1,800+ MCAT terms, defined.

Recrystallization purifies a solid by dissolving it in a minimum of hot solvent and cooling slowly, so the desired compound crystallizes while most impurities stay in solution. Key point: the ideal solvent dissolves the compound well whe...

The dependent variable is the outcome a researcher measures to see whether it changes in response to the independent variable. In an experiment, a difference in it between groups is the effect being tested. To spot it, ask what was measu...

A redox reaction transfers electrons: one species is oxidized (loses electrons, oxidation number rises) and another is reduced (gains electrons, oxidation number falls). The two occur together, and the oxidizing agent is the species that...

A test charge is a small, imaginary positive point charge used to probe an electric field without disturbing the source charges. The field at a point is the force per unit charge on it, E = F/q₀, so it points the way a positive test char...

Knowledge is information stored in long-term memory and organized into schemas. It is divided into declarative (explicit) knowledge of facts and events and procedural (implicit) knowledge of skills. Hippocampal damage can impair new decl...

Chyme is the acidic, semi-liquid mixture of partially digested food and gastric juice that leaves the stomach through the pyloric sphincter. Its acid in the duodenum triggers secretin, so the pancreas releases bicarbonate to neutralize it.

Epimers are diastereomers that differ in configuration at exactly one stereocenter and match at every other. The classic MCAT examples are D-glucose and D-galactose (C4 epimers) and D-glucose and D-mannose (C2 epimers). Anomers are epime...

Plane-polarized light has its electric field oscillating in a single plane, made by passing unpolarized light through a polarizing filter. Chiral, optically active compounds rotate that plane; the direction (+ clockwise, − counterclockwi...

The olfactory epithelium is the patch of tissue in the upper nasal cavity that holds olfactory receptor neurons, which are chemoreceptors that bind odorant molecules. Unlike vision, hearing, and touch, its signals reach the cortex and li...

Trigonal bipyramidal is the VSEPR electron geometry for five electron domains around a central atom (sp³d), with 120° and 90° angles. Most testable: lone pairs take equatorial positions, giving seesaw, T-shaped, or linear molecular shapes.

Atomic mass is the mass of one atom of a nuclide, measured in atomic mass units (u), where 1 u is 1/12 the mass of a carbon-12 atom (about 1.66 × 10⁻²⁷ kg). It is close to the mass number but slightly less than the sum of its parts, and...

Ion exchange chromatography separates charged molecules by their attraction to oppositely charged groups fixed on a resin. Testable point: a cation exchange resin is negatively charged and holds positive analytes, and the reverse for an...

Chemiosmosis is the use of a proton (H⁺) gradient across a membrane to make ATP: H⁺ flows down its gradient through ATP synthase, and that flow drives ADP + Pᵢ → ATP. Most testable: the electron transport chain only builds the gradient,...

The prefrontal cortex is the front part of the frontal lobe. It supports executive functions such as planning, decision making, judgment, and impulse control, partly by regulating limbic emotional responses. Damage (Phineas Gage) typical...

Prophase is the first stage of mitosis, when chromatin condenses into visible chromosomes and the spindle begins to form. Texts differ on whether nuclear envelope breakdown ends prophase or starts prometaphase. Testable point: homologs p...

Volumetric flow rate (Q) is the volume of fluid passing a cross-section per unit time, Q = Av, in m³/s. For an incompressible fluid with no leaks, Q is the same at every point along the pipe, so the fluid speeds up where the area narrows.

Alarm is the first stage of Selye's general adaptation syndrome, when a stressor triggers the sympathetic fight-or-flight response. The testable point is the order of the stages: alarm, then resistance, then exhaustion.

External validity is the extent to which a study's findings generalize beyond its own sample, setting, and conditions to other people, places, and times. The most testable point: an unrepresentative sample or an artificial lab setting lo...

Diamagnetic describes an atom, ion, or molecule whose electrons are all paired, so it has no net magnetic moment and is weakly repelled by an external magnetic field. Testable point: no unpaired electrons means diamagnetic; one or more m...

An absorption spectrum shows which wavelengths a substance absorbs. Because energy levels are quantized, an atom absorbs photons whose energy matches the gap between two levels, leaving dark lines. Those lines fall at the same wavelength...

Phosphorylase (glycogen phosphorylase) breaks α-1,4 glycosidic bonds at the ends of glycogen using inorganic phosphate, releasing glucose-1-phosphate in glycogenolysis. Testable point: glucagon and epinephrine activate it by phosphorylat...

Inhalation is the movement of air into the lungs, driven by muscles that enlarge the chest cavity and lower alveolar pressure below atmospheric pressure. The MCAT tests it in Biology, not as a separate Psychology or Sociology concept. Ke...

The stratum basale is the deepest layer of the epidermis, where stem-like keratinocytes divide by mitosis to replace cells shed from the surface. It also contains melanocytes and Merkel cells. The testable point: it is the epidermal laye...

An element is a pure substance whose atoms all have the same atomic number Z (the same number of protons), and it cannot be broken into simpler substances by chemical means. The most testable point: identity is set by proton count, not b...

