Glossary - MCAT Biochemistry Review

MCAT Biochemistry Review


Absolute configuration—The nomenclature system used for the three-dimensional arrangement of atoms in isomers; the most common systems are D/L and (R)/(S).

Acetal—A carbon atom bonded to an alkyl group, two –OR groups, and a hydrogen.

Acetyl-CoA—An important metabolic intermediate that links glycolysis and β-oxidation to the citric acid cycle; can also be converted into ketone bodies.

Activation—The conversion of a biomolecule to its active or usable form, such as activation of tRNA with an amino acid or activation of a fatty acid with CoA to form fatty acyl-CoA.

Activation energy—The energy required to change the state of a molecule or group of molecules to the transition state; the energy required for a reaction to occur.

Active site—The catalytically active portion of an enzyme.

Active transport—The movement of a molecule against its concentration gradient with energy investment; primary active transport uses ATP, whereas secondary active transport uses a favorable transport gradient of a different molecule.

Activity—The measure of the catalytic activity of an enzyme, also called the velocity or rate. It is often measured as a vmax and may be analyzed after protein isolation.

Activity analysis—The determination of the enzymatic activity of an isolated protein by interaction with a substrate; usually colorimetric in nature.

Adenosine triphosphate (ATP)—The primary energy molecule of the body; it releases energy by breaking the bond with the terminal phosphate to form ADP and inorganic phosphate.

Adipocyte—A cell specializing in fat storage.

Aerobic respiration—A collection of energy-producing metabolic processes that require oxygen, including the citric acid cycle, electron transport chain, and oxidative phosphorylation.

Aldose—A sugar in which the highest-order functional group is an aldehyde; can be categorized by number of carbons (triose, tetrose, pentose, hexose, etc.).

Allosteric enzymes—Enzymes that experience changes in their conformation as a result of interactions at sites other than the active site, called allosteric sites; conformational changes may increase or decrease enzyme activity.

α-Helix—An element of secondary structure, marked by clockwise coiling of amino acids around a central axis.

Alternative splicing—The production of multiple different but related mRNA molecules from a single primary transcript of hnRNA.

Amino acid—A dipolar compound containing an amino group (–NH2) and a carboxyl group (–COOH).

Amphipathic—Having both hydrophilic and hydrophobic regions.

Amphoteric—The ability to act as an acid or base.

Amplification—Increased transcription (and translation) of a gene in response to hormones, growth factors, and other intracellular conditions.

Anabolism—The series of metabolic processes that result in the consumption of energy and the synthesis of molecules.

Anaerobic respiration—The series of energy-producing metabolic processes that do not require oxygen, including glycolysis and fermentation.

Anomers—A subtype of epimers in which the chiral carbon with inverted configuration was the carbonyl carbon (anomeric carbon).

Antibody—A specialized protein molecule produced by lymphocytes for interaction with antigens; antibodies consist of two heavy and two light chains that have constant and variable regions. Antibodies, also called immunoglobulins, are mediators of the immune response.

Anticodon—A three-nucleotide sequence on a tRNA molecule that pairs with a corresponding mRNA codon during translation.

Antigen—The region of a molecule that interacts with an antibody; in most cases, antigens are proteins.

Apoenzyme—An enzyme devoid of the prosthetic group, coenzyme, or cofactor necessary for normal activity.

Apolipoproteins—Protein molecules responsible for the interaction of lipoproteins with cells and the transfer of lipid molecules between lipoproteins; also called apoproteins.

Aromaticity—The ability of a molecule to delocalize π electrons around a conjugated ring, creating exceptional stability.

Basal metabolic rate—The amount of energy consumed in a given period of time by a resting organism.

β-oxidation—The catabolism of fatty acids to acetyl-CoA.

β-pleated sheet—An element of secondary structure, marked by peptide chains lying alongside one another, forming rows or strands.

Bile—A mixture of salts, pigments, and cholesterol that acts to emulsify lipids in the small intestine.

Bradford protein assay—A colorimetric method of determining the concentration of protein in an isolate against a protein standard; relies on a transition of absorption between bound and unbound Coomassie Brilliant Blue dye.

Cahn–Ingold–Prelog system—The system used to name isomers ((E) vs. (Z); (R) vs. (S)), based on the atomic numbers of their substituents and their orientation in three-dimensional space.

Calorimeter—A device for measuring the heat change during the course of a reaction.

Carboxylation—The addition of carboxylic acid groups to a molecule.

Carotenoids—A group of molecules that are tetraterpenes (made of eight isoprene units).

Catabolism—The series of metabolic processes that result in the release of energy and the breakdown of molecules.

Catecholamines—Mediators of the sympathetic nervous system and adrenal gland; include epinephrine and norepinephrine.

Cell adhesion molecules—Specialized structural proteins that are involved in cell–cell junctions as well as transient cellular interactions; common cell adhesion molecules are cadherins, integrins, and selectins.

Central dogma of molecular biology—The major steps in the transfer of genetic information, from transcription of DNA to RNA to translation of that RNA to protein.

Centrifugation—The process of separating components on the basis of their density and resistance to flow by spinning a sample at very high speeds; the highest density materials form a solid pellet and the lowest density materials remain in the supernatant (liquid portion).

Ceramide—The simplest sphingolipid, with a single hydrogen as its head group.

Cerebroside—A sphingolipid containing a carbohydrate as a head group.

Chaperones—Proteins that assist in protein folding during posttranslational processing.

Chemiosmotic coupling—The utilization of the proton-motive force generated by the electron transport chain to drive ATP synthesis in oxidative phosphorylation.

