MCAT Chem/Phys key terms

A measure of the total heat content of a system at constant pressure, equal to internal energy plus pressure times volume; changes in enthalpy tell how much heat a reaction absorbs or releases.

A measure of disorder or the number of microscopic arrangements available to a system; it tends to increase in isolated systems, driving spontaneous processes toward greater dispersion of energy.

A thermodynamic quantity combining enthalpy, entropy, and temperature; a negative change means a process is spontaneous at constant temperature and pressure, while positive means nonspontaneous.

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts in the direction that partially offsets the disturbance and restores equilibrium.

A solution containing a weak acid and its conjugate base, or a weak base and its conjugate acid, that resists large pH changes when small amounts of strong acid or base are added.

An equilibrium constant describing how readily a weak acid donates a proton in water; larger values indicate a stronger acid, and its negative logarithm is pKa.

The equilibrium constant for dissolution of a sparingly soluble ionic solid, equal to the product of ion concentrations raised to their stoichiometric coefficients in a saturated solution.

The total enthalpy change of a reaction depends only on initial and final states, so it equals the sum of enthalpy changes for any sequence of steps leading from reactants to products.

A protein catalyst that speeds up a biochemical reaction by lowering its activation energy without being consumed, often showing high specificity for particular substrates and reaction types.

The substrate concentration at which an enzyme works at half its maximum rate; a smaller Km indicates tighter substrate binding and higher apparent affinity.

A form of enzyme inhibition in which a molecule resembling the substrate binds the active site, preventing substrate binding; it raises apparent Km but leaves maximum velocity unchanged at high substrate.

Control of enzyme activity by a molecule binding at a site separate from the active site, changing the enzyme's shape and either enhancing or reducing its catalytic function.

The cytoplasmic pathway that breaks one glucose into two pyruvate molecules, producing a small amount of ATP and NADH and serving as the first stage of cellular respiration.

The mitochondrial process in which electron transport creates a proton gradient that drives ATP synthase, producing most of the ATP made during aerobic metabolism.

A covalent bond joining the carboxyl group of one amino acid to the amino group of another, releasing water; it forms the backbone of proteins and has partial double-bond character.

Loss of a protein's functional three-dimensional shape caused by heat, extreme pH, or chemicals, typically disrupting weak interactions while leaving the amino acid sequence intact.

The net force on an object equals its mass times its acceleration; this relationship explains how forces change motion and is written as F = ma.

The net work done on an object equals its change in kinetic energy, linking forces acting over a distance to the object's speed.

In an isolated system with no external net force, the total momentum of all objects remains constant; this principle underlies collisions and recoil problems.

The acceleration of an object moving in a circle, directed toward the center, with magnitude equal to speed squared divided by radius; it changes direction, not speed.

A measure of a force's tendency to rotate an object about an axis, calculated as force times the perpendicular distance from the axis to the force's line of action.

The voltage across a conductor equals the current through it multiplied by its resistance; this relationship holds for many ohmic materials at constant temperature.

The ejection of electrons from a metal surface when light of sufficiently high frequency strikes it, demonstrating that light carries energy in discrete packets called photons.

The change in observed frequency of a wave when the source, the observer, or both move relative to the medium, causing higher pitch when approaching and lower when receding.

An electron-rich atom or molecule that donates an electron pair to form a new bond, often attacking an electron-poor carbon in substitution or addition reactions.

An electron-poor atom or molecule that accepts an electron pair to form a new bond, commonly acting as a target for nucleophiles in addition and substitution reactions.

A one-step substitution in which a nucleophile attacks a carbon from the side opposite a leaving group, causing inversion of stereochemistry and proceeding fastest at unhindered carbons.

When a hydrogen halide adds to an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogens, placing the halogen on the more substituted carbon.

The property of a molecule that cannot be superimposed on its mirror image, usually due to a carbon bonded to four different groups; such molecules rotate plane-polarized light.

A way of describing electron delocalization by drawing multiple valid Lewis structures; the real molecule is a hybrid with electron density spread over the participating atoms.

A property of cyclic, planar, fully conjugated molecules with a continuous ring of p orbitals containing 4n+2 pi electrons, making them unusually stable.

The maintenance of a relatively stable internal environment in an organism, achieved through physiological feedback mechanisms that counteract changes in temperature, pH, and other variables.

The movement of water across a semipermeable membrane from a region of lower solute concentration to one of higher solute concentration, equalizing water potential.

Movement of substances across a cell membrane against their concentration gradient using energy, typically from ATP hydrolysis, often carried out by pump proteins.

A rapid electrical signal traveling along a neuron membrane, caused by sequential changes in ion permeability that briefly reverse the membrane potential and then restore it.

The division of a eukaryotic cell's nucleus producing two genetically identical daughter cells, used for growth, repair, and asexual reproduction in many organisms.

A model describing allele and genotype frequencies that remain constant in a large population with random mating, no mutation, no migration, and no selection.

The process by which a cell converts an extracellular signal, such as a hormone, into an intracellular response through a series of molecular relay steps.