1,800+ MCAT terms, defined.

Symbolic culture is the nonmaterial part of culture: the shared symbols, language, beliefs, values, and rituals through which a society gives meaning to its world. The most testable point is the contrast with material culture, the physic...

Prokaryotic describes cells, bacteria and archaea, that lack a membrane-bound nucleus and other membrane-bound organelles. The most testable point is that transcription and translation occur together in the cytoplasm, often on polycistro...

Hydrostatic pressure is the pressure in a fluid at rest caused by the weight of the fluid above a point. It increases linearly with depth, P = P₀ + ρgh, and depends on depth, density and g, not on container shape or width.

A covalent bond forms when two atoms share electron pairs, typically between nonmetals. The most testable point: equal sharing gives a nonpolar bond, unequal sharing (an electronegativity difference) gives a polar bond, and more shared p...

A riot is a violent, disorderly outburst by a crowd, and the MCAT treats it as an example of collective behavior: spontaneous, loosely structured group action. The testable point is matching a riot to an explanation: contagion, convergen...

A dominant allele is expressed in the phenotype of a heterozygote, masking the recessive allele, so Aa looks like AA. Dominance describes expression, not how common or fit an allele is. Aa × Aa gives a 3:1 dominant to recessive phenotype...

A reducing agent donates electrons (or hydride) to another species and is itself oxidized. In organic chemistry, common ones are NaBH₄, LiAlH₄ and H₂ with a metal catalyst. The key test point: LiAlH₄ is stronger and reduces esters and ca...

Cultural relativism is the principle that a person's beliefs, values and practices should be understood within their own culture's context rather than judged by the standards of another culture. It is the opposite of ethnocentrism, and t...

Snell's law relates the angles of a light ray to the refractive indices of two media at a boundary: n₁ sin θ₁ = n₂ sin θ₂. Angles are measured from the normal. Light typically bends toward the normal when it enters a medium with higher n...

Le Châtelier's principle says a system at equilibrium that is disturbed shifts in the direction that partly offsets the disturbance. The most testable point: concentration and pressure changes shift the position but not K, while a temper...

A double bond is two shared electron pairs between atoms: one σ bond plus one π bond, which blocks rotation. In biochemistry, cis double bonds kink fatty acid chains, lowering melting point and raising membrane fluidity; the peptide bond...

Gestalt principles are rules describing how the visual system groups separate elements into organized wholes. They include proximity, similarity, continuity, closure, symmetry, common fate and figure-ground. The testable point is that th...

The humanistic perspective holds that people have free will and an inherent drive toward growth and self-actualization. Its key ideas are Maslow's hierarchy of needs and Rogers's unconditional positive regard, which helps the real self m...

A species is a group of organisms that can interbreed in nature and produce viable, fertile offspring, and that is reproductively isolated from other such groups (the biological species concept). Key test point: a mule shows that mating...

IR spectroscopy identifies functional groups by the infrared frequencies a molecule absorbs as its bonds stretch and bend. Stronger bonds and lighter atoms absorb at higher wavenumber. The most testable peaks are a broad O–H near 3300 cm...

The photoelectric effect is the ejection of electrons from a metal surface by light. Electrons come out when the photon energy hf is at least the work function. Above that threshold, frequency sets the maximum kinetic energy and intensit...

A galvanic (voltaic) cell uses a spontaneous redox reaction (E°cell > 0, ΔG° < 0) to produce electrical energy. Oxidation occurs at the anode (negative) and reduction at the cathode (positive), so electrons flow anode → cathode through t...

The psychoanalytic perspective, from Freud, explains personality and behavior as shaped by unconscious drives and conflicts, many rooted in early childhood. The most testable points are the id–ego–superego conflict and the defense mechan...

Eukaryotic describes cells with a membrane-bound nucleus and other membrane-bound organelles. Biochemically, they typically have linear chromosomes wrapped on histones, 80S ribosomes, and processed mRNA. Testable: transcription (nucleus)...

Social reproduction is the process by which a society's inequalities, especially class position, are passed from one generation to the next, often through schools and families. The testable link: Bourdieu's cultural capital helps explain...

An alkene is a hydrocarbon with at least one carbon–carbon double bond (one σ and one π bond). The sp² carbons are planar, and the electron-rich π bond makes alkenes typically react by electrophilic addition, such as HX adding by Markovn...

Atmospheric pressure is the pressure exerted by the weight of the air above a point. At sea level it is 1 atm = 101.3 kPa = 760 mmHg (760 torr), and it decreases as altitude increases. The most testable point is that absolute pressure in...

The reaction quotient Q has the same form as the equilibrium constant expression but uses the concentrations or pressures present at any moment. Compare Q with K: Q < K, the reaction proceeds forward; Q > K, in reverse; Q = K, at equilib...

Stereotype threat is the concern a person feels about confirming a negative stereotype of their group, which can impair performance on a task tied to that stereotype. It is a situational effect, not a measure of true ability, and it tend...

The pharynx is the muscular throat tube behind the nasal and oral cavities, shared by the respiratory and digestive tracts. Air passes on to the larynx and trachea, and food passes to the esophagus. The testable point is how swallowing k...

