1,800+ MCAT terms, defined.

Prosody is the rhythm, stress, pitch, and loudness of speech, which carry emotion and intent beyond the words themselves. Testable point: in most people prosody is processed largely by the right hemisphere, while core language areas (Bro...

Sociological theories are frameworks for explaining social patterns. Functionalism sees interdependent parts keeping society stable, conflict theory sees inequality and competition for resources, and symbolic interactionism sees meaning...

Heterozygous describes a genotype with two different alleles of a gene, one on each homologous chromosome (for example Aa). The most testable point: with complete dominance the dominant allele sets the phenotype, but incomplete dominance...

Autoionization of water is the reaction in which two water molecules transfer a proton to form H₃O⁺ and OH⁻: 2H₂O ⇌ H₃O⁺ + OH⁻. At 25 °C, Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, so pure water has pH 7 and the product stays fixed in any aqueous sol...

An electric dipole is a pair of equal and opposite charges, +q and −q, separated by a small distance d. Its dipole moment is p = qd, pointing from − to +. In a uniform field it has zero net force but feels a torque that turns it toward a...

Quaternary structure is the way two or more separate folded polypeptide chains (subunits) assemble into one functional protein. It is held together mainly by noncovalent forces, sometimes with disulfide bonds. A protein made of a single...

The arcuate nucleus is a hypothalamic cluster of neurons that senses hunger and satiety hormones in the blood. Ghrelin activates its appetite-stimulating neurons, while leptin and insulin inhibit those neurons and activate its appetite-s...

Ethers are organic compounds with an oxygen bonded to two carbon groups, R–O–R′. They cannot hydrogen-bond to each other, so they boil far lower than alcohols of similar mass, and open-chain ethers are fairly unreactive, which makes them...

The index of refraction, n = c/v, compares the speed of light in vacuum with its speed in a material, so n ≥ 1. Entering a higher-n medium, light slows and bends toward the normal (Snell's law: n₁ sinθ₁ = n₂ sinθ₂), while its frequency s...

Conditioning is learning in which behavior changes through association or consequences. Classical conditioning links two stimuli; operant conditioning links a behavior to reinforcement or punishment. Most testable: reinforcement, includi...

Standard deviation (SD) measures how spread out scores are around the mean, in the same units as the scores. A larger SD means more variability. In a roughly normal distribution, about 68% of scores fall within 1 SD of the mean and about...

Membrane transport is the movement of substances across a membrane, passively down a gradient (diffusion, facilitated diffusion, osmosis) or actively against it with energy. Most testable: active transport moves solute against its gradie...

Redox carriers are small coenzymes, mainly NAD⁺/NADH, FAD/FADH₂ and NADP⁺/NADPH, that shuttle electrons between reactions. NADH and FADH₂ carry electrons from fuel oxidation to the electron transport chain, while NADPH mostly supplies el...

In organic chemistry, R is a generic label for a carbon-containing group (usually alkyl) attached to a functional group, so R–OH means any alcohol. The testable point is that R's size and electron-donating character change reactivity. In...

A metal cation is a positively charged ion formed when a metal atom loses valence electrons, as in Na⁺, Ca²⁺ or Fe³⁺. It is smaller than its parent atom, and a higher positive charge on the same element gives a smaller ion.

The law of reflection says the angle of incidence equals the angle of reflection, both measured from the normal (the perpendicular to the surface), and the incident ray, reflected ray and normal lie in one plane. Measuring from the surfa...

A positive control is a group or condition given a treatment already known to produce the expected effect. It shows the study's procedure and measure can detect an effect. If it fails to show the effect, a null result for the new treatme...

A redox titration finds an unknown concentration by adding a standard oxidizing or reducing agent until the moles of electrons lost equal the moles gained (the equivalence point). The most testable point is to use the balanced equation's...

Lens power is the reciprocal of a lens's focal length, P = 1/f, measured in diopters (1 D = 1 m⁻¹). A converging lens has positive power and a diverging lens has negative power. A shorter focal length means a larger magnitude of power.

Free rotation is the twisting of one part of a molecule relative to another about a single (σ) bond, which interconverts conformations at room temperature. Double bonds, rings and partial double bonds (such as the amide C–N bond) restric...

Retroviruses (such as HIV) move genetic information RNA → DNA → RNA → protein: reverse transcriptase copies the RNA genome into DNA, which integrates into the host genome and is expressed by host machinery. The testable step is RNA → DNA.

Semantic memory is the part of explicit (declarative) memory that stores general knowledge, such as facts, concepts and word meanings, usually without recalling when or where they were learned. The key contrast is with episodic memory, w...

The pylorus is the narrow distal end of the stomach, ending in the pyloric sphincter, a smooth-muscle ring that meters chyme into the duodenum. Most testable: it is the stomach's exit, and emptying through it slows when fat or acid reach...

Anomia is difficulty retrieving the word for a known object, person, or place, while comprehension, fluency, and repetition are fairly well preserved. Its pure form is anomic aphasia, often linked to left temporal or parietal damage. It...

Standard enthalpy of formation (ΔH°f) is the enthalpy change when one mole of a compound forms from its elements in their standard states. It is zero for an element in its standard state. The MCAT uses it to find ΔH°rxn as products minus...

