1,800+ MCAT terms, defined.

A ligand-gated channel is a membrane ion channel that opens when a specific molecule binds to it. Ions then flow passively down their electrochemical gradient. The testable point is the effect on membrane potential: Na⁺ or Ca²⁺ influx us...

Square planar is a molecular geometry in which a central atom has four bonded atoms in one flat plane at 90° to each other. It arises from six electron domains, four bonds and two lone pairs on opposite sides (AX₄E₂), as in XeF₄.

Subjective norms are a person's perception of whether important others approve or disapprove of a behavior, weighed by how much the person wants to comply. In the theory of planned behavior they combine with attitude and perceived behavi...

Effusion is the escape of a gas through a tiny hole into a vacuum or a region of much lower pressure. At the same temperature, a lighter gas effuses faster: rate is inversely proportional to the square root of molar mass (Graham's law).

Electric field lines are a picture of an electric field: the tangent to a line shows the force direction on a positive test charge, and closer lines mean a stronger field. They leave positive charges and enter negative ones, and where th...

Dissociation is a disruption in the normal integration of consciousness, memory, identity, or perception, often tied to trauma or severe stress. It underlies dissociative amnesia, dissociative identity disorder, and depersonalization/der...

The mobile phase is the liquid solvent or gas that flows through the stationary phase in chromatography and carries the sample along. Compounds that favor it over the stationary phase move faster; on polar silica, nonpolar compounds usua...

A transmembrane protein is an integral membrane protein that crosses the lipid bilayer, with regions in the extracellular fluid, the membrane core, and the cytosol. Its membrane-spanning segment is usually nonpolar, most often an α-helix...

Mortality is the occurrence of death in a population, usually measured as a death rate per 1,000 people per year. With fertility and migration, it is one of three demographic processes that change population size. Falling mortality while...

Gene mapping places genes in order along a chromosome and estimates the distances between them. On the MCAT it uses recombination frequencies from crosses: the more often crossing over separates two linked genes, the farther apart they a...

Emf (electromotive force) is the voltage a cell can produce when no net current flows. It is energy per unit charge, not a force. A positive E°cell = E°cathode − E°anode means a spontaneous galvanic cell, and ΔG = −nFE links emf to free...

A charge's velocity is the vector v (speed and direction) of a moving charge. It matters because a magnetic field exerts a force F = qvB sinθ only on a moving charge, perpendicular to v, so it changes the charge's direction but not its s...

Broadbent's early selection theory holds that attention acts as a filter early in processing, passing one channel on physical features like pitch or location before meaning is analyzed. Hearing your own name in an ignored conversation is...

Vacuum distillation is distillation under reduced pressure, so the liquid boils at a lower temperature than its normal boiling point. The MCAT uses it for high-boiling or heat-sensitive compounds that could decompose if heated to their n...

In Piaget's theory, the formal operational stage is the last stage of cognitive development, typically beginning around age 12. Thinking becomes abstract, hypothetical and systematic. Concrete operational children reason logically only a...

The ideal gas constant R is the proportionality constant in PV = nRT. It is 8.314 J/(mol·K), or 0.0821 L·atm/(mol·K). It is the same for every ideal gas, and only its numerical value changes with the units, so match R to the units of P a...

An equipotential is a line, surface, or region where every point has the same electric potential, so the field does no work on a charge moved along it. Field lines are perpendicular to equipotentials and point toward lower potential.

A cytochrome is a heme-containing protein whose iron cycles between Fe³⁺ and Fe²⁺ to pass electrons one at a time down the electron transport chain. Most testable: cytochrome c carries electrons from Complex III to Complex IV, which redu...

The gambler's fallacy is the mistaken belief that after a streak of one outcome in independent random events, the opposite outcome is now "due." Each trial is independent, so the probability does not change. It is often explained by the...

K+ channels are membrane proteins that let K+ diffuse down its gradient, usually out of the cell. Leak channels hold the resting potential near EK, and voltage-gated K+ channels open after depolarization to repolarize the neuron, so bloc...

Molecular geometry is the 3D shape formed by a molecule's atoms around a central atom, predicted by VSEPR from the repulsion of electron domains. Lone pairs shape it but are not named in it, so H₂O has tetrahedral electron geometry but b...

A harmonic is a standing-wave frequency that is a whole-number multiple of the fundamental (lowest) frequency, fn = nf₁. Strings fixed at both ends and pipes open at both ends allow every integer n; a pipe closed at one end allows odd n...

The middle ear is the air-filled chamber between the eardrum and the cochlea's oval window. Its ossicles (malleus, incus, stapes) carry and amplify sound vibrations from air to cochlear fluid. Testable point: it conducts sound; transduct...

kcat is the turnover number: the number of substrate molecules one enzyme active site converts to product per second when the enzyme is saturated with substrate. It equals Vmax divided by total enzyme concentration, so unlike Vmax it doe...

Retinal means "of the retina," the light-sensitive layer at the back of the eye that holds the rods and cones. The most testable use is retinal disparity: the slight difference between the two eyes' images, a binocular depth cue that is...

