1,800+ MCAT terms, defined.

Metabolic states are the body's fuel-handling modes set by time since the last meal: absorptive (fed), postabsorptive (fasting), and prolonged fasting. Insulin drives storage in the fed state; glucagon drives release of glucose and other...

Collective behavior is relatively spontaneous, loosely organized action by a large group responding to a shared situation, such as crowds, riots, panics, fads and rumors. The MCAT often asks which account fits: contagion, convergence, or...

The H-zone is the central part of the A band in a sarcomere that holds only thick (myosin) filaments, with no thin-filament overlap. It narrows when the muscle contracts and can disappear in strong contraction, while the A band keeps its...

Specific rotation, [α], is the observed rotation of plane-polarized light by a chiral compound, scaled to a 1 dm path and 1 g/mL concentration so it is a characteristic constant. Enantiomers have equal magnitude and opposite sign; a race...

A source charge is the charge that creates the electric field and potential being analyzed, as opposed to the small test charge placed there to probe it. The field and potential at a point depend on the source charge and the distance, no...

A lead-acid battery is a rechargeable battery with a Pb anode, a PbO₂ cathode and H₂SO₄ electrolyte, giving about 2 V per cell. On discharge both electrodes turn to PbSO₄ and the acid is consumed; recharging runs the reaction backward as...

Accuracy is how close a measurement or finding is to the true value. In research design it is closely tied to validity, meaning the measure captures what it claims to. A measure can be reliable (consistent) yet inaccurate, because a syst...

A premature stop codon is a UAA, UAG, or UGA that appears in the reading frame before the normal end of a gene, so translation ends early and a shortened protein is made. It is usually caused by a nonsense mutation, which changes a sense...

Cultural means relating to culture: a group's shared beliefs, values, symbols, language, norms, and objects, learned through socialization and passed on across generations. The most testable point is that culture is learned, not biologic...

Young's double-slit experiment passes coherent light through two narrow slits so the overlapping waves form alternating bright and dark fringes on a screen, showing that light behaves as a wave. Bright fringes occur where the path differ...

One mole is the amount of a substance containing 6.022 × 10²³ elementary entities (atoms, molecules, ions, or formula units), a number called Avogadro's number. It is a count, not a mass: equal moles mean equal numbers of particles, and...

Ecstasy is MDMA, a synthetic amphetamine derivative that textbooks classify as a stimulant, a hallucinogen, or both. It mainly increases serotonin in the synapse by promoting its release and blocking its reuptake, which produces euphoria...

The I-band is the light region of a striated muscle sarcomere that contains only thin filaments (actin) and is bisected by the Z-line. During contraction the I-band gets shorter, while the A-band stays the same length. That contrast is t...

Reverse transcription is the making of DNA from an RNA template, catalyzed by reverse transcriptase. Retroviruses such as HIV use it to copy their RNA genome into DNA that integrates into the host genome. Key point: the template is RNA a...

A diffraction grating is a surface with many equally spaced parallel slits or lines that produces sharp bright maxima where d sinθ = mλ. Because the angle depends on wavelength, it splits white light into colors, with longer wavelengths...

Myelin is a fatty insulating sheath wrapped in segments around many axons. It is made by oligodendrocytes in the CNS and Schwann cells in the PNS, and it speeds action potentials by saltatory conduction. Loss of myelin, as in multiple sc...

The reticular activating system (RAS) is a network of neurons in the brainstem's reticular formation that projects through the thalamus to the cortex to regulate arousal, wakefulness, and alertness. Most testable: it sets the level of ar...

Speed of sound is how fast a sound wave travels through a medium. The medium's stiffness and density set it, not the source. It is generally greatest in solids and least in gases, and when sound changes media its frequency stays fixed wh...

The azimuthal (angular momentum) quantum number, l, identifies an electron's subshell and the shape of its orbital. For a given n, l is an integer from 0 to n − 1, with l = 0, 1, 2, 3 labeled s, p, d, f. It also fixes the allowed ml valu...

Secularization is the process by which religion loses influence over social institutions, public life, and often individual practice, usually linked to modernization, science, and rationalization. Classic theory predicts religion decline...

G2 is the second gap phase of interphase, after S phase and before mitosis. The cell grows, prepares for division, and checks at the G2/M checkpoint that replicated DNA is intact. Testable point: G2 cells have 4n DNA content but the same...

A ribosome is a membrane-less ribonucleoprotein machine with a small and a large subunit, each built from rRNA and proteins, that translates mRNA into protein. Prokaryotes have 70S ribosomes (30S + 50S); eukaryotes have 80S (40S + 60S)....

A portal system sends blood through two capillary beds in series, joined by a portal vein, before it returns to the heart. This is a biology topic, not organic chemistry. Testable point: in the hepatic portal system, most absorbed nutrie...

Division of labor is the splitting of work into specialized roles, so no one person produces everything. Durkheim (functionalism) held that it makes members interdependent and creates organic solidarity in complex societies. Marx saw it...

A non-conservative force does work that depends on the path taken, so no potential energy can be defined for it. Friction and air drag are the standard examples; they typically convert mechanical energy to thermal energy, so mechanical e...

