±), so the optical rotations cancel and the mixture is optically inactive. It typically forms when a stereocenter is created from a planar, achiral intermediate, as in...

Wernicke's encephalopathy is an acute brain disorder caused by thiamine (vitamin B1) deficiency, most often in chronic alcohol use. It causes confusion, unsteady gait and abnormal eye movements, and often progresses to Korsakoff syndrome...

Dilution means adding solvent to a solution so the solute concentration falls while the moles of solute stay the same. The key relationship is M₁V₁ = M₂V₂, so adding solvent lowers molarity in inverse proportion to the volume increase.

Intensity of sound is the power a sound wave delivers per unit area, in W/m². It grows with the square of the wave's amplitude, and for a point source it falls as 1/r². The most testable point: a 10 dB rise in sound level means a tenfold...

Voltage-gated channels are ion-selective membrane proteins that open or close in response to changes in membrane potential, letting ions move down their electrochemical gradients. In a neuron, Na⁺ channels open at threshold and drive the...

In attribution theory (Kelley's covariation model), consensus is the extent to which other people respond the same way as the actor in the same situation. High consensus points toward a situational attribution; low consensus points towar...

Excitatory describes input that makes a postsynaptic neuron more likely to fire, usually by depolarizing it toward threshold (an EPSP). Whether it fires depends on summation of excitatory and inhibitory inputs at the axon hillock, and th...

Average kinetic energy is the mean translational energy of the particles in a sample, equal to (3/2)kBT for an ideal gas. It is proportional to absolute temperature in kelvins and does not depend on the gas's identity or mass, so at the...

Dispersion is the spreading of white light into its colors because a medium's index of refraction depends on wavelength. In most transparent materials, such as glass and water, shorter wavelengths have a larger n, so violet bends more th...

A mass spectrometer ionizes molecules, sorts the ions by mass-to-charge ratio (m/z), and records their relative abundance. The most testable point: the molecular ion peak (highest main m/z, apart from small isotope peaks) usually gives t...

Deindividuation is a loss of self-awareness and personal accountability when a person is in a group or anonymous, so behavior often departs from usual standards. The classic account stresses lost self-awareness; the social identity view...

The Q cycle is how Complex III passes electrons from ubiquinol (QH₂), a two-electron carrier, to cytochrome c, a one-electron carrier, while adding to the proton gradient. Per QH₂ oxidized, 2 cytochrome c are reduced and 4 H⁺ are release...

Hund's rule says electrons fill degenerate orbitals (such as the three 2p orbitals) one at a time with parallel spins before any orbital gets a second electron. The most testable point is that it sets the number of unpaired electrons, wh...

Lev Vygotsky proposed a sociocultural theory of cognitive development: learning is driven by social interaction and culture, and language is the main tool of thought. Most testable: the zone of proximal development, the gap between what...

Thermoregulation is the maintenance of a fairly stable core body temperature by negative feedback, with the hypothalamus as the integrating center. Most testable: heat is lost by skin vasodilation and sweating, and conserved by vasoconst...

An alkyne is a hydrocarbon with a carbon–carbon triple bond (one σ, two π), and its sp carbons give a linear geometry. Most testable: a terminal alkyne's sp C–H is far more acidic (pKa about 25) than alkene or alkane C–H, so a strong bas...

Carl Rogers was a humanistic psychologist who developed person-centered therapy, in which the therapist offers empathy, genuineness, and unconditional positive regard to support growth toward self-actualization. Most testable: a gap betw...

An allosteric site is a binding site on a protein, separate from the active site, where a regulator (effector) binds and changes the protein's shape and activity. Effectors can activate or inhibit, and cooperative allosteric enzymes typi...

Two quantities are proportional when their ratio is constant, so y = kx: doubling x doubles y, and the graph of y versus x is a straight line through the origin. Most testable: gas-law temperature must be in kelvin, because 0 °C is not a...

The Lorentz force is the total electromagnetic force on a charge q moving at velocity v through electric field E and magnetic field B. On the MCAT it usually means the magnetic part, F = qvB sinθ, which is perpendicular to v, so it bends...

Health outcomes are the measurable results of health in people or populations, such as morbidity, mortality, and life expectancy. On the MCAT they are the result that social factors like socioeconomic status and access to care predict; h...

A diol has two hydroxyl (–OH) groups. Vicinal diols (OH on adjacent carbons) form from alkenes by syn addition with OsO₄ or by anti addition via an epoxide. Geminal diols (both OH on one carbon) are usually unstable hydrates of carbonyls.