1,800+ MCAT terms, defined.

Pedigree analysis uses a family tree to work out how a trait is inherited (autosomal or X-linked, dominant or recessive) and to infer family members' genotypes. Most testable: two unaffected parents with an affected child usually means t...

Goal displacement is when an organization, often a bureaucracy, treats its rules and procedures as ends in themselves, so its original purpose is neglected. The testable point: rigid rule-following, or the organization's own survival, ta...

The anode's reduction potential is the tabulated standard potential (in volts) for the anode half-reaction written as a reduction, even though oxidation actually happens there. In a galvanic cell it is the lower (more negative) of the tw...

The magnification equation, m = −i/o = hᵢ/hₒ, gives the ratio of image size to object size for a mirror or lens. The sign shows orientation: for a real object, negative m is an inverted, real image and positive m is an upright, virtual i...

An electron-withdrawing group pulls electron density toward itself through sigma bonds (inductive effect) or through pi bonds (resonance). It stabilizes nearby negative charge, so it usually makes acids stronger and bases weaker, and it...

The inferior colliculus is a midbrain structure that relays auditory information from the brainstem up to the thalamus and helps orient the head toward sounds. Most testable point: it is for hearing, while the superior colliculus is for...

An exergonic reaction has a negative ΔG, so it is thermodynamically spontaneous and releases free energy available to do work. The testable point: ΔG < 0 says nothing about speed, which depends on activation energy and catalysts.

Klüver–Bucy syndrome is a cluster of behavioral changes, including loss of fear and aggression, hyperorality and hypersexuality, that follows bilateral temporal lobe damage involving the amygdala. The most testable point is that amygdala...

The mass number (A) of an atom is the number of protons plus neutrons in its nucleus. It is a whole number written as the upper left superscript of an isotope. Isotopes of one element share the atomic number but differ in mass number.

A concave (diverging) lens is thinner at its center than its edges and spreads parallel rays apart. It has a negative focal length and, for a real object, always forms a virtual, upright, reduced image on the same side as the object.

Plasma is the liquid, cell-free part of blood, about 55% of its volume. It is mostly water with dissolved proteins, ions, nutrients, hormones, wastes and gases. Its proteins, mainly albumin, create the oncotic pressure that draws fluid b...

Passive aggression is the indirect expression of hostility through seemingly cooperative or neutral behavior, such as procrastinating, sulking, or deliberate inefficiency, instead of open confrontation. It is aimed at the real source of...

An aryl group is a substituent made by removing one hydrogen from an aromatic ring, so it attaches through a ring carbon; phenyl, C₆H₅−, is the simplest, and Ar is the general abbreviation. Testable point: aryl halides generally resist S...

Social psychology is the study of how an individual's thoughts, feelings, and behavior are influenced by other people and the social situation. The most testable point is that situational factors, such as group size or authority, often s...

A flavoprotein is a protein with a tightly bound flavin cofactor, FAD or FMN (made from riboflavin, vitamin B2), that takes part in redox reactions. Most testable: FAD is reduced to FADH₂, as in succinate dehydrogenase, and FADH₂ yields...

Avogadro's constant, NA, is the number of particles in one mole of a substance, about 6.022 × 10²³ per mole. It converts between moles and a count of atoms, molecules or ions. The testable point: count the right kind of particle, since o...

A converging (convex) lens is thicker at the center and, in a lower-index medium such as air, bends parallel rays to a focal point; f is positive. It forms a real, inverted image when the object is beyond f, but a virtual, upright, enlar...

Informal norms are unwritten, shared expectations for behavior, learned through socialization and usually enforced by peers through stares, gossip, or exclusion. The key contrast is with formal norms, which are written down and enforced...

Constitutional (structural) isomers have the same molecular formula but different atom-to-atom connectivity, so they are different compounds with different properties. The key test: if the bonding order differs, they are constitutional;...

Synaptic transmission is the passing of a signal from a presynaptic neuron to a postsynaptic cell. At a chemical synapse, Ca²⁺ entry triggers neurotransmitter release, which binds receptors and causes an EPSP or IPSP. Electrical synapses...

Situational means explained by a person's external circumstances rather than by traits. In attribution theory, a situational (external) attribution credits behavior to the environment or context. Testable point: observers often overlook...

The cell potential (Ecell) is the voltage between an electrochemical cell's two electrodes, the driving force on electrons. It equals the cathode's reduction potential minus the anode's. A positive E°cell means the reaction is spontaneou...

NADP⁺ is the oxidized form of nicotinamide adenine dinucleotide phosphate, an electron carrier that accepts a hydride to become NADPH. NADPH is the reducing agent for biosynthesis, and it is made mainly by the pentose phosphate pathway (...

Prevalence is the proportion of a population that has a condition at a given time or over a period, counting both new and existing cases. The key test point is that it differs from incidence, which counts only new cases, so recovery or d...

In organic chemistry, recombination is the joining of two free radicals into a stable molecule with a new covalent bond. It is a termination step of a radical chain reaction, since two radicals are consumed and none are produced. In biol...

EMF (electromotive force), or Ecell, is the cell potential: the voltage an electrochemical cell can drive, found as E°cathode − E°anode from reduction potentials. A positive Ecell means a spontaneous (galvanic) reaction, because ΔG = −nF...

Young's double-slit experiment sends coherent light through two narrow slits. The overlapping waves form alternating bright and dark fringes on a screen, which is evidence that light is a wave. Bright fringes occur where the path differe...

The accessory olfactory bulb receives input from the vomeronasal organ and processes pheromones in many mammals, sending signals to the amygdala and hypothalamus. In humans it is generally considered vestigial, though some researchers de...

Tumour suppressors are genes whose proteins restrain the cell cycle, aid DNA repair, or trigger apoptosis; p53 and Rb are the classic examples. Their loss of function usually requires both alleles to be inactivated, unlike an oncogene, w...

Activity theory is a sociological theory of aging holding that older adults who stay active and keep their social roles, or replace lost ones, tend to have greater life satisfaction. It is the usual contrast to disengagement theory, whic...

A symport (cotransport) is a membrane transporter that moves two different substances across the membrane in the same direction at the same time. It is usually secondary active transport: the downhill movement of one solute, often Na⁺, d...

A carbonyl compound with a C=C between the α and β carbons, so C=C–C=O is conjugated. Resonance makes the β carbon electrophilic, so nucleophiles can add at the carbonyl carbon (1,2) or the β carbon (1,4, conjugate or Michael addition)....

Henry's law says that at constant temperature, the concentration of a gas dissolved in a liquid is directly proportional to that gas's partial pressure above the liquid. Doubling the partial pressure doubles the dissolved amount. It fits...

Ferromagnetic materials (iron, cobalt, nickel) contain domains of aligned atomic magnetic moments, so they are strongly attracted to magnets and can stay magnetized after the field is removed. Paramagnetic materials are only weakly attra...

Cognitive flexibility is the ability to shift between tasks, mental sets, or strategies and to adapt thinking when rules or circumstances change. It is an executive function linked to the prefrontal cortex. The exam often contrasts it wi...