1,800+ MCAT terms, defined.

An operon is a cluster of genes, typical of prokaryotes, that share one promoter and operator and are transcribed together as one mRNA. The most testable point: the lac operon is inducible (allolactose releases the repressor), while the...

H₂O is water, a bent polar molecule that hydrogen bonds, dissolves ions and polar molecules, and acts as a reactant in hydrolysis or a product in condensation. Most testable: nonpolar groups cluster away from water (hydrophobic effect),...

Hydrolysis is the cleavage of a bond by water, with H added to one fragment and OH to the other. It is the reverse of condensation (dehydration synthesis). Testable point: it splits peptide, glycosidic, ester and ATP phosphoanhydride bon...

The isoelectric point (pI) is the pH at which a molecule, such as an amino acid or protein, has no net charge. Below the pI it carries a net positive charge, and above it a net negative charge. For an amino acid with a nonionizable side...

CO₂ is carbon dioxide, the fully oxidized carbon end product of fuel breakdown, released in decarboxylation steps such as pyruvate dehydrogenase and the citric acid cycle. In blood it travels mostly as bicarbonate, so a rise in CO₂ lower...

A glycosidic bond is the covalent C–O–C link, formed by condensation, between a sugar's anomeric carbon and an oxygen of another molecule, usually another sugar's OH. Its α or β configuration matters: amylase cleaves α-1,4 links but not...

Triacylglycerols (triglycerides) are glycerol joined to three fatty acids by ester bonds. They are the body's main long-term energy store, kept in adipocytes, and give more energy per gram than carbohydrate because their carbons are more...

An inducer is a small molecule that turns on transcription of an operon, usually by binding the repressor and causing it to release the operator. In the lac operon, allolactose (an isomer of lactose) is the natural inducer. A corepressor...

A peptide bond is the covalent amide bond joining the carboxyl carbon of one amino acid to the amino nitrogen of the next, formed by a condensation (dehydration) reaction that releases H₂O. Resonance gives it partial double-bond characte...

Km is the Michaelis constant: the substrate concentration at which an enzyme works at half of Vmax. A lower Km means the enzyme reaches half-maximal rate at lower substrate, which usually indicates higher apparent affinity for the substr...

A cofactor is a nonprotein helper, either a metal ion or a small organic molecule (a coenzyme, often vitamin-derived), that an enzyme needs to be active. The enzyme without it is an apoenzyme; with it, a holoenzyme. Testable point: losin...

A repressor is a regulatory protein that binds DNA and lowers transcription of a gene, in bacteria usually by binding the operator and blocking RNA polymerase. Testable point: an inducer (allolactose in lac) pulls the repressor off DNA,...

Motor proteins (myosin, kinesin, dynein) use energy from ATP hydrolysis to move along cytoskeletal filaments. Myosin works on actin; kinesin and dynein work on microtubules, with most kinesins moving toward the plus end and dynein toward...

Gel electrophoresis separates charged macromolecules, such as DNA or proteins, by driving them through a porous gel in an electric field. With uniform charge per mass, as for DNA fragments or proteins in SDS-PAGE, smaller molecules trave...

Telomeres are repeated, noncoding DNA sequences (TTAGGG in humans) at the ends of linear eukaryotic chromosomes. They shorten with each round of replication because DNA polymerase cannot fully copy the lagging-strand end, unless telomera...

Alternative splicing joins the exons of one eukaryotic pre-mRNA in different combinations, producing different mature mRNAs and protein isoforms from a single gene. It is a main reason a cell can make more distinct proteins than it has g...

Lipid rafts are small, dynamic regions of the plasma membrane enriched in cholesterol and sphingolipids. They are thicker and more ordered (less fluid) than the surrounding bilayer and are thought to cluster certain signaling proteins. C...

Anomers are two cyclic sugar stereoisomers that differ in configuration at the anomeric carbon, the former carbonyl carbon made a new stereocenter by ring closure. In a D-sugar, α has the anomeric OH trans to the CH₂OH group and β has it...

Facilitated diffusion is passive movement of polar or charged solutes across a membrane through channel or carrier proteins, down their gradient with no ATP spent. Carrier-mediated transport saturates at a maximum rate, while simple diff...

Phosphodiester bonds are the covalent links between nucleotides in DNA and RNA. A phosphate joins the 3′ carbon of one sugar to the 5′ carbon of the next, forming the sugar-phosphate backbone. Heating or denaturing breaks the hydrogen bo...

Denaturation is the loss of a protein's native 3D shape (secondary, tertiary and quaternary structure) when noncovalent forces or disulfide bonds are disrupted, usually abolishing function. Peptide bonds are not broken, so primary struct...

An inhibitor is a molecule that binds an enzyme and lowers its reaction rate. The MCAT sorts inhibitors by their effect on Km and Vmax: competitive raises Km with Vmax unchanged, noncompetitive lowers Vmax with Km unchanged.

