1,800+ MCAT terms, defined.

DNA ligase is the enzyme that seals a nick in the sugar-phosphate backbone by forming a phosphodiester bond between a 3′ OH and an adjacent 5′ phosphate. It joins Okazaki fragments on the lagging strand, and it also seals nicks in DNA re...

Effective nuclear charge (Zeff) is the net positive charge a valence electron feels after inner electrons partly shield it from the full nuclear charge. It rises left to right across a period and changes little down a group, which explai...

The force of gravity is the attractive force between any two masses, given by Newton's law of universal gravitation. It is proportional to both masses and falls off as 1/r², so doubling the center-to-center distance cuts the force to one...

Pyrophosphate (PPi, P₂O₇⁴⁻) is two phosphate groups joined by a phosphoanhydride bond. It is released when ATP is cleaved to AMP and when a nucleotide is added to DNA or RNA. Its hydrolysis to two Pi is strongly exergonic, so it helps dr...

Heuristics are mental shortcuts, or rules of thumb, that let people judge or solve problems quickly with little effort. They often give good answers but can produce systematic errors, unlike an algorithm, a step-by-step procedure that gi...

Ligaments are bands of dense regular connective tissue, rich in collagen, that connect bone to bone across a joint and stabilize it. The classic MCAT contrast is that ligaments join bone to bone, while tendons join muscle to bone.

A weak base accepts a proton from water only partly (B + H₂O ⇌ BH⁺ + OH⁻), so it has a small Kb and most of it stays unprotonated at equilibrium. Most testable: a smaller Kb (larger pKb) means a weaker base and a stronger conjugate acid,...

Mass defect is the amount by which a nucleus's measured mass is less than the combined mass of its separate protons and neutrons. The missing mass appears as the nuclear binding energy (E = Δmc²), and a larger defect per nucleon usually...

A glycoprotein is a protein with short, often branched carbohydrate chains covalently attached, added in the ER and Golgi. The most testable point: sugars link to asparagine (N-linked) or to serine/threonine (O-linked), and they aid fold...

Extinction is the weakening of a learned response when the conditioned stimulus appears without the unconditioned stimulus (classical) or when a behavior is no longer reinforced (operant). It suppresses learning rather than erasing it, s...

A tosylate is the ester of an alcohol with p-toluenesulfonic acid (R–OTs). It converts a poor leaving group, OH⁻, into an excellent one, the tosylate anion, so the carbon can undergo SN2 (or E2) reactions. Tosylation keeps the carbon's c...

An electrolytic cell uses an external power source to force a nonspontaneous redox reaction (ΔG > 0, E°cell < 0). Oxidation is still at the anode and reduction at the cathode, but the anode is positive, the opposite of a galvanic cell.

Final velocity (v) is an object's velocity at the end of the time interval being studied. In constant-acceleration kinematics, v = v₀ + at. It is a vector, so its sign gives direction, and at a projectile's peak the vertical component is...

Observational learning is acquiring a behavior by watching a model and the consequences the model receives, without being directly reinforced yourself. In Bandura's Bobo doll study, children imitated aggression more when the model was re...

Solomon Asch's studies showed that people often go along with a unanimous group's clearly wrong answer on a simple line-matching task. It is the classic demonstration of conformity driven by normative social influence, and one dissenter...

HCl is the strong acid secreted by parietal cells of the stomach, giving gastric juice a pH of about 1.5–2. It denatures proteins, kills many microbes, and activates pepsinogen to pepsin, an enzyme that works best at low pH.

Methyl is the –CH₃ group, a small nonpolar, hydrophobic carbon group. In biochemistry it is the side chain of alanine and the group added in methylation. Methylation of cytosine in DNA is typically linked to reduced transcription, withou...

Trigonal planar describes a central atom with three electron groups and no lone pairs (AX₃), so all bond angles are about 120° in one plane. It goes with sp² hybridization, as in BF₃, carbocations, and the carbonyl carbon.

A concentration cell is a galvanic cell with identical electrodes and the same ion in both half-cells at different concentrations, so E° = 0 and the voltage comes from the Nernst equation. The dilute side is the anode, the concentrated s...

Pitch is how high or low a sound seems to a listener, and it is set mainly by the wave's frequency: higher frequency means higher pitch. Amplitude changes loudness, not pitch, and a sound's frequency stays the same when it crosses betwee...

Kohlberg proposed that moral reasoning develops in a fixed order through three levels (preconventional, conventional, postconventional), each with two stages. The most testable point is that the stage is set by the reasoning a person giv...

Labeling theory says deviance comes from society's reaction to an act, not from the act itself. A label can be internalized and may lead to further deviance (secondary deviance, per Lemert). It is a symbolic interactionist view, tied to...

A conjugate base is what remains when a Brønsted–Lowry acid donates a proton (HA → H⁺ + A⁻). The stronger the acid, the weaker its conjugate base, and Ka × Kb = Kw for a conjugate pair.

An antinode is a point on a standing wave where the oscillation amplitude is maximum, because the two opposite-moving waves are always in phase there. Adjacent antinodes are λ/2 apart, with a node midway between them.

Ring strain is the extra potential energy of a cyclic molecule compared with a strain-free open-chain analog, arising from angle, torsional and steric strain. Cyclopropane is the most strained, while the cyclohexane chair is essentially...

