1,800+ MCAT terms, defined.

The tricuspid valve is the three-leaflet atrioventricular valve between the right atrium and right ventricle. It opens passively when atrial pressure exceeds ventricular pressure and closes at the start of ventricular systole, preventing...

Charged particles (electrons, protons, ions) carry net charge q, in multiples of e = 1.60 × 10⁻¹⁹ C, and feel forces in electric and magnetic fields. Most testable: the electric force qE can do work, while the magnetic force qvB sinθ is...

An electrolyte is a substance that releases mobile ions in water, so the solution conducts electricity. Strong electrolytes dissociate essentially completely, weak ones only partly, and nonelectrolytes like glucose do not form ions. More...

Change blindness is failing to notice a large change in a visual scene when it happens during a brief interruption, such as a blink, cut, or flicker. It shows that perception relies on selective attention, so people often miss changes in...

A hydrolase is an enzyme that cleaves a bond by adding water across it; proteases, lipases, nucleases, phosphatases and amylase are examples. Testable point: hydrolases use water, while lyases cleave without water or oxidation and ligase...

A coercive organization secures compliance mainly through force or the threat of it, and its members usually did not choose to join. Prisons are the standard example. The testable point is the contrast with utilitarian organizations (pay...

Electric potential difference (voltage), ΔV, is the change in electric potential energy per unit charge between two points, measured in volts (J/C). Released in an electric field, a positive charge tends to move toward lower potential an...

The gas constant R is the proportionality constant in the ideal gas law, with units of energy per mole per kelvin. Its value is 8.314 J/(mol·K), or 0.0821 L·atm/(mol·K); the number depends only on the units, so choose the version that ma...

Cognitive distortions are habitual, automatic, exaggerated thought patterns, such as catastrophizing or all-or-nothing thinking, that bias how a person reads events and can maintain depression and anxiety. The MCAT tests naming the disto...

Oogenesis is the formation of female gametes by meiosis in the ovary. Primary oocytes arrest in prophase I, and the ovulated secondary oocyte arrests in metaphase II. Meiosis II is completed only if a sperm penetrates it.

Silica gel is porous silicon dioxide (SiO₂) with polar Si–OH groups on its surface. It is the usual polar stationary phase in TLC and column chromatography, so polar compounds stick more strongly and move slowly (low Rf), while nonpolar...

Ecological validity is how well a study's setting, tasks, and procedures resemble real-life conditions, so its findings can be expected to apply outside the research setting. Tightly controlled lab studies often gain internal validity bu...

An organic molecule is a carbon-based compound, typically containing C–H bonds, with carbon bonded to H, O, N, S, or P. Biological macromolecules (carbohydrates, lipids, proteins, nucleic acids) are organic, and their behavior depends la...

Ultrasound is sound with frequency above about 20,000 Hz, beyond human hearing. In imaging, pulses reflect off tissue boundaries and the echo time gives depth (d = vt/2). Higher frequency gives finer resolution but less penetration.

Hybridisation is the mixing of an atom's valence s and p orbitals into equivalent hybrid orbitals that form sigma bonds and hold lone pairs. Count sigma bonds plus lone pairs on the atom: 2 is sp, 3 is sp², 4 is sp³. Pi bonds are not cou...

Evolutionary game theory models how behavioral strategies spread in a population when the fitness payoff of a strategy depends on what others are doing. A strategy that no rare alternative can beat once it is common is an evolutionarily...

Bioenergetics is the study of how cells transform and transfer energy, using thermodynamics and free energy (ΔG). Most testable: cells couple exergonic reactions, such as ATP hydrolysis, to endergonic ones so the overall ΔG is negative.

A flashbulb memory is a vivid, detailed memory of the circumstances in which you learned of a surprising, emotionally significant event. Emotional arousal (amygdala activity) strengthens encoding, but these memories are often less accura...

Tendons are cords of dense, collagen-rich connective tissue that attach skeletal muscle to bone and transmit the force of contraction to the skeleton. The testable point: tendons connect muscle to bone, whereas ligaments connect bone to...

A voltmeter measures the potential difference between two points in a circuit and is connected in parallel with the element being measured. An ideal voltmeter has infinite resistance so it draws no current; real ones have very large resi...

Gas molecules are the particles described by kinetic molecular theory: far apart, in constant random motion, with negligible volume and no intermolecular forces in an ideal gas. Average kinetic energy depends only on temperature, so at t...

A side chain (R group) is the variable group on an amino acid's α carbon. It sets the amino acid's size, polarity, and charge. Testable point: side chains, not the repeating backbone, drive tertiary structure, with hydrophobic ones usual...

The frustration-aggression hypothesis says that blocking a person's goal produces frustration, which raises the urge to aggress. The original 1939 version claimed a strict link; Berkowitz's revision says frustration only makes aggression...

HPLC separates a mixture by pumping a liquid mobile phase at high pressure through a column of fine stationary-phase particles; compounds attracted more to the stationary phase elute later. Testable point: in normal phase the least polar...

Cohesion is the attraction between molecules of the same substance, such as hydrogen bonding among water molecules. It produces surface tension. The testable point is cohesion versus adhesion: when adhesion to the container wall exceeds...