Peptide bond formation joins the carboxyl group of one amino acid to the amino group of another, forming an amide bond and releasing H₂O (a condensation, or nucleophilic acyl substitution). Resonance gives the bond partial double-bond ch...

A class system ranks people by economic position such as wealth, income, occupation and education, so social mobility is possible. It is more open than a caste system, which rests on ascribed status. Marx defines class by relation to the...

2,3-BPG (2,3-bisphosphoglycerate) is made in red blood cells and binds deoxyhemoglobin, stabilizing the T state and lowering O₂ affinity. More BPG shifts the oxygen-dissociation curve right (higher P50), so hemoglobin unloads more O₂ in...

Cultural lag is the gap that arises when nonmaterial culture (norms, values, laws) changes more slowly than material culture (technology). Proposed by William Ogburn, it explains social problems that appear while rules and beliefs catch...

A lever is a rigid bar that pivots about a fixed point (the fulcrum), so a small effort force can balance or move a larger load. At rotational equilibrium the torques balance: Feffort × deffort = Fload × dload. A longer effort arm lowers...

Distinctiveness is whether a person acts this way only in this particular situation or across many situations. It is one cue in Kelley's covariation model of attribution. High distinctiveness points to a situational (external) cause; low...

The stratum spinosum is the "spiny" layer of the epidermis, lying between the stratum basale and the stratum granulosum. Its living keratinocytes are linked by many desmosomes, which give the layer mechanical strength. The usual test poi...

An aldol reaction joins two aldehydes or ketones: the enolate (from the α carbon) attacks the carbonyl carbon of a second molecule, forming a β-hydroxy carbonyl. The testable point is that the new C–C bond links an α carbon to a carbonyl...

The just world hypothesis (Melvin Lerner) is the belief that the world is fair, so people get what they deserve. Most testable point: it often leads observers to blame victims, attributing misfortune to the person's character or choices...

The Hill coefficient (n) measures cooperativity of ligand binding to a protein with several binding sites. n = 1 means noncooperative binding (hyperbolic curve, as in myoglobin), n > 1 means positive cooperativity (sigmoidal curve, as in...

Kinetic molecular theory models an ideal gas as particles in random motion with negligible volume, no intermolecular forces, and elastic collisions. Its key result: average kinetic energy depends on temperature in kelvins alone, so at on...

Turbulent flow is irregular, swirling fluid motion with eddies, in contrast to smooth laminar layers. It becomes likely at a high Reynolds number (fast, wide, dense, or low-viscosity fluid), and Poiseuille's law no longer applies.

Karl Marx was a 19th-century theorist behind conflict theory, which holds that society is shaped by struggle between classes over resources. The most testable point is his split between the bourgeoisie, who own the means of production, a...

A lactam is a cyclic amide: the N–C=O group sits inside a ring. It forms when an amino acid's amine attacks its own carboxyl group and water is lost. Ring size is named by Greek letter (β = 4-membered, γ = 5, δ = 6), and the strained β-l...

Moral development is how a person's reasoning about right and wrong changes over time. In Kohlberg's theory it moves through three levels, preconventional, conventional and postconventional, and the stage is set by the reasoning given, n...

Retroviruses (such as HIV) are enveloped RNA viruses that carry reverse transcriptase, which copies their RNA genome into DNA. That DNA integrates into the host chromosome as a provirus. The testable point is the reversed flow of informa...

A dipole-dipole force is the intermolecular attraction between the δ+ end of one polar molecule and the δ− end of another. For molecules of similar size, it is usually stronger than London dispersion alone, so polar compounds tend to hav...

A non-conservative force is one whose work depends on the path taken, so the work over a closed loop is not zero and no potential energy can be defined for it. Friction and air resistance are the standard examples, and they usually drain...

An isomerase is an enzyme that converts a molecule into one of its isomers by rearranging atoms within that same molecule, so substrate and product share a molecular formula. Glycolysis is the usual setting: phosphoglucose isomerase and...

Sensory processing is how the nervous system detects a stimulus, transduces it into neural signals, and sends them to the brain, where they are interpreted as perception. Most testable: bottom-up processing is stimulus-driven, while top-...

Net charge is the sum of the positive and negative charges on a molecule at a given pH, set by which ionizable groups are protonated. Key test point: a molecule is net positive when pH is below its pI, zero at the pI, and net negative ab...

A reaction mechanism is the series of elementary steps that add up to the overall reaction, including any intermediates. Key MCAT point: a proposed mechanism must sum to the balanced equation and agree with the experimental rate law, whi...

Photon energy is the energy carried by one photon, a single quantum of electromagnetic radiation. It is E = hf = hc/λ, so it rises with frequency and falls with wavelength. Brightness changes the number of photons, not the energy of each...

Denial is an ego defense mechanism in psychoanalytic theory in which a person refuses to accept a real, threatening fact and acts as though it is not true. The most testable point: denial rejects external reality, whereas repression push...

Internal validity is how confidently a study shows that the independent variable, rather than some other factor, caused the change in the dependent variable. Random assignment, control groups and control of confounds raise it, and tight...

Immunity is the body's resistance to pathogens and foreign material, built from fast, nonspecific innate defenses and slower, antigen-specific adaptive responses. The most testable point is that memory, meaning a faster and stronger seco...