The fluid mosaic model pictures the cell membrane as a phospholipid bilayer with proteins embedded in it, and both lipids and proteins can drift sideways within the layer. Most testable point: unsaturated (kinked) tails and higher temper...

Milgram's obedience experiment showed that many ordinary people (about 65% in the baseline version) would deliver what they believed were severe shocks to another person when an authority figure told them to. The testable point: obedienc...

A recessive allele's trait is masked when a dominant allele is present (under complete dominance), so it shows in homozygous recessive individuals (aa), or in males for X-linked genes. Testable: Aa × Aa carrier parents give 1/4 affected...

Wernicke's aphasia is a language disorder caused by damage to Wernicke's area, usually in the left temporal lobe. Comprehension is impaired, and speech is fluent but often meaningless. The testable contrast is Broca's aphasia, where spee...

The Bohr model pictures the hydrogen electron in circular orbits with fixed, quantized energies. Light is absorbed or emitted when the electron jumps between levels, and the photon energy equals the energy gap. It fits one-electron speci...

Projectile motion is the curved, two-dimensional path of an object launched into the air and moving under gravity, with air resistance neglected. Horizontal and vertical motions are independent: horizontal velocity is constant, while ver...

Hydration is the addition of water (H and OH) across a C=C bond, turning an alkene into an alcohol. Acid-catalyzed hydration goes through a carbocation, so OH adds to the more substituted carbon (Markovnikov) and rearrangements are possi...

Sensation is the detection of physical or chemical stimuli by sensory receptors and their transduction into neural signals sent to the brain. It comes before perception, which is the brain's interpretation of that input, and the MCAT oft...

The hydrophobic effect is the tendency of nonpolar groups to cluster together in water. It is driven mainly by the entropy gain of the surrounding water, not by strong attraction between the nonpolar groups. It is a major driving force f...

Halogens are the Group 17 (VIIA) elements: F, Cl, Br, I and At. Each has seven valence electrons, so it readily gains one electron to form a halide ion, X⁻. Most testable: going down the group, electronegativity and oxidizing strength de...

A scalar is a quantity described by a magnitude and unit alone, with no direction. Distance, speed, mass, energy, work and temperature are examples. Testable point: scalars add as ordinary numbers, while vectors need direction, and a sca...

Wernicke's area is a region of the temporal lobe, usually in the left hemisphere, involved in understanding spoken and written language. Damage causes Wernicke's (receptive) aphasia: speech is fluent but often meaningless, and comprehens...

A steroid hormone is a lipid-soluble hormone made from cholesterol, such as cortisol, aldosterone, estrogen, progesterone, and testosterone. It typically crosses the cell membrane, binds an intracellular receptor, and changes gene transc...

A geminal diol (gem-diol, or hydrate) has two OH groups on the same carbon. It forms when water adds to the C=O of an aldehyde or ketone. For most carbonyls the equilibrium favors the carbonyl, but small aldehydes and carbonyls with elec...

Maslow's hierarchy of needs is a humanistic theory of motivation that ranks needs from physiological and safety up through love and belonging, esteem, and self-actualization. Lower needs generally must be largely met before higher ones d...

Elastic potential energy is energy stored in a deformed elastic object, such as a stretched or compressed spring. For an ideal spring U = ½kx², so it depends on the square of the displacement from equilibrium: doubling the stretch quadru...

Northern blotting detects a specific RNA (usually mRNA) in a sample. RNA is separated by gel electrophoresis, transferred to a membrane, and hybridized to a labeled complementary probe. Band presence, size, and intensity show whether a g...

Situational attribution (external attribution) explains a person's behavior by outside factors such as circumstances, luck, or other people, not by traits. The testable point is that observers underuse it for others' behavior (fundamenta...

A sensory receptor is a specialized cell or nerve ending that detects a type of stimulus and converts it into a neural signal (sensory transduction). Testable point: receptor type sets which stimulus is detected, and the brain area recei...

Anaerobic metabolism makes ATP without oxygen, typically by glycolysis followed by fermentation (lactate in animal cells, ethanol and CO₂ in yeast). Fermentation regenerates NAD⁺ so glycolysis can continue, giving a net 2 ATP per glucose.

Keto-enol tautomerization is the rapid, acid- or base-catalyzed interconversion of a carbonyl compound and its enol by moving a proton and shifting the double bond. For simple aldehydes and ketones the keto form is usually favored at equ...

A reaction rate is the change in concentration of a reactant or product per unit time (mol/L·s). The most testable point: the rate law, rate = k[A]^m[B]^n, has orders found from experimental data, not from the balanced equation's coeffic...

Gauge pressure is pressure measured relative to the surrounding atmospheric pressure: Pgauge = Pabs − Patm. A gauge open to the air reads zero. The most testable point is that gas laws need absolute pressure, so add Patm to a gauge value...

Neuroticism is the Big Five (five-factor) trait describing a tendency toward negative emotions such as anxiety, worry, and moodiness, with strong reactions to stress. The low end is emotional stability. It is a normal-range personality d...

Recombinant DNA is DNA made by joining sequences from different sources, usually a gene cut and ligated into a plasmid vector, then copied in host cells such as bacteria. Testable point: antibiotic resistance selects cells that took up t...