A substrate is the molecule an enzyme binds at its active site and converts to product. Raising substrate concentration speeds the reaction until the enzyme saturates at Vmax, and Km is the [S] at which the rate is half of Vmax.

In biochemistry, synthesis is anabolism: building larger molecules from smaller precursors, as in protein, glycogen, fatty acid and DNA synthesis. It is usually endergonic, so cells typically couple it to ATP hydrolysis or other exergoni...

Ion channels are integral membrane proteins with a water-filled pore that lets specific ions cross the bilayer by facilitated diffusion, down their electrochemical gradient and without ATP. Many are gated (voltage, ligand or mechanical),...

Mutarotation is the gradual change in optical rotation when a pure anomer of a sugar dissolves and equilibrates with its other anomer through the open-chain form. Starting from either pure α- or β-D-glucose, the rotation drifts to the sa...

A reducing sugar has a free anomeric carbon (hemiacetal or hemiketal) that can open to an aldehyde and be oxidized, reducing an agent such as Cu²⁺ in Benedict's or Ag⁺ in Tollens' reagent. Testable point: sucrose is non-reducing because...

SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis) separates denatured proteins mainly by size. SDS gives proteins a similar charge-to-mass ratio, so smaller proteins migrate farther toward the positive electrode (anode).

The active site is the pocket or cleft on an enzyme where the substrate binds and the reaction is catalyzed. Its shape and chemistry match the substrate (lock-and-key or induced fit). Competitive inhibitors bind here, and denaturation th...

The cytosol is the water-based fluid inside the plasma membrane but outside the organelles, holding enzymes, ions and metabolites. Its most testable point is pathway location: glycolysis and fatty acid synthesis occur here, while the cit...

The anomeric carbon is the former carbonyl carbon of a sugar, which becomes a new stereocenter (a hemiacetal or hemiketal carbon) when the sugar cyclizes. Its OH can point either way, giving α and β anomers. A free anomeric OH makes a su...

The double helix is the structure of double-stranded DNA: two antiparallel strands coiled together, with sugar-phosphate backbones outside and bases paired inside by hydrogen bonds (A–T, G–C). Most testable: pairing gives Chargaff's rule...

Hydrophobic means "water-fearing": a nonpolar molecule or group that cannot hydrogen bond with water and tends to cluster away from it. The key testable point is that this clustering is driven mainly by a gain in the entropy of water, no...

The mitochondrial matrix is the compartment enclosed by the inner mitochondrial membrane. It houses pyruvate dehydrogenase, most citric acid cycle enzymes and β-oxidation. The electron transport chain pumps H⁺ out of it, so the matrix is...

Mismatch repair is a post-replication pathway that removes wrongly paired but chemically normal bases from the newly made DNA strand and refills the gap. The testable point is strand choice: bacteria mark the parent strand by methylation...

REDOX is short for reduction-oxidation: reactions in which electrons move from one species to another. The species that loses electrons is oxidized and the one that gains them is reduced. In metabolism, fuels are oxidized as NAD⁺ and FAD...

A kinase is an enzyme that transfers a phosphate group from ATP to a substrate. Protein kinases phosphorylate serine, threonine, or tyrosine side chains to change a protein's activity. Phosphorylation can activate or inhibit, depending o...

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...

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...

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)...

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...

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)...

An uncompetitive inhibitor binds only to the enzyme–substrate complex, not to free enzyme, so it cannot be outcompeted by more substrate. It lowers both Vmax and apparent Km by the same factor, which gives parallel lines on a Lineweaver–...

ATP hydrolysis is the reaction ATP + H₂O → ADP + Pi, which is strongly exergonic (ΔG°′ about −30 kJ/mol). Cells couple it to endergonic processes, and the energy release comes from the products being more stable, not from the bond breaki...

A coenzyme is a small organic, nonprotein molecule, often derived from a vitamin, that an enzyme needs for activity and that carries electrons or chemical groups between reactions (NAD⁺, FAD, coenzyme A). It is one type of cofactor; meta...

Restriction enzymes (restriction endonucleases) are bacterial enzymes that cut double-stranded DNA at specific recognition sequences, often palindromic. Many leave sticky ends, so fragments cut by the same enzyme can be joined by DNA lig...

Fermentation is the anaerobic regeneration of NAD⁺ from NADH by passing electrons to pyruvate (giving lactate) or to acetaldehyde (giving ethanol in yeast). It lets glycolysis keep making its net 2 ATP per glucose without oxygen; the fer...

NH₃ is ammonia, the nitrogen waste released when amino acids are broken down. At physiological pH it exists mostly as ammonium, NH₄⁺. It is toxic, so the liver converts it to urea in the urea cycle for excretion.

The lac operon is a set of E. coli genes (lacZ, lacY, lacA) transcribed from one promoter to use lactose. The repressor blocks it until allolactose binds the repressor. Maximal expression also needs low glucose, so that cAMP–CAP can acti...

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...