1,800+ MCAT terms, defined.

A virtual image is a point from which light rays only appear to diverge; no light actually passes through it, so it cannot be projected on a screen. Testable point: a virtual image has a negative image distance (i < 0) in the standard si...

Mania is a distinct period of abnormally elevated or irritable mood with increased energy and activity. A manic episode marks bipolar I disorder, whereas the milder hypomania, paired with a major depressive episode, marks bipolar II.

An NMR spectrum plots radio-frequency absorption against chemical shift (δ, ppm) for nuclei such as ¹H or ¹³C in a magnetic field. Each set of equivalent nuclei gives one signal, and its position, area and splitting reveal structure; n e...

A Haworth projection is a flat drawing of a cyclic sugar, with the ring viewed edge-on and each substituent pointing up or down. For D-sugars, groups on the right in the Fischer projection point down, and the α anomer has its anomeric OH...

The orbitofrontal cortex is the lowest part of the prefrontal cortex, just above the eye sockets. It helps weigh rewards and consequences, guide decisions, and restrain impulses. Damage often produces impulsive, socially inappropriate be...

G1 (Gap 1) is the first phase of interphase, after mitosis and before S phase. The cell grows and makes proteins and organelles. The testable point is the G1 checkpoint (restriction point): p53 and Rb normally block entry into S phase if...

Light refraction is the bending of a light ray as it crosses a boundary between two media, caused by a change in the light's speed. Entering a medium with a higher index of refraction, the ray bends toward the normal; entering a lower on...

Role exit is the process of disengaging from a role that was central to your identity, such as spouse, employee, or patient, and building a new identity as an "ex." The testable point is Ebaugh's four stages: first doubts, seeking altern...

In noncompetitive inhibition, an inhibitor binds an allosteric site (not the active site) and lowers the amount of working enzyme. In the pure case Vmax decreases, Km is unchanged, and adding more substrate cannot reverse it.

Rote rehearsal (maintenance rehearsal) is repeating information over and over without working with its meaning, which keeps it active in short-term memory. The testable point: it is shallow processing and usually gives weaker long-term m...

ΔH is the enthalpy change, the heat absorbed or released by a system at constant pressure. ΔH < 0 is exothermic (heat released) and ΔH > 0 is endothermic (heat absorbed). It can be estimated from bonds broken minus bonds formed.

Convection is heat transfer by the bulk motion of a fluid (liquid or gas): warmer fluid, usually less dense, rises while cooler, denser fluid sinks, forming a current. It needs a fluid that can flow, so it cannot occur through a solid or...

Kinesins are motor proteins that use ATP hydrolysis to walk along microtubules, carrying vesicles and organelles. Most kinesins move toward the plus end, usually toward the cell periphery (anterograde transport in axons), while dynein mo...

Self-determination theory (Deci and Ryan) says motivation and well-being depend on three innate psychological needs: autonomy, competence, and relatedness. Settings that support them tend to foster intrinsic motivation, while controlling...

Creatine phosphate is a high-energy phosphate store in muscle cells that rapidly regenerates ATP by giving its phosphate group to ADP. It is the fastest ATP source and needs no oxygen, but it runs out within seconds, so glycolysis and ae...

H₂SO₄ is sulfuric acid, a diprotic strong acid. Its first proton dissociates essentially completely, but the second, from HSO₄⁻, only partly (Ka2 near 10⁻²). Each mole can still neutralize 2 mol of OH⁻ in a titration.

A dielectric is an insulating material placed between capacitor plates. Its molecules polarize and reduce the electric field inside, so capacitance is multiplied by the dielectric constant κ (κ ≥ 1). Whether Q or V changes depends on whe...

Nuclear spin is an intrinsic angular momentum of some nuclei, such as ¹H and ¹³C, that makes them act like tiny magnets. In a magnetic field its spin states differ in energy, and NMR detects the radio-frequency energy absorbed when nucle...

Tolerance is a reduced response to a drug after repeated use, so a larger dose is needed for the original effect. Along with withdrawal, it is a marker of physiological dependence. Part of it can be conditioned to environmental cues.

A DNA library is a collection of DNA fragments cloned into vectors and held in host cells. A genomic library covers the whole genome, including introns; a cDNA library is made from mRNA, so it has only expressed sequences and no introns....

Assortative mating is non-random partner choice in which people pair with others who resemble them, such as in education, age, ethnicity, religion, or values. The testable point: similarity tends to predict attraction and partnering, not...

The thermodynamic product is the most stable product, the one with the lowest Gibbs free energy. It dominates when a reaction is reversible and reaches equilibrium, usually at higher temperature or longer time. The faster-forming kinetic...

Inertia is an object's tendency to resist changes in its velocity, and mass is its measure. By Newton's first law, an object at rest stays at rest and a moving one keeps constant velocity unless a net external force acts. For the same ne...

Cued recall is retrieving a stored item when given a hint, such as a category, first letter, or one word of a studied pair, instead of producing it with no help. It usually beats free recall but is typically harder than recognition, and...

Mechanical digestion is the physical breakdown of food into smaller pieces without breaking covalent bonds. It includes chewing, stomach churning and segmentation in the small intestine. The testable point is that it increases surface ar...

