1,800+ MCAT terms, defined.

Simple diffusion is passive movement of a solute down its concentration gradient directly through the lipid bilayer, with no transport protein or ATP. Small nonpolar molecules like O₂ and CO₂ cross this way, and the rate rises in proport...

Chemical equilibrium is the state of a reversible reaction in which forward and reverse rates are equal, so concentrations stay constant (not necessarily equal). In organic chemistry it governs esterification, acetal formation and tautom...

An adiabatic process is one in which no heat is exchanged between the system and its surroundings (q = 0). Energy can still change by work, so ΔU = −W. An ideal gas that expands adiabatically and does work cools, and one that is compress...

Wave speed is how fast a wave pattern, such as a crest, advances through a medium: v = fλ. At MCAT level the medium sets it, not the source. When a wave enters a new medium, frequency stays constant while speed and wavelength change in p...

The looking-glass self is Charles Horton Cooley's idea that we build a self-concept by imagining how others see and judge us, then reacting to that imagined judgment. The testable point is that it rests on our perception of others' views...

Motor function is the nervous system's control of movement: the primary motor cortex (precentral gyrus, frontal lobe) sends commands down the spinal cord to skeletal muscle. Testable point: each hemisphere mainly controls the opposite si...

Harmonics are the standing-wave frequencies a string or air column can sustain: whole-number multiples of the lowest one, the fundamental (first harmonic). Strings and open pipes support every integer multiple, but a pipe closed at one e...

An elimination reaction removes a leaving group and a hydrogen from adjacent carbons, forming a C=C π bond (for example, alkyl halide → alkene). E2 is one concerted step with a strong base; E1 goes through a carbocation. The more substit...

A micelle is a spherical cluster of amphipathic molecules, such as soaps, detergents or bile salts, in water, with polar heads facing the water and nonpolar tails packed inside. It forms by the hydrophobic effect, which raises the entrop...

The serial position effect is the tendency to recall items at the start (primacy) and end (recency) of a list better than items in the middle. Primacy reflects rehearsal and long-term storage; recency reflects short-term memory. A distra...

Heritability is the share of variation in a trait, within a given population and environment, that is due to genetic differences (all genetic variance in broad-sense, additive only in narrow-sense). It is not how genetic a trait is in on...

Meditation is a set of practices, such as focusing on the breath or observing thoughts without judgment, that produce a relaxed, awake, altered state of consciousness. The most testable point: EEG typically shows more alpha waves (someti...

Charles's law says that for a fixed amount of gas at constant pressure, volume is directly proportional to absolute (Kelvin) temperature, so V₁/T₁ = V₂/T₂. The most testable point is that T must be in kelvins: doubling the Kelvin tempera...

The Rydberg formula gives the wavelength of the photon emitted or absorbed when a hydrogen electron changes between levels n₁ and n₂. Most testable point: a bigger energy gap means a shorter wavelength, so transitions to n = 1 (Lyman) ar...

Acetone (propanone, CH₃COCH₃) is the simplest ketone. It is a polar aprotic solvent: it accepts hydrogen bonds at its carbonyl oxygen but has no O–H or N–H to donate. Its most testable role is favoring SN2 reactions by leaving nucleophil...

Freud's psychosexual theory says personality forms through childhood stages (oral, anal, phallic, latency, genital), each centered on a body zone where libido is focused. Unresolved conflict at a stage can cause fixation that shapes adul...

An oxidoreductase is an enzyme that catalyzes an oxidation-reduction reaction: one substrate loses electrons while another gains them, often with a cofactor such as NAD⁺ or FAD. It is EC class 1, and the testable skill is spotting one fr...

A coordinate covalent bond is a covalent bond in which one atom, the donor with a lone pair, supplies both shared electrons to an acceptor with an empty orbital. The donor is the Lewis base and the acceptor is the Lewis acid. Once formed...

Energy dissipation is the conversion of organized energy, such as mechanical or electrical energy, into thermal energy that spreads out and is hard to recover. Total energy is still conserved, but the mechanical energy of the system typi...

Secure attachment is an infant's bond with a caregiver in which the child uses the caregiver as a secure base, is upset by separation, and is quickly comforted on reunion. In Ainsworth's Strange Situation it is the most common pattern an...

A peptide hormone is a hormone made of amino acids, such as insulin, glucagon, or ADH. Being polar, it generally cannot cross the cell membrane, so it binds a surface receptor and acts through second messengers like cAMP. That contrasts...

A chemical formula uses element symbols and subscripts to show which atoms a substance contains and in what ratio. The empirical formula is the simplest whole-number ratio; the molecular formula gives the actual atom counts and is a whol...

Magnetism is the interaction produced by moving electric charges, described by magnetic fields (B, in tesla). A field exerts force F = qvB sinθ on a moving charge, perpendicular to its velocity, so it changes the charge's direction but d...

An α-helix is a secondary structure in which the polypeptide backbone coils into a right-handed spiral held by hydrogen bonds between backbone C=O and N–H groups four residues apart. Testable point: the R groups point outward, and prolin...

Homeostasis is the maintenance of a stable internal state around a set point. In psychology it underlies drive reduction theory: a need creates a drive, and behavior that reduces the drive restores balance. It fits hunger and thirst but...

