1,800+ MCAT terms, defined.

The electromagnetic spectrum is the full range of electromagnetic waves, ordered by frequency and wavelength: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. All travel at c in vacuum, so higher frequency means shorter wa...

An uncompetitive inhibitor binds only to the enzyme–substrate complex, not to free enzyme, so it cannot be outcompeted by more substrate. It lowers both Vmax and apparent Km by the same factor, which gives parallel lines on a Lineweaver–...

Distress is the negative form of stress: the response to a demand that a person appraises as threatening or beyond their coping resources. It contrasts with eustress, the positive, motivating kind. The testable point is that appraisal, n...

The immune system is the network of cells, tissues, and molecules that detects and destroys pathogens and abnormal cells. It has a fast, nonspecific innate arm and a slower, specific adaptive arm; adaptive memory cells make a second expo...

Conductors are materials, such as metals, whose charges move freely, so they carry current with low resistance. In electrostatic equilibrium, the electric field inside a conductor is zero, excess charge sits on its surface, and the whole...

An electrophile is an electron-pair acceptor: a species with a positive or partial positive charge, or an empty orbital, that bonds to a nucleophile's electron pair. On the MCAT, carbonyl carbons, carbocations and carbons bearing leaving...

Korsakoff's syndrome is a memory disorder caused by thiamine (vitamin B1) deficiency, most often seen with chronic alcohol use. It produces severe anterograde amnesia, often with some retrograde amnesia and confabulation. Implicit memory...

ATP hydrolysis is the reaction ATP + H₂O → ADP + Pi, which is strongly exergonic (ΔG°′ about −30 kJ/mol). Cells couple it to endergonic processes, and the energy release comes from the products being more stable, not from the bond breaki...

Priming is when exposure to one stimulus (the prime) changes how a later stimulus is processed, usually without awareness. It is a form of implicit memory, explained by spreading activation in a semantic network. Most testable: a related...

A rate law is an experimentally determined equation relating a reaction's rate to reactant concentrations raised to powers (orders): rate = k[A]^m[B]^n. The orders come from experiment, not from the balanced equation, except for a single...

Newton's first law (the law of inertia) says an object keeps its velocity, whether at rest or moving at constant speed in a straight line, unless a nonzero net external force acts on it. Most testable: constant velocity, including zero,...

Dishabituation is the recovery of a response that had declined through habituation, usually after a new or different stimulus is presented. In infant studies, renewed looking time after a change suggests the baby can tell the new stimulu...

Somatic describes the body's non-reproductive cells, as opposed to germ-line cells that give rise to gametes. Most somatic cells are diploid (2n) and divide by mitosis, so a somatic mutation is not passed to offspring through gametes.

The Cahn-Ingold-Prelog rules rank the four groups on a stereocenter by atomic number, working outward from the center until a difference appears. With the lowest priority group pointing away, 1 → 2 → 3 clockwise is R and counterclockwise...

A coenzyme is a small organic, nonprotein molecule, often derived from a vitamin, that an enzyme needs for activity and that carries electrons or chemical groups between reactions (NAD⁺, FAD, coenzyme A). It is one type of cofactor; meta...

A photon is a quantum (packet) of electromagnetic radiation with energy E = hf = hc/λ. Energy rises with frequency and falls with wavelength, and it does not depend on the light's intensity, which only sets how many photons arrive.

The right hand rule gives the direction of the magnetic force on a moving positive charge (fingers along v, curl toward B, thumb along F) and of the field around a current-carrying wire. For a negative charge, reverse the force direction.

Social class is a category of people who hold a similar position in a society's hierarchy, usually defined by income, wealth, education, and occupation. Marx defined it by relation to the means of production; Weber also added prestige an...

Isomers are different compounds with the same molecular formula but different arrangements of atoms. The MCAT sorts them into constitutional isomers, which differ in connectivity, and stereoisomers, which share connectivity but differ in...

Spatial refers to location, distance, direction, and arrangement. In psychology it covers perceiving and remembering where things are: the dorsal "where" visual stream (parietal lobe) and hippocampal spatial memory. In sociology, spatial...

The sympathetic nervous system is the autonomic division that drives fight-or-flight responses, such as faster heart rate and slower digestion. Its fibers have short preganglionic and long postganglionic neurons, and most postganglionic...

An oxidation number is the charge an atom would have if all bonding electrons went to the more electronegative atom. It tracks electron transfer in redox reactions. Oxidation is an increase in oxidation number and reduction is a decrease.

Restriction enzymes (restriction endonucleases) are bacterial enzymes that cut double-stranded DNA at specific recognition sequences, often palindromic. Many leave sticky ends, so fragments cut by the same enzyme can be joined by DNA lig...

Infrared spectroscopy identifies functional groups by the infrared frequencies a molecule absorbs as its bonds stretch and bend. Peaks are read in wavenumber (cm⁻¹); the most testable features are the strong C=O peak near 1700 cm⁻¹ and t...

Mass number (A) is the total number of protons plus neutrons in a nucleus. It is a whole number that identifies an isotope, and isotopes of one element share the same atomic number Z but have different mass numbers.

