1,800+ MCAT terms, defined.

Malate is a four-carbon citric acid cycle intermediate, formed when fumarase hydrates fumarate and oxidized by malate dehydrogenase to oxaloacetate, producing NADH. Testable point: fumarate → malate is a hydration with no electron carrie...

Weber's law says the just noticeable difference (JND) between two stimuli is a constant proportion of the original stimulus intensity, not a fixed amount. The stronger the starting stimulus, the larger the change needed to notice it. It...

The limiting reagent is the reactant that is completely used up first, so it sets the maximum amount of product (the theoretical yield). To find it, divide each reactant's moles by its balanced-equation coefficient; the smallest quotient...

A transverse wave is one in which the oscillation (of the medium, or of the fields in light) is perpendicular to the direction the wave travels. Light and waves on a string are transverse; sound in air is longitudinal. Transverse waves c...

Bandura's social learning theory says people learn behaviors by observing models and the consequences the models receive, not only through their own reinforcement or punishment. Most testable: in the Bobo doll study, children imitated an...

Cell differentiation is the process by which a less specialized cell becomes a specialized cell type with its own structure and function. It results from differential gene expression, and in most cells the DNA sequence itself stays the s...

Stereoisomers have the same molecular formula and the same atom-to-atom connectivity but different three-dimensional arrangements. They are either enantiomers (non-superimposable mirror images) or diastereomers (all other stereoisomers,...

Ivan Pavlov was the Russian physiologist who discovered classical conditioning, in which a neutral stimulus repeatedly paired with an unconditioned stimulus comes to trigger a conditioned response. The most testable skill is labeling the...

The Na+/K+ ATPase is a membrane enzyme that hydrolyzes ATP to pump 3 Na+ out of the cell and 2 K+ in, each against its concentration gradient (primary active transport). Most testable: the 3:2 ratio and the Na+ gradient it builds, which...

An exothermic process releases heat from the system to the surroundings, so the system's enthalpy falls and ΔH is negative. The most testable point is that exothermic describes heat flow, not spontaneity: ΔG also depends on TΔS, so an ex...

Simple harmonic motion is oscillation about an equilibrium position where the restoring force is proportional to displacement and points back toward equilibrium (F = −kx). For an ideal spring, and a pendulum at small angles, the period d...

In Freud's psychoanalytic theory, the ego is the part of personality that follows the reality principle, mediating between the id's urges and the superego's moral standards. On the MCAT, the ego is the realistic mediator and the user of...

Feminist theory is a sociological perspective that examines how gender inequality is built into institutions and everyday interaction, often through patriarchy (male dominance). It grows out of conflict theory and treats gender as largel...

The sarcolemma is the plasma membrane of a muscle fiber. It carries the action potential across the cell surface and into the T-tubules, which leads the sarcoplasmic reticulum to release Ca²⁺ and the fiber to contract.

A spontaneous process is thermodynamically favorable: it proceeds forward from the given conditions without continuous outside work. At constant T and P, this means ΔG < 0. Spontaneous does not mean fast, because rate is set by kinetics.

Gravity is the attractive force between objects with mass, proportional to the product of the masses and inversely proportional to the square of the distance between their centers. Near Earth's surface, freely falling objects share the s...

A binding protein is a protein that reversibly and specifically binds a ligand without chemically changing it, as hemoglobin does with O₂. Affinity is measured by Kd: a lower Kd means tighter binding, and Kd equals the ligand concentrati...

Endorphins are neuropeptides the body makes that act like opioids, binding opioid receptors to reduce pain and produce feelings of pleasure or well-being. The MCAT links them to pain modulation, stress, exercise, and the reward effects o...

Mass spectrometry ionizes a sample, separates the ions by mass-to-charge ratio (m/z), and plots their relative abundance. The molecular ion peak, usually at the highest m/z apart from small isotope peaks, gives the molecular mass. The ta...

A galvanic (voltaic) cell uses a spontaneous redox reaction to produce electrical energy. It has E°cell > 0 and ΔG < 0, with oxidation at the anode, reduction at the cathode, and electrons flowing through the wire from anode to cathode.

Conservation of momentum says the total momentum (p = mv) of a system stays constant when the net external force on it is zero. In collisions and explosions, total momentum before equals total momentum after, even when kinetic energy is...

Source monitoring is the process of deciding where a memory came from: who told you, where you saw it, or whether you imagined it. Its failures, called source monitoring errors or source misattribution, are a testable reason memory is re...

A threshold is the minimum stimulus intensity needed to produce a response. In sensation, the absolute threshold is the weakest stimulus detected about 50% of the time, and the difference threshold is the smallest change noticed about 50...

Inhibitory describes a synaptic input that makes the postsynaptic cell less likely to fire an action potential, typically by hyperpolarizing it (an IPSP) through Cl⁻ influx or K⁺ efflux. Whether a transmitter is inhibitory depends on the...

An alkane is a saturated hydrocarbon: carbon and hydrogen joined by single bonds, with the formula CnH2n+2 when acyclic. It is nonpolar and held together by London dispersion forces, so boiling point rises with chain length and falls wit...

