1,800+ MCAT terms, defined.

An enol has an OH group on a carbon of a C=C double bond (C=C–OH). It is a tautomer of a ketone or aldehyde. For most simple carbonyl compounds the keto form is strongly favored at equilibrium, though enol content rises with conjugation,...

A hallucinogen is a psychoactive drug that alters perception, mood and thought, often producing hallucinations. On the MCAT, LSD, psilocybin and mescaline are the standard examples, and LSD and psilocybin act largely on serotonin receptors.

Reduction potential (E°red) is a measure, in volts, of how strongly a species tends to gain electrons, compared with the standard hydrogen electrode (0 V). The more positive it is, the stronger the oxidizing agent. In a galvanic cell, th...

A disulfide bond is a covalent S–S bond formed when the thiol groups (–SH) of two cysteine side chains are oxidized. It stabilizes tertiary and quaternary structure, mainly in extracellular proteins, and reducing agents such as β-mercapt...

In Freud's psychoanalytic theory, libido is the psychic energy of the life (sexual, pleasure-seeking) instincts, which arises in the id. The most testable point is that libido is invested in different body areas across the psychosexual s...

Mood is a sustained, relatively low-intensity emotional state that is often diffuse and not tied to one specific trigger. The testable contrast is with emotion, which is typically briefer, more intense, and directed at a particular stimu...

An allele is one alternative version of a gene, found at the same locus on homologous chromosomes. A diploid individual has two alleles per gene, and the pair (genotype), together with dominance relationships, determines the phenotype.

E2 is a bimolecular elimination: a base removes a β-hydrogen in one concerted step as the leaving group departs, forming an alkene. The rate depends on both substrate and base, and the H and leaving group must be anti-periplanar.

Molarity (M) is the moles of solute per liter of total solution (mol/L). Because solution volume changes with temperature, molarity does too, while molality does not. Diluting keeps moles constant: M₁V₁ = M₂V₂.

Amplitude is the maximum displacement of a wave or oscillating object from its equilibrium position. Energy and intensity scale with amplitude squared, so doubling the amplitude quadruples them, while frequency and wave speed do not change.

Bottom-up processing is perception that starts with the raw sensory input (features such as edges, color and motion) and builds up to a recognized whole. It is data-driven, unlike top-down processing, which uses expectations and context....

A rate-limiting enzyme catalyzes the slowest step of a metabolic pathway, so it largely sets the pathway's overall flux. It usually acts at an irreversible, regulated step, so changing its activity alters pathway output far more than cha...

Self-actualization is the drive to realize one's full potential, the top level of Maslow's hierarchy of needs. In Maslow's model it generally becomes the main motivator only after lower needs are reasonably met; Rogers also used the rela...

An E1 reaction is a two-step, unimolecular elimination: the leaving group departs first to form a carbocation, then a weak base removes a β-hydrogen to form an alkene. Its rate depends only on the substrate, and it competes with SN1 thro...

Internal energy (U) is the total kinetic and potential energy of a system's particles, and it is a state function. Its change follows the first law, ΔU = q + w (w is work on the system; physics texts write Q − W with W by the system). Fo...

The index of refraction, n = c/v, compares the speed of light in vacuum to its speed in a medium. It is dimensionless, equals 1 for vacuum, and is greater than 1 for ordinary media. Light entering a higher-n medium slows and bends toward...

Gender is the set of social and cultural roles, expectations, and identities a society links to being masculine, feminine, or another identity. It differs from sex, which is biological. The testable point is that gender is socially const...

A filament is a long, thin polymer of protein subunits. The MCAT uses it for cytoskeletal microfilaments (actin) and intermediate filaments (keratin, lamins), and for the thin (actin) and thick (myosin) filaments of muscle, which slide p...

A vitamin is an organic nutrient needed in small amounts that the body generally cannot make enough of, so it comes from the diet. Many B vitamins become coenzymes (NAD⁺, FAD, CoA). A key split is water-soluble (B, C) versus fat-soluble...

A second-order reaction has a rate law whose concentration exponents add to 2, such as rate = k[A]² or rate = k[A][B]. Most testable: for rate = k[A]², a plot of 1/[A] versus time is a straight line with slope k, and k has units M⁻¹s⁻¹.

Conservation of energy says the total energy of an isolated system stays constant; energy changes form or moves between objects. Mechanical energy (KE + PE) is constant when only conservative forces do work; friction and other nonconserv...

The bystander effect is the tendency for a person to be less likely to help someone in need when other people are present. It is explained by diffusion of responsibility and by reading others' inaction as a sign nothing is wrong (plurali...

Optical activity is the ability of a chiral substance to rotate the plane of plane-polarized light. Enantiomers rotate it by equal amounts in opposite directions, so a racemic mixture shows no net rotation. The sign (+ or −) is measured,...

Social institutions are enduring, organized patterns of norms, roles, and values that meet basic societal needs, such as family, education, religion, government, economy, and health care. Functionalism sees them as sustaining stability,...

Standard reduction potential (E°red) is the voltage of a reduction half-reaction under standard conditions (1 M, 1 atm, 25 °C), measured relative to the standard hydrogen electrode, defined as 0 V. The more positive it is, the stronger t...