Reaction formation is a psychoanalytic defense mechanism in which a person hides an unacceptable impulse or feeling by adopting the opposite attitude or behavior, often in an exaggerated way. The testable clue is an excessive, insistent...

Complete dominance means one allele fully masks the other in a heterozygote, so Aa looks the same as AA. The most testable point: an Aa × Aa cross typically gives a 3:1 dominant to recessive phenotype ratio and a 1:2:1 genotype ratio.

An electron acceptor is a species that takes up electrons. In organic chemistry it usually means an electrophile (Lewis acid) that accepts an electron pair from a nucleophile to form a bond. Look for a partial positive atom or an empty o...

Gibbs free energy (G = H − TS) is the energy available to do useful work at constant temperature and pressure. A negative ΔG means a reaction is spontaneous as written, zero means equilibrium, and ΔG = ΔH − TΔS shows how temperature can...

Parallel resistors connect across the same two nodes, so each branch has the same voltage and the current splits among them. Adding a resistor in parallel lowers the equivalent resistance, which is smaller than the smallest single resistor.

Social identity theory (Tajfel and Turner) says part of your self-concept comes from the groups you belong to, and that people tend to raise self-esteem by favoring their ingroup over outgroups. Testable point: even arbitrary group membe...

Nucleotide excision repair removes bulky, helix-distorting DNA damage, such as UV-induced thymine dimers, by cutting out a short stretch of the damaged strand. DNA polymerase refills the gap using the intact strand as template, and ligas...

A social script is a learned mental outline of the usual sequence of actions and roles in a familiar situation, such as ordering at a restaurant. It is a kind of event schema that guides expectations and behavior, and a departure from it...

Aliphatic describes organic compounds, or parts of molecules, that are not aromatic. They can be straight, branched, or ring-shaped (alicyclic), and saturated or unsaturated. Alkanes, alkenes, alkynes and cyclohexane are aliphatic; benze...

A calorimeter is an insulated device that measures heat released or absorbed in a process by the temperature change of a known mass of water or other material. Most testable: heat lost by the system equals heat gained by the surroundings...

Resistance to flow is how strongly a tube opposes the movement of a viscous fluid, so a pressure drop is needed to keep the fluid moving. For steady laminar flow it scales as 1/r⁴, so halving a tube's radius raises its resistance 16-fold.

Twin studies compare monozygotic (identical) twins, who share nearly all their genes, with dizygotic (fraternal) twins, who share about half, to separate genetic from environmental influence on a trait. If identical pairs match more ofte...

Substrate concentration, [S], is the molar amount of substrate available to an enzyme. For a typical Michaelis-Menten enzyme, rate rises with [S] in a hyperbola and levels off at Vmax once the active sites are saturated. Km is the [S] at...

A covalent bond forms when two atoms share one or more pairs of valence electrons, usually between nonmetals. The most testable point: more shared pairs (single, double, triple) make a shorter, stronger bond, and unequal sharing makes th...

Crystallized intelligence is the knowledge and skill a person has accumulated through learning and experience, such as vocabulary and general facts. The testable point: it tends to be maintained or to grow into later adulthood, unlike fl...

A cyanohydrin has an –OH group and a nitrile (–C≡N) on the same carbon. It forms when cyanide ion adds to the carbonyl carbon of an aldehyde or ketone. Testable point: the product has one more carbon, and its nitrile can be hydrolyzed to...

Iconic memory is visual sensory memory: a brief, detailed image of what was just seen that lasts well under a second unless attention moves it into working memory. The most testable point is Sperling's partial-report experiment, which sh...

Thermodynamic control means a reversible reaction reaches equilibrium, so the major product is the most stable one (lowest Gibbs free energy), not the one that forms fastest. Higher temperature and longer time favor it; the faster-formin...

Positive charge is the kind of electric charge a proton carries; an object is positively charged when it has a net deficit of electrons. Positive charges repel each other and attract negative charges, and electric field lines point away...

The pinna (auricle) is the visible, cartilage-based flap of the outer ear. It collects sound waves and funnels them into the ear canal toward the eardrum, and its folds help the brain judge a sound's elevation and whether it is in front...

Cytokinesis is the division of the cytoplasm and plasma membrane that splits one cell into two daughter cells, usually beginning in late anaphase or telophase. Animal cells pinch in two with an actin–myosin contractile ring; plant cells...

Splicing is the removal of introns from eukaryotic pre-mRNA and the joining of the remaining exons into mature mRNA, done in the nucleus by the spliceosome. Most testable: alternative splicing lets one gene make several different proteins.

A carboxylate ion (RCOO⁻) is the conjugate base formed when a carboxylic acid loses its proton. Resonance delocalizes the negative charge over both oxygens, which explains the acidity of carboxylic acids and why carboxylate salts dissolv...

Adulthood is the lifespan period after adolescence, usually split into early, middle, and late adulthood. Physical function generally peaks early and declines gradually. Most testable: Erikson's conflicts are intimacy vs isolation, gener...

A reaction is zeroth order in a reactant when its rate does not depend on that reactant's concentration, so rate = k. Most testable point: a plot of [A] versus time is a straight line with slope −k, unlike the curved plots for first and...