The speed of light, c, is the speed of electromagnetic waves in a vacuum, 3.00 × 10⁸ m/s, the same for every wavelength. The key use is c = fλ: frequency and wavelength are inversely related, so shorter wavelength means higher frequency...

Electromagnetic waves are transverse waves of oscillating electric and magnetic fields that need no medium and travel at c = 3.00 × 10⁸ m/s in vacuum. Every type has the same speed in vacuum, so higher frequency means shorter wavelength...

Parkinson's disease is a progressive neurodegenerative disorder in which dopamine-producing neurons of the substantia nigra die, causing resting tremor, rigidity, and slowed movement. Most testable: it involves too little dopamine, the o...

Self-serving bias is the tendency to attribute your own successes to internal, dispositional causes and your own failures to external, situational causes. It is usually explained as protecting self-esteem. It concerns explaining your own...

Mole fraction (X) is the moles of one component divided by the total moles of all components in a mixture. It is unitless, and the mole fractions in a mixture sum to 1. The most testable use is that it sets partial pressure (Dalton's law...

An insulator is a material with very few free charge carriers, so it has very high resistivity and passes almost no current. The testable point: it resists current, not the electric field, and a dielectric between capacitor plates is an...

Nonpolar describes a bond with equal or nearly equal electron sharing, or a molecule with no net dipole. A molecule can be nonpolar even with polar bonds if their dipoles cancel by symmetry (CO₂, CCl₄). Nonpolar substances dissolve in no...

An integral protein is embedded in the phospholipid bilayer by hydrophobic interactions between its nonpolar side chains and the lipid tails. Most span the membrane, though some sit in one leaflet. The testable point: it is released only...

A depressant is a psychoactive drug that reduces central nervous system activity, lowering arousal, anxiety and inhibition. Alcohol, barbiturates and benzodiazepines are the standard examples, and they act largely by enhancing GABA, the...

Cytokines are small secreted signaling proteins, made largely by immune cells, that bind receptors on target cells to change their behavior. The most testable point: they coordinate immune responses (interleukins, interferons, TNF), usua...

The mesolimbic pathway is a dopamine circuit running from the ventral tegmental area (VTA) in the midbrain to limbic targets, especially the nucleus accumbens. It is the core of the brain's reward system, and many addictive drugs and rew...

In collision theory, reactant particles must collide to react, but only collisions with energy of at least the activation energy (Ea) and a suitable orientation form products. Rate tracks the frequency of these effective collisions. High...

The joule (J) is the SI unit of energy, work, and heat: 1 J = 1 N·m = 1 kg·m²/s². One joule is the work done when a 1 N force moves an object 1 m along the force. Expect to convert it to other units and to recognize it as the unit of ene...

Size-exclusion (gel filtration) chromatography separates molecules by size on a column of porous beads. Large molecules cannot enter the pores, take a shorter path, and elute first; small molecules enter the pores and elute later. Bigges...

Language development is the sequence by which children move from cooing and babbling to words, phrases and grammatical sentences, with comprehension ahead of production. The most testable point is contrasting the learning (Skinner), nati...

A glycerophospholipid is a glycerol backbone with two fatty acid tails (ester bonds) and a phosphate linked to a polar head group, so it is amphipathic. It is the main lipid type of cell membranes and forms bilayers with the tails inward...

Ka is the acid dissociation constant: the equilibrium constant for HA + H₂O ⇌ H₃O⁺ + A⁻. A larger Ka (smaller pKa) means a stronger acid that dissociates more, so Ka and pKa move in opposite directions.

Valence is the positive or negative quality of an emotion or stimulus: how pleasant or attractive versus unpleasant or aversive it is. It is separate from arousal (intensity), so the key skill is sorting emotions by valence without confu...

A genotype is the specific combination of alleles an individual carries for a gene, such as AA, Aa, or aa. It differs from the phenotype, the observable trait. Different genotypes can produce the same phenotype, as when AA and Aa both sh...

A hydroxyl is an –OH group covalently bonded to carbon, as in alcohols and phenols. It is polar and both donates and accepts hydrogen bonds, which raises boiling point and water solubility. Testable point: –OH is a poor leaving group unl...

Concrete operational is Piaget's third stage of cognitive development, roughly ages 7 to 11, when children reason logically about concrete, real objects and events. The most testable point is conservation: they grasp that quantity stays...

The half-equivalence point is where half of a weak acid (or weak base) in a titration has been neutralized, so [HA] = [A⁻]. There pH = pKa (for a weak base, pOH = pKb), at the midpoint of the buffer region.

A sound wave is a mechanical, longitudinal wave of alternating compressions and rarefactions that needs a medium. The source fixes its frequency, so at a boundary the frequency stays constant while speed and wavelength change.

Secondary active transport moves one solute against its gradient by coupling it to another solute (often Na⁺) moving down its gradient through the same carrier. The carrier does not hydrolyze ATP, but ATP is used indirectly by primary ac...

Identity is a person's sense of who they are, made up of personal identity (individual traits, abilities, history) and social identity (membership in groups such as gender, race, religion or class). Identity describes the self; self-este...