A voltage drop is the decrease in electric potential across a circuit element, such as a resistor, as current passes through it. It follows V = IR. In series the drops typically add up to the source emf, while parallel branches share the...

A semantic network is a model of long-term memory in which concepts are nodes joined by associative links. Activating one node spreads activation to linked nodes, so related concepts are retrieved faster, which is the basis of priming.

Lysogenic describes a viral life cycle in which a phage's DNA joins the host genome as a prophage and is copied with it, without making virions or lysing the cell at that time. Stress can induce the prophage to enter the lytic cycle.

Acid dissociation is the reversible loss of a proton from an acid, HA, to water or another base, giving the conjugate base A⁻ and H₃O⁺. Its extent is measured by Ka or pKa. In organic chemistry, a more stable conjugate base means a large...

Treisman's attenuation theory is a model of selective attention in which an early filter weakens unattended input instead of blocking it. Highly meaningful stimuli, such as your own name, have low thresholds and can still break through....

DNA methylation is the addition of a methyl group (–CH₃) to a base in DNA, typically carbon 5 of cytosine in CpG sequences, by DNA methyltransferases. Methylation of a promoter usually lowers transcription without changing the base seque...

PO₄³⁻ is the phosphate ion, the fully deprotonated form of triprotic phosphoric acid (H₃PO₄). It is tetrahedral, with P at +5. Most testable: near pH 7.4 the buffer pair is H₂PO₄⁻/HPO₄²⁻ (pKa ≈ 7.2), not PO₄³⁻.

The spring constant k measures a spring's stiffness: the restoring force it exerts per meter of stretch or compression, in N/m. In Hooke's law, F = −kx, so a larger k means a greater force for the same displacement.

VTA is the ventral tegmental area, a midbrain region whose dopamine-releasing neurons project to the nucleus accumbens and prefrontal cortex (the mesolimbic and mesocortical pathways). It is a core part of the reward pathway, and drugs o...

Aldol addition is a reaction in which the enolate of one aldehyde or ketone attacks the carbonyl carbon of a second molecule, forming a β-hydroxy aldehyde or ketone. Most testable point: the new C–C bond joins the α-carbon of the nucleop...

Correspondent inference theory (Jones and Davis) explains when observers conclude that someone's behavior reflects a matching underlying trait. Inference is strongest when the act was freely chosen, intended, and unexpected or socially u...

The mitotic spindle is the microtubule array that attaches to chromosomes at their kinetochores and separates them during mitosis. Its microtubules typically grow from centrosomes at opposite poles, and in anaphase the kinetochore microt...

The frequency factor (A, the pre-exponential factor) in the Arrhenius equation reflects how often reactant collisions occur with a suitable orientation. It multiplies e^(−Ea/RT), the fraction of collisions with enough energy, to give k....

Fission is the splitting of a heavy nucleus, such as uranium-235, into smaller nuclei, usually after it absorbs a neutron, releasing energy and more neutrons. The energy comes from a small mass loss (E = Δmc²), because the products have...

An embedded protein is an integral membrane protein, held in the phospholipid bilayer by nonpolar amino acids that contact the lipid tails. Many span the whole bilayer (transmembrane), but some sit only partly in it. It is removed from t...

Endogenous means produced or driven from within the organism, as opposed to exogenous, which comes from the external environment. The MCAT uses it for endogenous opioids (endorphins), internally generated circadian rhythms, and voluntary...

A better leaving group departs more easily with the bonding electron pair because it is more stable afterward. Weak bases, the conjugate bases of strong acids (I⁻, Br⁻, tosylate), are generally good leaving groups, while strong bases lik...

Latent heat is the heat absorbed or released when a substance changes phase at constant temperature, calculated with q = mL. On a heating curve it is the flat plateau, where added energy changes intermolecular attractions instead of temp...

Internal resistance (r) is the resistance inside a real battery that makes its terminal voltage lower than its emf when it delivers current. Terminal voltage is V = ε − Ir, so it falls as current rises and equals the emf when no current...

Institutional facts are facts that exist only because people collectively agree to give something a status, such as money, marriage, or property. They contrast with brute facts, which hold regardless of belief. Testable point: they are s...

In Freudian psychoanalytic theory, the pleasure principle is the id's drive to seek immediate gratification of urges and avoid tension or pain, without regard for reality or morality. The testable point: the id follows the pleasure princ...

An allergic reaction is an overreaction of the immune system to a harmless antigen (an allergen). In the classic immediate form, a first exposure makes IgE that coats mast cells, and re-exposure triggers histamine release. Some delayed a...

Nucleotide synthesis is how cells make nucleotides, either de novo from small precursors or by salvaging free bases. Most testable: purine rings are built on PRPP (ribose-5-phosphate activated by ATP), while pyrimidine rings are built fi...

A hydration reaction adds H and OH across a C=C double bond to make an alcohol. With aqueous acid it goes through a carbocation and follows Markovnikov's rule, so OH ends up on the more substituted carbon, and rearrangements are possible...

Bromine (Br, Z = 35) is a group 17 halogen that exists as diatomic Br₂, a reddish-brown liquid at room temperature. Its most testable points are the halogen trends: it is a weaker oxidizing agent and less electronegative than chlorine, b...