MCAT Bio/Biochem key terms

A small molecule built around a central carbon bearing an amino group, a carboxyl group, a hydrogen, and a variable side chain; the side chain's chemistry determines how the unit behaves inside a protein.

A molecule carrying both a positive and a negative charge at the same time, giving it no net charge overall. Amino acids exist this way near neutral pH because their acidic and basic groups are both ionized.

The pH at which a molecule's positive and negative charges exactly balance, so its average net charge is zero. At this pH a dissolved amino acid or protein moves least in an electric field.

The covalent link formed when the carboxyl group of one amino acid reacts with the amino group of another, releasing water. The resulting bond is flat and rigid, which limits how the backbone can bend.

The exact linear order of amino acids in a polypeptide chain, read from the free amino end to the free carboxyl end. This sequence is encoded by DNA and dictates all higher levels of folding.

A common protein folding pattern in which the backbone coils into a right-handed spiral held together by hydrogen bonds between every fourth residue. Side chains project outward from the spiral.

A protein folding pattern in which stretches of backbone line up side by side as flat strands, linked by hydrogen bonds between strands. Strands may run in the same or opposite directions.

Local, repeating folding patterns such as helices and sheets that a polypeptide adopts, stabilized mainly by hydrogen bonds between backbone atoms rather than side chains.

The overall three-dimensional shape of a single folded polypeptide, produced by interactions among side chains such as hydrophobic clustering, hydrogen bonds, salt bridges, and disulfide links.

A covalent cross-link between the sulfur atoms of two cysteine side chains. It can lock parts of a protein together and is usually formed in the oxidizing environment outside the cell.

The tendency of nonpolar groups to cluster together in water because water molecules prefer to hydrogen-bond with each other. This clustering is a major driving force in protein folding and membrane assembly.

Loss of a protein's functional folded shape caused by heat, extreme pH, or chemicals, without breaking the peptide bonds. The chain may unfold and clump, and activity is usually lost.

A protein (or occasionally RNA) that speeds up a specific biochemical reaction by lowering its activation energy, without being consumed. Its shape creates a binding site tailored to particular reactants.

The pocket or groove on an enzyme where substrate binds and chemistry happens. Its shape and chemical groups position reactants precisely and stabilize the reaction's transition state.

Any substance that increases the rate of a reaction by providing a lower-energy pathway, while emerging unchanged at the end. It does not alter the overall energy difference between reactants and products.

The energy barrier that reactants must overcome for a reaction to proceed. Enzymes reduce this barrier, so more molecules can react at a given temperature.

The substrate concentration at which an enzyme works at half its maximum rate, written Km. A small value suggests the enzyme binds substrate tightly and reaches saturation easily.

A reversible block in which an inhibitor resembling the substrate occupies the enzyme's active site. Raising substrate concentration can outcompete it, so maximum rate is unchanged but more substrate is needed.

Control of an enzyme by a molecule binding at a site away from the active site, shifting the protein between more and less active shapes. Such regulators may be activators or inhibitors.

A control mechanism in which the end product of a metabolic pathway shuts down an earlier enzyme in that same pathway. This prevents wasteful buildup of product.

A small organic helper molecule that associates loosely with an enzyme and carries chemical groups or electrons during catalysis. Many are derived from vitamins.

A molecule composed of carbon, hydrogen, and oxygen, often with the hydrogen and oxygen in a two-to-one ratio. Simple sugars link into chains that store energy or serve as structural material.

A single sugar unit that cannot be broken into smaller sugars, such as glucose or fructose. It typically contains three to seven carbons and exists in ring form in solution.

The covalent linkage joining two sugar units, formed with the loss of a water molecule. Its geometry affects whether the resulting polysaccharide is digestible or structural.

A broadly defined class of molecules that dissolve poorly in water but readily in nonpolar solvents, including fats, steroids, and phospholipids. Their water-shunning nature suits them for membranes and energy storage.

A long hydrocarbon chain ending in a carboxyl group. Saturated chains have no double bonds and pack tightly, while unsaturated chains kink at double bonds and stay more fluid.

A lipid with a phosphate-containing, water-loving head and two water-fearing tails. In water these molecules self-assemble into bilayers, the basic fabric of cell membranes.

The building block of nucleic acids, made of a nitrogenous base, a five-carbon sugar, and one or more phosphate groups. Nucleotides also serve as energy currency and signaling molecules.

The covalent link that joins the sugar of one nucleotide to the phosphate of the next, forming the sugar-phosphate backbone of DNA or RNA. It gives the strand directionality.

The specific hydrogen bonding between complementary nitrogenous bases, such as adenine with thymine and guanine with cytosine in DNA. This pairing lets one strand template the other.

Adenosine triphosphate, the cell's main short-term energy currency. Breaking its terminal phosphate bond releases usable energy that powers endergonic reactions and transport.

The sequence of reactions in the cytosol that splits one glucose into two pyruvate molecules, producing a small amount of ATP and reducing equivalents. It works with or without oxygen.

An anaerobic route that regenerates the electron carrier NAD+ by transferring electrons to pyruvate or a derivative, yielding lactate or ethanol. It lets glycolysis keep running without oxygen.

A cyclic set of mitochondrial reactions that fully oxidizes acetyl groups derived from fuel molecules, capturing energy as reduced carriers and a little ATP while releasing carbon dioxide.

The process in which electrons passed down a membrane protein chain pump protons, and the returning protons drive ATP synthesis. Oxygen is the final electron acceptor in this pathway.

A series of membrane-embedded protein complexes that pass electrons from carriers to a final acceptor, releasing energy used to move protons across the membrane.

Use of a proton gradient across a membrane to do work, chiefly driving ATP synthesis as protons flow back through a channel enzyme. It couples electron transport to energy storage.

A small molecule that accepts electrons from one reaction and delivers them to another, shuttling reducing power. Common examples include NADH and FADH2.

The thin lipid bilayer studded with proteins that encloses a cell and controls what enters and leaves. Its interior is oily, so small nonpolar molecules cross more easily than charged ones.

A double layer of phospholipids with tails facing inward and heads facing water on both sides. It forms a stable barrier that seals off compartments.

Passive movement of a substance across a membrane through a channel or carrier protein, always down its concentration gradient and requiring no energy input.

Movement of a substance across a membrane against its concentration gradient, powered by energy such as ATP hydrolysis or an existing ion gradient.

A double-membrane organelle where the citric acid cycle and oxidative phosphorylation occur. Its folded inner membrane houses the electron transport machinery.

Division of a cell's nucleus that produces two genetically identical daughter nuclei, usually followed by splitting of the cytoplasm. It underlies growth and repair in body tissues.

A two-round division that halves the chromosome number and shuffles genetic material, producing gametes. Each resulting cell carries one set of chromosomes rather than two.

The ordered sequence of growth, DNA replication, and division a cell passes through. Checkpoints verify that each stage is complete and accurate before the next begins.

Exchange of matching chromosome segments between paired homologous chromosomes during meiosis. It creates new combinations of alleles on a single chromosome.