Pretend play is make-believe in which a child lets an object, action, or role stand for something else, such as a banana for a phone. In Piaget's theory it shows symbolic thought, a hallmark of the preoperational stage (about ages 2 to 7).

Capitalism is an economic system based on private ownership of the means of production, market competition, and the pursuit of profit. The most testable point is Marx's conflict-theory view: it divides society into owners (bourgeoisie) a...

Glyceraldehyde is the three-carbon aldose (an aldotriose), HOCH₂–CHOH–CHO, with one stereocenter. It is the reference for D/L sugar nomenclature, and its phosphate, G3P, is a key glycolysis intermediate. D/L is structural, not the sign o...

A Michael addition is the conjugate (1,4) addition of a nucleophile, usually a stabilized enolate, to the β carbon of an α,β-unsaturated carbonyl, forming a new C–C bond. Test point: attack is at the β carbon, not the carbonyl carbon, th...

A dielectric is an insulator placed between capacitor plates; it polarizes, weakens the field, and raises capacitance by the factor κ (C = κC₀). With the battery connected, V stays fixed and Q rises; if the battery is disconnected, Q sta...

Qsp is the ion product: the solubility product expression evaluated with the ion concentrations present right now, not necessarily at equilibrium. Compare it with Ksp. If Qsp > Ksp, a precipitate forms; if Qsp < Ksp, more solid can disso...

Harlow raised infant rhesus monkeys with a wire surrogate that gave milk and a cloth surrogate that gave none; the infants clung mostly to the cloth one. It shows attachment depends strongly on contact comfort, not just feeding, challeng...

Kinaesthesia (kinesthesia) is the sense of the position and movement of body parts, based on receptors in muscles, tendons and joints. Many texts treat it as proprioception; others use proprioception for position and kinesthesia for move...

Electrostatics is the study of electric charges at rest and the forces, fields, and potential energies they produce. The most testable point: like charges repel, opposite charges attract, and the force between point charges falls off as...

Dalton's law says the total pressure of a mixture of ideal gases equals the sum of the partial pressures of its components, each acting as if it alone filled the container. Most testable: partial pressure = mole fraction × total pressure.

Relative configuration is the arrangement of groups at a stereocenter compared with a reference compound, as in D/L labeling against glyceraldehyde, rather than an R/S assignment. If no bond to the stereocenter is broken, its configurati...

Telomere shortening is the loss of repeat DNA from the ends of linear chromosomes with each round of replication, because DNA polymerase cannot fully copy the lagging-strand end. Most somatic cells have little telomerase, so critically s...

In Goffman's dramaturgical approach, the front stage is the social setting where a person performs a role for an audience, managing behavior to fit that audience's expectations. The testable point is the contrast with the back stage, whe...

Diastole is the phase of the cardiac cycle in which the heart muscle relaxes and the chambers fill with blood. In ventricular diastole the semilunar valves are closed and the AV valves open. Diastolic pressure is the lower number in a bl...

Parvocellular cells are the small cells of the lateral geniculate nucleus (and the retinal ganglion cells feeding them) that carry color and fine detail. They start the "what" pathway to the temporal lobe. Magnocellular cells, in contras...

A membrane potential is the voltage across a cell membrane, set by uneven ion distribution and selective permeability. An ion's electrochemical gradient combines its concentration gradient and the electrical gradient, so its net flow dep...

A concentration gradient is a difference in a species' concentration between two regions, such as across a membrane. Net diffusion of an uncharged species runs down the gradient, from high to low, until it levels out. In a concentration...

A vicinal diol (glycol) has two OH groups on adjacent carbons, as in ethylene glycol. The most testable point is how it forms from an alkene: OsO₄ or cold, dilute KMnO₄ gives syn addition, while epoxide formation then ring opening gives...

Attentional capture is the involuntary pull of attention toward a stimulus, a form of bottom-up (stimulus-driven) processing, even when you were focused on something else. Sudden, intense or personally relevant stimuli, like a loud noise...

Enzyme inhibition is a decrease in an enzyme's reaction rate caused by an inhibitor, which can bind reversibly or irreversibly. Most testable: a competitive inhibitor raises apparent Km with Vmax unchanged, while a pure noncompetitive in...

Polarization means restricting a transverse wave's oscillation, for light the electric field, to one plane. A polarizer cuts unpolarized light to half its intensity. Polarized light meeting a second polarizer at angle θ follows I = I₀cos...

pOH is the negative base-10 logarithm of the hydroxide ion concentration, pOH = −log[OH⁻]. At 25 °C, pH + pOH = 14, so a pOH below 7 means a basic solution, and each 1-unit drop in pOH means a tenfold rise in [OH⁻].

Aversive conditioning reduces attraction to a stimulus or behavior by pairing it with something unpleasant. In the classical account, the target (such as alcohol paired with nausea) becomes a conditioned stimulus that triggers avoidance;...

Systole is the contraction phase of the cardiac cycle, when a chamber squeezes blood onward. Ventricular systole is the usual meaning: rising ventricular pressure closes the AV valves (first heart sound), then opens the semilunar valves...

The carbonyl carbon is the carbon double-bonded to oxygen (C=O). Oxygen pulls electron density away, so this sp² carbon is partially positive and electrophilic. Nucleophiles attack it in nucleophilic addition (aldehydes, ketones) and nuc...