A chemical bond is an attractive interaction that holds atoms together in a compound, lowering their total energy. The most testable point is classifying bonds (ionic, covalent, metallic) by electronegativity difference and predicting ho...

Erikson's psychosocial stages are eight lifespan stages, each built around a social crisis between two outcomes, such as trust vs mistrust. Know the order and pairings: vignettes are matched to a stage by age and task, though ages are ap...

In NMR, a deshielded nucleus has less surrounding electron density, so it feels more of the applied magnetic field and absorbs at higher frequency. It appears downfield (left, larger δ in ppm). Electronegative neighbors and π systems typ...

RNA interference (RNAi) is gene silencing after transcription: short double-stranded RNAs (siRNA or miRNA) guide the RISC complex to a complementary mRNA, which is then cleaved or blocked from translation. The testable point is that prot...

Orgasm is the peak of sexual arousal and the third phase of the Masters and Johnson sexual response cycle (excitement, plateau, orgasm, resolution). Most testable: resolution follows it, and males typically have a refractory period in wh...

Granules are small cytoplasmic packets of stored material. Secretory granules are membrane-bound, form via the ER and Golgi, and release their contents by exocytosis. Granulocytes (neutrophils, eosinophils, basophils) are white blood cel...

Spectra (plural of spectrum) show how light is spread over wavelength or frequency. In MCAT physics the key case is atomic line spectra: electrons moving between quantized energy levels emit or absorb photons with ΔE = hf, so each elemen...

The kelvin (K) is the SI unit of absolute temperature, with 0 K at absolute zero. One kelvin is the same size as one Celsius degree, and T(K) = t(°C) + 273.15. Gas laws and kinetic theory need temperature in kelvins, not Celsius.

Episodic memory is explicit (declarative) long-term memory for personally experienced events tied to a particular time and place, such as your last birthday. The MCAT pairs it with semantic memory, which holds general facts; both are exp...

A nucleophile attacks when it donates an electron pair (a lone pair or π bond) to form a new bond to an electron-poor atom, the electrophile, commonly a carbon with a partial positive charge. Testable point: the bond forms at the electro...

In Marxist conflict theory, the bourgeoisie is the capitalist class that owns the means of production (factories, land, capital). Its interests conflict with the proletariat, who must sell their labor for wages. The MCAT tests this as th...

In optics, light rays converge when they bend toward one another and meet at a point. Converging elements (convex lenses in air, concave mirrors) have positive focal length f. Testable point: they can form a real image or a virtual one,...

A molecular formula gives the actual number of atoms of each element in one molecule, such as C₆H₁₂O₆ for glucose. It is a whole-number multiple of the empirical formula, so you need the molar mass to get from the empirical formula to th...

Degradation is the enzyme-catalyzed breakdown of a biomolecule, such as a protein, mRNA, or fuel molecule, into smaller units, often by hydrolysis. Most testable: many proteins are tagged with ubiquitin and destroyed by the proteasome, s...

Functional attitudes theory (Daniel Katz) says people hold attitudes because the attitudes meet psychological needs. The four functions are knowledge, instrumental (adaptive), ego-defensive, and value-expressive. The usual question gives...

Peristalsis is a wave of alternating smooth muscle contraction and relaxation that pushes contents along a tube, such as the esophagus and intestines. It is involuntary and mainly propulsive (one-way), unlike segmentation, which mixes wi...

A spectator ion is a dissolved ion that is present unchanged on both sides of a reaction, so it is left out of the net ionic equation. Testable point: in a precipitation or strong acid–strong base reaction, the ions that do not form the...

Static pressure is the pressure a fluid exerts on its surroundings through random molecular collisions, the P term in Bernoulli's equation. For an ideal fluid in steady flow at constant height, static pressure is lower where the fluid mo...

Diethyl bromomalonate, BrCH(CO₂Et)₂, is diethyl malonate with one α-hydrogen replaced by Br. Unlike diethyl malonate, which is the nucleophile once deprotonated, its C–Br carbon is an electrophile that can undergo SN2 displacement of Br⁻.

The macula (macula lutea) is the central region of the retina around the fovea, rich in cones, giving sharp color vision at the center of gaze. The same word also names the hair-cell patches in the utricle and saccule that sense linear a...

The myelin sheath is a lipid-rich insulating wrapping around many axons, made by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. It speeds conduction because the action potential jumps b...

Sexual orientation is an enduring pattern of emotional, romantic, and sexual attraction to people of the same gender, a different gender, more than one gender, or no gender. The most testable point is that it is separate from gender iden...

A voltaic (galvanic) cell uses a spontaneous redox reaction to produce electrical current. Oxidation occurs at the anode and reduction at the cathode, and E°cell is positive (ΔG° negative). Electrons flow from anode to cathode.

Air resistance is the drag force air exerts on a moving object, directed opposite its motion relative to the air and growing with speed. Most testable: a falling object speeds up until drag equals its weight, then falls at constant termi...

The acrosome reaction is the release of digestive enzymes from the acrosome at the sperm head after the sperm contacts the egg's zona pellucida. The enzymes let the sperm digest a path to the egg's plasma membrane. It is a step in fertil...