1,800+ MCAT terms, defined.

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...

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...

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.

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...

Malate is a four-carbon citric acid cycle intermediate, formed when fumarase hydrates fumarate and oxidized by malate dehydrogenase to oxaloacetate, producing NADH. Testable point: fumarate → malate is a hydration with no electron carrie...

The Na+/K+ ATPase is a membrane enzyme that hydrolyzes ATP to pump 3 Na+ out of the cell and 2 K+ in, each against its concentration gradient (primary active transport). Most testable: the 3:2 ratio and the Na+ gradient it builds, which...

A binding protein is a protein that reversibly and specifically binds a ligand without chemically changing it, as hemoglobin does with O₂. Affinity is measured by Kd: a lower Kd means tighter binding, and Kd equals the ligand concentrati...

An integral protein is embedded in the phospholipid bilayer by hydrophobic interactions between its nonpolar side chains and the lipid tails. Most span the membrane, though some sit in one leaflet. The testable point: it is released only...

A glycerophospholipid is a glycerol backbone with two fatty acid tails (ester bonds) and a phosphate linked to a polar head group, so it is amphipathic. It is the main lipid type of cell membranes and forms bilayers with the tails inward...

Secondary active transport moves one solute against its gradient by coupling it to another solute (often Na⁺) moving down its gradient through the same carrier. The carrier does not hydrolyze ATP, but ATP is used indirectly by primary ac...

A lyase is an enzyme (EC class 4) that breaks C–C, C–O, or C–N bonds without water and without redox, typically leaving a double bond. It can also run in reverse, adding a group across a double bond. On the MCAT, tell it apart from hydro...

Dehydration (a condensation reaction) joins two molecules by removing the elements of water, so H₂O is released. It forms peptide, glycosidic and ester bonds. Linking n monomers into a linear chain releases n − 1 waters, and the reverse...

A post-translational modification is a chemical change made to a protein after the ribosome has made it, such as phosphorylation, glycosylation, or cleavage. It can alter activity, location, or stability without changing the gene. Phosph...

Branching enzyme is a glycogen-synthesis enzyme that moves a block of about seven glucose residues from the end of an α-1,4 chain to an interior glucose, forming an α-1,6 linkage. Branching makes glycogen more soluble and gives more chai...

DNA ligase is the enzyme that seals a nick in the sugar-phosphate backbone by forming a phosphodiester bond between a 3′ OH and an adjacent 5′ phosphate. It joins Okazaki fragments on the lagging strand, and it also seals nicks in DNA re...

A glycoprotein is a protein with short, often branched carbohydrate chains covalently attached, added in the ER and Golgi. The most testable point: sugars link to asparagine (N-linked) or to serine/threonine (O-linked), and they aid fold...

Methyl is the –CH₃ group, a small nonpolar, hydrophobic carbon group. In biochemistry it is the side chain of alanine and the group added in methylation. Methylation of cytosine in DNA is typically linked to reduced transcription, withou...

The fluid mosaic model pictures the cell membrane as a phospholipid bilayer with proteins embedded in it, and both lipids and proteins can drift sideways within the layer. Most testable point: unsaturated (kinked) tails and higher temper...

The hydrophobic effect is the tendency of nonpolar groups to cluster together in water. It is driven mainly by the entropy gain of the surrounding water, not by strong attraction between the nonpolar groups. It is a major driving force f...

Northern blotting detects a specific RNA (usually mRNA) in a sample. RNA is separated by gel electrophoresis, transferred to a membrane, and hybridized to a labeled complementary probe. Band presence, size, and intensity show whether a g...

Recombinant DNA is DNA made by joining sequences from different sources, usually a gene cut and ligated into a plasmid vector, then copied in host cells such as bacteria. Testable point: antibiotic resistance selects cells that took up t...

A disulfide bond is a covalent S–S linkage formed when the thiol (–SH) groups of two cysteine side chains are oxidized. It stabilizes tertiary and quaternary structure, especially in secreted and extracellular proteins, and is broken by...

Hydrophilic means water-loving: a polar or charged molecule or region that interacts favorably with water by hydrogen bonding or ion–dipole forces. Testable point: in folded soluble proteins, hydrophilic side chains tend to face the aque...

A transferase is an enzyme that moves a functional group, such as a phosphate or amino group, from a donor molecule to an acceptor molecule. Kinases and aminotransferases are examples. A testable point: a phosphatase is a hydrolase, not...

Voltage-gated channels are ion-selective membrane proteins that open or close in response to changes in membrane potential, letting ions move down their electrochemical gradients. In a neuron, Na⁺ channels open at threshold and drive the...

The Q cycle is how Complex III passes electrons from ubiquinol (QH₂), a two-electron carrier, to cytochrome c, a one-electron carrier, while adding to the proton gradient. Per QH₂ oxidized, 2 cytochrome c are reduced and 4 H⁺ are release...

An allosteric site is a binding site on a protein, separate from the active site, where a regulator (effector) binds and changes the protein's shape and activity. Effectors can activate or inhibit, and cooperative allosteric enzymes typi...

A domain is a compact region of a single polypeptide that folds into a stable unit and often carries a specific function, such as binding a ligand, binding DNA, spanning a membrane or catalysis. Many domains can fold independently, so de...