The Venturi effect is the drop in a flowing fluid's pressure where a tube narrows and the fluid speeds up. By continuity the fluid moves faster in the narrow part, and by Bernoulli's principle its pressure is lower there (ideal fluid, ho...

In Freud's psychosexual theory, the phallic stage (about ages 3 to 6) is when the child's libido centers on the genitals. The Oedipus complex (and, in Freud's account, the Electra complex) arises here, and its resolution through identifi...

Cilia are short, hair-like projections of eukaryotic cells with a core of microtubules in a 9+2 pattern. Dynein arms slide the microtubules to bend the cilium, so motile cilia beat to move fluid or mucus. Many cells also have a nonmotile...

Affinity chromatography purifies a molecule by its specific, reversible binding to a ligand fixed to the column beads. Components that don't bind wash through first, and the target is eluted later by free ligand or a change in pH or salt.

The reticular formation is a network of neurons running through the brainstem (core of the medulla, pons and midbrain) that regulates arousal, alertness and the sleep-wake cycle. Its ascending activating role is the most testable point:...

An anion is an atom or group of atoms that has gained one or more electrons, giving it a net negative charge. Anions are larger than their parent atoms, and in electrochemical cells they migrate toward the anode.

The continuity equation states that for an incompressible fluid in steady flow, the volume flow rate is the same at every point along a pipe, so A₁v₁ = A₂v₂. The fluid speeds up where the cross-section narrows and slows where it widens.

In Freud's psychoanalytic theory, the unconscious is the level of mind holding wishes, urges and memories outside awareness, often because they are threatening. The testable point: it is said to shape behavior indirectly, through dreams,...

An aldol is a β-hydroxy aldehyde or ketone, formed when an enolate's α carbon attacks the carbonyl carbon of another molecule (aldol addition). Heating often dehydrates it to a conjugated α,β-unsaturated carbonyl (aldol condensation).

Nitrogenous bases are the nitrogen-containing rings in nucleotides: purines (A, G; two rings) and pyrimidines (C, T, U; one ring). In double-stranded DNA, A pairs with T and G with C (A with U in RNA), so A = T and G = C.

Red blood cells (erythrocytes) carry O₂ on hemoglobin. Mature mammalian RBCs have no nucleus or mitochondria, so they make ATP by glycolysis alone and do not use the O₂ they carry. Low O₂ delivery raises erythropoietin, which increases R...

Dalton's law states that the total pressure of a gas mixture equals the sum of the partial pressures of its components, each being the pressure that gas would exert alone in the same container. Most testable: partial pressure = mole frac...

A free body diagram is a sketch of one object, drawn as a point or box, showing only the forces acting on it as labeled arrows. Its most testable use is to sum the force components along each axis to get net force (ΣF = ma), which is zer...

Recognition is a memory retrieval task in which you identify information as previously encountered when it is presented to you, as in multiple choice or a police lineup. It is usually easier than recall, which requires you to produce the...

Affinity is the strength of attraction between a molecule and another substance, such as the stationary phase in chromatography. A compound with greater affinity for the stationary phase than for the mobile phase moves more slowly and el...

Anger is a basic emotion, usually a high-arousal, negative response to a perceived threat, insult or blocked goal. It has physiological, cognitive and behavioral components. The exam often tests that its facial expression is recognized a...

Terminal velocity is the constant speed a falling object reaches in a fluid when the upward drag force (plus any buoyant force) equals its weight, so net force and acceleration are zero. Testable point: gravity still acts, but it is bala...

Vapor pressure is the pressure of a vapor in dynamic equilibrium with its liquid (or solid) in a closed container, where evaporation and condensation rates are equal. It rises with temperature, and a liquid boils when its vapor pressure...

A phosphate group is a phosphorus bonded to four oxygens (–OPO₃²⁻ when attached to a molecule), usually linked by an ester bond to an –OH. Its negative charge is why phosphorylation can switch a protein's activity up or down, depending o...

Rational choice theory explains behavior as individuals weighing costs and benefits and choosing the option that best serves their own goals. Most testable point: it builds social patterns up from individual calculations (micro level), a...

Organelles are specialized compartments inside a cell, each with a specific job. Most eukaryotic organelles are membrane-bound, which lets incompatible reactions run in separate conditions. Most testable: match each organelle to its func...

An energy level is one of the specific, quantized energies a bound electron in an atom can have. Electrons move between levels by absorbing or emitting a photon with energy equal to the gap, ΔE = hν. Higher n means higher energy, and the...

Bernoulli's equation is conservation of energy for an ideal fluid (incompressible, nonviscous, steady flow): P + ½ρv² + ρgh is constant along a streamline. At the same height, faster flow means lower pressure, as in a narrowed pipe.

Stereochemistry is the study of how atoms are arranged in three dimensions and how that affects properties and reactions. Stereoisomers share connectivity but differ in spatial arrangement. Enantiomers differ in optical rotation; diaster...

REM is rapid eye movement sleep, the stage where the EEG looks nearly awake (fast, low-amplitude waves), dreams are most vivid, and skeletal muscles are essentially paralyzed. That mix is why it is called paradoxical sleep. Dreams also o...