The retro-aldol reaction is the reverse of aldol addition: a β-hydroxy aldehyde or ketone splits at the bond between its α and β carbons into two carbonyl compounds, with an enolate as the departing group. Glycolysis uses it when aldolas...

General intelligence (g) is Spearman's idea that one broad underlying mental ability contributes to performance on many different cognitive tasks, because scores on varied tests tend to correlate positively. The key contrast is with Gard...

Sialic acid is a family of nine-carbon acidic sugars, usually found at the ends of glycan chains on cell-surface glycoproteins and glycolipids. Its carboxylate is negatively charged at physiological pH, and viral neuraminidase cleaves it.

Vaporization is the change from liquid to gas. It is endothermic because energy is absorbed to overcome intermolecular attractions. On a heating curve the temperature stays constant during the change, and the heat absorbed equals n × ΔHvap.

Newton's third law says that if object A exerts a force on object B, then B exerts a force on A that is equal in magnitude and opposite in direction. The two forces act on different objects, so they do not cancel when you find the net fo...

Strong linguistic determinism is the strong form of the Sapir-Whorf hypothesis: language determines thought, so people cannot think about ideas their language has no words for. The MCAT contrasts it with the weaker linguistic relativity...

Guilt is a negative, self-conscious emotion felt when you judge that your behavior broke your own moral standards or harmed someone. Testable point: guilt usually targets a specific act, while shame targets the whole self. Erikson's stag...

Allele frequency is the fraction of all copies of a gene in a population that are a given allele. The MCAT usually finds it with Hardy-Weinberg (p + q = 1), starting from the recessive phenotype: q = √q². This holds only if the populatio...

An electrode is a conductor, usually a metal or graphite, where a redox half-reaction occurs in an electrochemical cell. The anode is always where oxidation occurs and the cathode is always where reduction occurs, in both galvanic and el...

An ohmmeter is a meter that measures the resistance of a component, in ohms. It uses its own internal battery to drive a current through the component, so the component must be isolated from the circuit's power source and any parallel br...

Succinyl CoA synthetase is the citric acid cycle enzyme that converts succinyl CoA to succinate and uses the released thioester energy to make GTP (or ATP, depending on the isoform) from GDP (or ADP) and Pi. It is the cycle's substrate-l...

Mitral cells are the main output neurons of the olfactory bulb. They receive input from olfactory receptor neurons in glomeruli and send axons along the olfactory tract to olfactory cortex and limbic areas, without the usual thalamic rel...

Beer's law says a solution's absorbance of light is directly proportional to the concentration of the absorbing species and to the path length. The most testable point is that doubling concentration doubles absorbance (A = εlc), a linear...

Molar mass is the mass of one mole of a substance, usually in g/mol. It equals the sum of the atomic masses in the formula and is numerically the same as the formula mass in amu. It is the conversion factor between grams and moles: n = m/M.

A pressure difference (ΔP) is the pressure at one point minus the pressure at another. It is what drives fluid flow, from higher toward lower pressure. For viscous flow in a tube, flow rate is proportional to ΔP and to r⁴, so small radiu...

Sensitization is a form of nonassociative learning in which a strong, noxious or threatening stimulus makes an organism respond more strongly to later stimuli, often including different, milder ones. It is the counterpart of habituation,...

An out-group is a social group a person does not belong to or identify with, in contrast to their in-group. People tend to favor the in-group and judge the out-group more negatively, and often see out-group members as more alike than in-...

The autonomic nervous system controls involuntary effectors (smooth muscle, cardiac muscle, glands) through the sympathetic (fight or flight) and parasympathetic (rest and digest) divisions. Most organs get both, and the two usually have...

Cytosine deamination is the hydrolytic loss of cytosine's amino group, which converts it to uracil. In DNA, uracil is removed by base excision repair; if it persists through replication, it pairs with adenine and a G·C pair becomes A·T (...

A halide here is an organic compound with a halogen (F, Cl, Br, I) bonded to carbon, such as an alkyl halide R–X or an acyl halide RCOX. The most testable point is leaving-group ability: I⁻ > Br⁻ > Cl⁻ > F⁻, so R–I reacts fastest in subs...

A molecular orbital is a wavefunction for an electron spread over a whole molecule, formed by combining atomic orbitals. Bonding MOs lie lower in energy and antibonding MOs higher, so bond order = ½(bonding − antibonding electrons) predi...

Image distance (i) is the distance from a mirror or lens to its image, measured along the principal axis. Its sign is the testable point: positive i means a real image, negative i means a virtual image, found from 1/f = 1/o + 1/i.

Agents of socialization are the groups and institutions that teach people the norms, values, and roles of their society. The main ones are family, peers, school, mass media, and religion. The MCAT often asks you to match a scenario to th...

Moral reasoning is the thinking a person uses to decide and justify what is right or wrong. In Kohlberg's theory it develops through three levels: preconventional, conventional and postconventional. The exam scores the reasoning given, n...

A lattice is the regular, repeating three-dimensional arrangement of particles in a crystalline solid, described by its unit cell. Most testable: shared corner and face particles give 1, 2 and 4 particles per cell for simple, body-center...