A monosaccharide is a single sugar unit, a polyhydroxy aldehyde (aldose) or ketone (ketose) with the formula (CH₂O)n, usually n = 3 to 7. It cannot be hydrolyzed into smaller sugars. Most testable: glucose and galactose are aldohexoses,...

In biochemistry, aromatic usually describes amino acids whose side chains contain a planar ring of delocalized π electrons: phenylalanine, tyrosine, and tryptophan. They mostly sit in protein interiors, and Trp and Tyr absorb UV light ne...

Isocitrate is a six-carbon citric acid cycle intermediate formed from citrate by aconitase. Isocitrate dehydrogenase converts it to α-ketoglutarate, releasing CO₂ and making NADH; this is a key regulated, rate-limiting step of the cycle.

A prosthetic group is a non-amino-acid cofactor, such as heme or FAD, that stays tightly bound to a protein, often covalently, and is needed for its function. Unlike a loosely bound coenzyme such as NAD⁺, it usually remains attached thro...

An mRNA transcript is the RNA copy of a gene, made by RNA polymerase, that ribosomes read to build a polypeptide. It is complementary to the template strand and matches the coding strand with U in place of T. Eukaryotic transcripts are p...

A zwitterion is a molecule carrying both a positive and a negative charge, so its net charge is zero. An amino acid near neutral pH is the standard case (−NH₃⁺ and −COO⁻). Testable point: the zwitterion predominates at the isoelectric po...

A thiol is an R–SH group, the sulfur analog of an alcohol; in proteins it is the cysteine side chain. Oxidation joins two thiols into a disulfide bond (R–S–S–R) that stabilizes tertiary and quaternary structure, and reducing agents break...

A second messenger is a small intracellular molecule or ion (cAMP, IP₃, DAG, Ca²⁺) released after an extracellular first messenger binds a surface receptor; it relays and amplifies the signal. Testable: hydrophilic hormones usually signa...

Histone acetylation is the transfer of an acetyl group from acetyl-CoA to lysine residues on histone tails, done by histone acetyltransferases (HATs). It neutralizes the lysine's positive charge, loosens DNA-histone binding, and generall...

Integrins are transmembrane receptors, each made of an α and a β subunit, that bind extracellular matrix proteins such as fibronectin, collagen and laminin and connect to the cytoskeleton inside. Testable point: integrins link cell to ma...

Urea, (NH₂)₂C=O, is the main nitrogen waste from amino acid breakdown in humans. The liver makes it from toxic ammonia in the urea cycle, and the kidneys excrete it. Testable point: the liver makes urea, and it is not uric acid, which co...

Sigmoidal means S-shaped. In biochemistry it marks cooperative binding, as in hemoglobin's O₂ curve or the rate curve of many allosteric enzymes. Non-cooperative binding, as in myoglobin or Michaelis-Menten enzymes, gives a hyperbola ins...

Structural proteins give cells and tissues shape, support and strength. Examples are collagen, elastin, keratin, and the cytoskeletal proteins actin and tubulin. Collagen and keratin are fibrous, but actin and tubulin are globular subuni...

The extracellular matrix (ECM) is the network of secreted proteins and polysaccharides between animal cells, especially in connective tissue. Collagen gives tensile strength, elastin gives recoil, and proteoglycans resist compression. In...

Inhibition is a decrease in an enzyme's reaction rate caused by an inhibitor binding to it, reversibly or irreversibly. Most testable: a competitive inhibitor raises apparent Km and leaves Vmax unchanged, while a pure noncompetitive inhi...

Actin filaments (microfilaments) are the thinnest cytoskeletal fibers, polymers of globular G-actin that form polar F-actin strands. They shape the cell, drive crawling and cytokinesis, and with myosin power muscle contraction. Testable:...

A dehydrogenase is an oxidoreductase that oxidizes its substrate by removing hydrogen (as hydride plus a proton) and passing the electrons to a coenzyme such as NAD⁺ or FAD. The coenzyme is reduced, to NADH or FADH₂, and that NADH or FAD...

Selectins are cell-surface adhesion proteins that bind specific carbohydrate groups on other cells' glycoproteins in a Ca²⁺-dependent way. They mediate the weak, transient binding that makes leukocytes roll along blood vessel walls befor...

A zymogen is an inactive enzyme precursor that is activated when a protease cuts specific peptide bonds, as in trypsinogen → trypsin. Because the cleavage is covalent, activation is irreversible, and it protects the cells that make prote...

Epimers are diastereomers that differ in configuration at exactly one stereocenter and match at every other. The classic MCAT examples are D-glucose and D-galactose (C4 epimers) and D-glucose and D-mannose (C2 epimers). Anomers are epime...

Chemiosmosis is the use of a proton (H⁺) gradient across a membrane to make ATP: H⁺ flows down its gradient through ATP synthase, and that flow drives ADP + Pᵢ → ATP. Most testable: the electron transport chain only builds the gradient,...

Phosphorylase (glycogen phosphorylase) breaks α-1,4 glycosidic bonds at the ends of glycogen using inorganic phosphate, releasing glucose-1-phosphate in glycogenolysis. Testable point: glucagon and epinephrine activate it by phosphorylat...