Alzheimer's disease is a progressive neurodegenerative disorder and a leading cause of dementia, marked by gradual memory loss and cognitive decline. It is linked to beta-amyloid plaques and tau tangles, with early hippocampal damage, so...

Cognition is the set of mental processes used to acquire, store, and use information, including attention, memory, language, reasoning, and problem solving. The MCAT most often tests it through Piaget's stages of cognitive development an...

Metabolism is the sum of all enzyme-catalyzed reactions in a cell or organism. Catabolism breaks molecules down and captures energy as ATP and reduced carriers; anabolism builds molecules and uses that energy. Pathways are linked by ATP...

Fermentation is the anaerobic regeneration of NAD⁺ from NADH by passing electrons to pyruvate (giving lactate) or to acetaldehyde (giving ethanol in yeast). It lets glycolysis keep making its net 2 ATP per glucose without oxygen; the fer...

Nucleophilic substitution replaces a leaving group on an electrophilic carbon with a nucleophile. It runs by SN2 (one concerted step, inversion, rate depends on both reactants) or SN1 (carbocation intermediate, racemization, rate depends...

The rate-determining step is the slowest elementary step in a multistep mechanism, and it limits how fast the overall reaction goes. The overall rate law follows the molecularity of this step, with any intermediates replaced by the react...

A neutron is an electrically neutral particle in the nucleus with a mass of about 1 u, slightly greater than a proton's. It sets the isotope without changing the element or charge. Testable point: N = A − Z, and β⁻ decay turns a neutron...

Sensory adaptation is the decrease in sensitivity of sensory receptors (and the neural response) to a constant, unchanging stimulus over time. It lets you notice changes rather than steady background. Responses to a stimulus that changes...

A belief is the acceptance that a claim is true, whether or not the evidence is solid. Beliefs form the cognitive component of an attitude. The testable point is that beliefs can persist after their supporting evidence is discredited (be...

A non-spontaneous process has ΔG > 0 under the given conditions, so it will not proceed on its own and needs an energy input or coupling to a favorable process. Its reverse is spontaneous, and ΔG says nothing about how fast either direct...

Equivalent resistance is the single resistance that could replace a network of resistors and draw the same current from the same source. Series resistances add; for parallel, reciprocals add, so adding a parallel branch lowers the total.

sp² describes an atom whose one s and two p orbitals mix into three equivalent hybrids, trigonal planar with angles near 120°. The leftover p orbital forms a π bond (or is empty in a carbocation). Testable: three electron groups (σ bonds...

NH₃ is ammonia, the nitrogen waste released when amino acids are broken down. At physiological pH it exists mostly as ammonium, NH₄⁺. It is toxic, so the liver converts it to urea in the urea cycle for excretion.

Conflict theory is a macro-level sociological view that society is shaped by competition among groups with unequal power for scarce resources like wealth and status. Institutions tend to protect dominant groups, and tension drives change...

A sperm is the mature male gamete: a haploid (n) cell with a head (nucleus and acrosome), a mitochondria-rich midpiece and a flagellum. It is made by spermatogenesis in the seminiferous tubules, where one primary spermatocyte (2n) yields...

In Freud's psychodynamic theory, defense mechanisms are unconscious strategies the ego uses to reduce anxiety from conflict among the id, superego and reality. The MCAT usually gives a short vignette and asks you to name the mechanism, s...

A cation is a positively charged ion, formed when an atom loses electrons. It is smaller than its parent atom, and in both galvanic and electrolytic cells cations migrate toward the cathode.

Faraday's law of induction says a changing magnetic flux through a loop of wire induces an emf, proportional to the rate of change of flux and to the number of turns. The key point is that the flux must change; a steady field gives no em...

Tautomerization is the rapid interconversion of two constitutional isomers that differ in the position of a hydrogen and a double bond, most often keto ⇌ enol. It is catalyzed by acid or base. For simple aldehydes and ketones the keto fo...

A reflex is an automatic, involuntary response to a specific stimulus, often routed through a simple spinal circuit without input from the brain. The most testable point: infant reflexes (rooting, sucking, grasp, Moro, Babinski) are pres...

The lac operon is a set of E. coli genes (lacZ, lacY, lacA) transcribed from one promoter to use lactose. The repressor blocks it until allolactose binds the repressor. Maximal expression also needs low glucose, so that cAMP–CAP can acti...

Keq is the equilibrium constant: the ratio of product to reactant concentrations (each raised to its coefficient) once forward and reverse rates are equal. It depends only on temperature. Keq > 1 favors products; Keq < 1 favors reactants.

Kirchhoff's rules are two conservation laws for analyzing circuits. The junction rule says current into a junction equals current out (charge is conserved). The loop rule says potential changes around any closed loop sum to zero (energy...

Conscious means aware of yourself and your surroundings, including current thoughts, sensations and perceptions. The MCAT treats consciousness as a range of states (alert, drowsy, asleep, hypnotized, drug-altered). In Freud's model, the...

Gas exchange is the passive diffusion of O₂ into the blood and CO₂ out of it across a thin, moist respiratory surface such as alveoli, gills or tracheae. Each gas moves down its own partial pressure gradient, and the rate rises with surf...