The centromere is the constricted region of a chromosome where sister chromatids are joined and where the kinetochore forms for spindle attachment. Chromosomes are counted by centromeres, so sisters become separate chromosomes only when...

Decay is the spontaneous change of an unstable nucleus into a more stable one by emitting alpha, beta or gamma radiation. Each nucleus decays at random, but a sample's undecayed amount falls exponentially, halving every half-life.

A weak acid only partially ionizes in water, so HA ⇌ H⁺ + A⁻ lies mostly toward HA and Ka is small (Ka < 1). The most testable point: a weaker acid has a smaller Ka, a larger pKa, and a stronger conjugate base.

A ketal is a carbon bonded to two OR groups and two carbon groups, R₂C(OR′)₂, made by reacting a ketone with two alcohols (or one diol) under acid catalysis. The most testable point: ketals resist base, nucleophiles and hydrides, so they...

Projection is a psychoanalytic (Freudian) defense mechanism in which a person attributes their own unacceptable thoughts, feelings or impulses to someone else, reducing anxiety. Testable point: the feeling is assigned to the other person...

Formal charge is the charge an atom would have in a Lewis structure if every bonding pair were shared equally. The best Lewis structure usually has formal charges closest to zero, with any negative charge on the more electronegative atom.

Coulomb's law gives the electrostatic force between two point charges: it is proportional to the product of the charges and inversely proportional to the square of their separation. Like charges repel and opposite charges attract. Doubli...

Photoreceptors are the rods and cones, retinal cells that transduce light into neural signals. Rods work in dim light and give no color information; cones need brighter light and give color and sharp detail, and are concentrated in the f...

Endocrine signaling is when ductless glands release hormones into the blood, which carry them to distant target cells that have the matching receptor. Most testable point: endocrine signals travel through the bloodstream, unlike exocrine...

Replication is the copying of DNA before cell division. It is semiconservative: each daughter helix has one old strand and one new strand, built 5′→3′ by DNA polymerase, continuously on the leading strand and in Okazaki fragments on the...

Gas chromatography (GC) separates a vaporized mixture as an inert carrier gas moves it through a column coated with a stationary phase. Components that are more volatile and interact less with the stationary phase generally elute first,...

A stressor is an event, condition or demand that triggers the stress response. Stress is the reaction, not the stimulus. Whether an event acts as a stressor depends largely on appraisal, so the same event can be a threat for one person a...

Buoyant force is the upward force a fluid exerts on an object in it. By Archimedes' principle, it equals the weight of the fluid displaced, so it depends on fluid density and displaced volume. For a given displaced volume it ignores the...

Graham's law says that at the same temperature, a gas's effusion rate (escape through a tiny hole) is inversely proportional to the square root of its molar mass, so lighter gases effuse faster. It is also applied, approximately, to diff...

A social network is a web of social ties linking individuals or groups, through which support, information, resources, and influence flow. Testable point: strong ties (close family, friends) tend to give emotional support, while weak tie...

Nucleophilic addition is a reaction in which a nucleophile attacks the electrophilic carbon of a C=O (or other polar π bond), breaking the π bond and forming a tetrahedral intermediate. Aldehydes are generally more reactive than ketones.

A reading frame is the way an mRNA is divided into consecutive, non-overlapping three-nucleotide codons, set by the start codon AUG. Testable point: an insertion or deletion that is not a multiple of three shifts the frame (a frameshift)...

Attribution theory is the social psychology account of how people explain behavior, as dispositional (internal traits) or situational (external circumstances). Most testable: observers often favor dispositional explanations for others (f...

A gene is a segment of DNA carrying the information for a functional product, usually a protein but sometimes a functional RNA such as tRNA or rRNA. Most testable: a gene is transcribed into RNA, and in eukaryotes introns are spliced out...

A first-order reaction has a rate proportional to the concentration of one reactant raised to the first power (rate = k[A]), so doubling [A] doubles the rate. Its most testable features are a constant half-life, t½ = 0.693/k, and a strai...

Magnetic describes fields and forces from moving charges (currents) and from the intrinsic magnetic moments in magnets. A magnetic field exerts a force on a moving charge perpendicular to both its velocity and the field, so it changes di...

Sanctions are social reactions that enforce norms: rewards (positive sanctions) for conforming and punishments (negative sanctions) for deviating. They can be formal (laws, fines) or informal (praise, gossip, ostracism), and they are a k...

A chiral center (stereocenter) is an atom, most often an sp³ carbon, bonded to four different groups, so the center has two possible configurations (R and S). A molecule with one such center is nonsuperimposable on its mirror image; n ce...

Cognitive behavioral therapy (CBT) is a structured psychotherapy that treats problems such as depression and anxiety by identifying and changing maladaptive thoughts and behaviors. Its key idea is that thoughts influence emotions and act...

An ideal gas is a model in which particles have negligible volume and no attractions or repulsions, so PV = nRT applies. Real gases behave most ideally at high temperature and low pressure, and small nonpolar gases such as He come closest.