Relative densities compare one material's density with another's, often as specific gravity (ρ/ρwater, unitless). The key test point: an object denser than the surrounding fluid sinks, a less dense one floats, and a floating object's sub...

Planar means the atoms of a group lie in one flat plane. In organic chemistry this is the geometry around sp² centers (trigonal planar, bond angles near 120°), such as alkene and carbonyl carbons, carbocations, benzene and amide bonds. M...

An interactionist (symbolic interactionism) view holds that society and the self are built through everyday interactions and the shared meanings of symbols. It is a micro-level theory, unlike the macro-level functionalist and conflict th...

In semiconservative replication, each daughter DNA double helix contains one original (parental) strand and one newly synthesized strand. The Meselson–Stahl ¹⁵N/¹⁴N density experiment supported it over the conservative and dispersive mod...

Total pressure is the pressure a gas mixture exerts on its container, equal to the sum of the partial pressures of its components (Dalton's law, exact for ideal gases). Each partial pressure is its mole fraction times the total pressure,...

Nociceptors are sensory receptors, usually free nerve endings, that respond to potentially damaging stimuli such as extreme heat, strong pressure or chemicals from injured tissue. Fast myelinated A-delta fibers carry sharp pain; slower u...

Thrombocytes are platelets: anucleate cell fragments budded from bone marrow megakaryocytes. They stick to damaged vessel walls and aggregate into a plug, and they support the clotting cascade that makes fibrin. Impaired platelets or clo...

Torsional strain is the extra energy a molecule has when bonds on adjacent atoms are eclipsed instead of staggered, because the bonding electron pairs repel. Testable point: around a C–C single bond, staggered conformations are lowest in...

Reciprocal determinism is Albert Bandura's model in which personal factors (thoughts, beliefs, self-efficacy), behavior, and environment influence one another in both directions. The testable point is that causation runs in loops, so a p...

Standard free energy ΔG° is the Gibbs free energy change for a reaction when all species are in their standard states (1 M solutes, 1 atm gases, usually 298 K). It sets the equilibrium constant: ΔG° = −RT ln Keq, so a negative ΔG° means...

Angular displacement (Δθ) is the net angle through which an object rotates about an axis, usually measured in radians. Counterclockwise is conventionally positive. The most testable link is s = rΔθ, which holds only when Δθ is in radians.

Prokaryotic transcription is the making of RNA from a DNA template by a single RNA polymerase, which a sigma factor directs to the promoter. Testable point: bacterial mRNA is not processed, is often polycistronic, and is translated while...

A cross-sectional study is an observational design that measures variables in a sample at one point in time, often comparing groups such as different age groups. It is quick and inexpensive, but it generally cannot show causation or time...

The sensory register is the first stage of memory in the Atkinson–Shiffrin model. It briefly holds raw sensory input, as iconic (visual) memory for a fraction of a second and echoic (auditory) memory for a few seconds. Input that is atte...

The cortical reaction is the egg's release of cortical granule contents by exocytosis after a sperm fuses with it. The enzymes modify the zona pellucida (vitelline layer in sea urchins) so no more sperm can bind. It is the slow block to...

A Brønsted-Lowry acid is a proton (H⁺) donor, and a Brønsted-Lowry base is a proton acceptor. The most testable point in organic chemistry is that a stronger acid has a more stable conjugate base, so a lower pKa means a stronger acid.

Alkaline earth metals are the Group 2 elements (Be, Mg, Ca, Sr, Ba, Ra). They have two valence electrons (ns²) and typically form M²⁺ cations. Down the group, atomic radius and reactivity increase while ionization energy decreases.

Chemical potential energy is energy stored in the arrangement of electrons and nuclei in a substance's bonds. It is released, often as heat, when a reaction forms products with lower potential energy. Breaking bonds takes in energy; form...

Autonomy is the capacity to govern oneself and make one's own choices. The MCAT most often uses it in Erikson's second stage, autonomy versus shame and doubt (toddler years), and in self-determination theory, where autonomy is a basic ne...

Zymogen activation is the conversion of an inactive enzyme precursor (a zymogen or proenzyme) into an active enzyme, typically by cutting specific peptide bonds. The cleavage is covalent and irreversible. The classic example is trypsinog...

In Piaget's theory, the concrete operational stage (roughly ages 7 to 11) is when children begin to reason logically about real, tangible objects and events. The most testable point is conservation: a child understands that quantity stay...

A basic buffer is a weak base plus its conjugate acid (e.g., NH₃/NH₄⁺) that resists pH change by neutralizing added acid or added base. Testable point: pOH = pKb + log([conjugate acid]/[base]), so pOH = pKb when the two are equal.

Circularly polarized light is a transverse wave whose electric field keeps constant magnitude and rotates around the direction of travel, made of two perpendicular components of equal amplitude 90° out of phase. Plane-polarized light is...

Convergence is a binocular depth cue: the eyes rotate inward to fixate a near object, and the brain uses feedback from the eye muscles to judge distance. The more the eyes turn inward, the closer the object seems. It needs both eyes.

Hypermutation (somatic hypermutation) is a very high rate of point mutations in the variable-region genes of antibodies in activated B cells. Selection of the B cells whose mutated antibodies bind antigen more tightly produces affinity m...