Chiral—Describes a molecule with a nonsuperimposable mirror image.

Chiral center—A carbon atom bound to four different substituents; a chiral compound must have at least one chiral center.

Cholesterol—A molecule containing four linked aromatic rings; cholesterol provides both fluidity and stability to cell membranes and is the precursor for steroid hormones.

Chromatography—The process of separating molecules by their interactions with a stationary phase and a mobile phase; most chromatographic methods rely on the chemical similarity of molecules, with the exception of size-exclusion chromatography.

Citric acid cycle—A metabolic pathway that produces GTP, energy carriers, and carbon dioxide as it burns acetyl-CoA; also called the Krebs cycle or tricarboxylic acid (TCA) cycle; can share intermediates with many other metabolic processes including fatty acid and cholesterol synthesis, gluconeogenesis, amino acid metabolism, and others.

Closed system—A system capable of exchanging energy, but not matter, with the environment.

Coding strand—The strand of DNA that is not used as a template during transcription; also called the sense strand.

Codon—A three-nucleotide sequence in an mRNA molecule that pairs with an appropriate tRNA anticodon during translation.

Coenzyme—An organic molecule that helps an enzyme carry out its function.

Cofactor—An inorganic molecule or ion that helps an enzyme carry out its function.

Colligative properties—Physical properties that change according to the concentration of solutes, but not their identity. Colligative properties include vapor pressure depression, boiling point elevation, freezing point depression, and osmotic pressure.

Competitive inhibition—A decrease in enzyme activity that results from the interaction of an inhibitor with the active site of an enzyme; competitive inhibition can be overcome by addition of excess substrate.

Condensation reaction—A reaction in which the removal of a water molecule accompanies the formation of a bond; also called a dehydration reaction.

Conformational coupling—A less-accepted mechanism of ATP synthase activity in which the protons cause a conformational change that releases ATP from ATP synthase.

Conjugated protein—A protein that derives part of its function from covalently attached molecules (prosthetic groups).

Cooperativity—The interaction between subunits of a multi-subunit protein in which binding of substrate to one subunit increases the affinity of other subunits for the substrate; unbinding of substrate from one unit decreases the affinity of other subunits for the substrate.

Corepressor—A species that binds with a repressor, allowing the complex to bind to the operator region of an operon, stopping transcription of the relevant gene.

Cristae—The infoldings of the inner mitochondrial membrane that increase the surface area available for electron transport chain complexes.

C-terminus—The free carboxyl end of a polypeptide.

Degeneracy—A characteristic of the genetic code, in which more than one codon can specify a single amino acid.

Denaturation—The loss of secondary, tertiary, or quaternary structure in a protein, leading to loss of function.

Deoxyribonucleic acid (DNA)—A nucleic acid found exclusively in the nucleus that codes for all the of the genes necessary for life; transcribed to mRNA and always read 5′ to 3′.

Desmosomes—Cell–cell junctions that anchor layers of epithelial cells to one another.

Diastereomers—Compounds with at least one—but not all—chiral carbons in inverted configurations; differ in physical properties.

Diprotic—Containing two hydrogens (acid), or being able to pick up two hydrogens (base).

Disulfide bond—A covalent interaction between the –SH groups of two cysteine residues; an element of tertiary and quaternary structure.

Edman degradation—A stepwise process for determining the amino acid sequence in an isolated protein.

Electrochemical gradient—An uneven separation of ions across a biological membrane, resulting in potential energy.

Electrophoresis—The process of separating compounds on the basis of size and charge using a porous gel and an electric field; protein electrophoresis generally uses polyacrylamide, while nucleic acid electrophoresis generally uses agarose.

Elongation—The three-step cycle that is repeated for each amino acid being added to a protein during translation.

Enantiomers—Compounds that are nonsuperimposable mirror images; have the same physical and chemical properties except for rotation of plane-polarized light and interaction with a chiral environment.

Endocytosis—The transport of molecules into a cell through the invagination of cell membrane and the formation of a vesicle; phagocytosis is the endocytosis of solid, pinocytosis is the endocytosis of liquid.

Endothermic reaction—A reaction that requires heat (positive ΔH).

Enhancer—A collection of several response elements that allow for the control of one gene's expression by multiple signals.

Enthalpy—The overall change in heat of a system during a reaction.

Entropy—The disorder of a system; systems in which entropy is increased are generally favored.

Enzyme—A biological molecule with catalytic activity; includes many proteins and some RNA molecules.

Epimers—A subtype of diastereomers that differ in absolute configuration at exactly one chiral carbon.

Euchromatin—The looser, less-dense collections of DNA that appear light-colored under a microscope; transcriptionally active.

Exocytosis—The transport of molecules out of a cell by release from a transport vesicle; the vesicle fuses to the cell membrane during secretion.

Exon—A portion of hnRNA that is spliced with other exons to form mature mRNA.

Exothermic reaction—A reaction that releases heat (negative ΔH).

Facilitated diffusion—The movement of solute molecules across the cell membrane down their concentration gradients through a transport protein or channel; used for ions and large or polar molecules.

Fatty acid—A monocarboxylic acid without additional substituents; fatty acids may be saturated (all single bonds) or unsaturated (contain at least one double bond); natural fatty acids are in the cis conformation.

Feedback inhibition—The inhibition of an enzyme by its product (or a product further down in a metabolic pathway); used to maintain homeostasis.

Feed-forward activation—The stimulation of an enzyme by an intermediate that precedes the enzyme in a metabolic pathway.

Fermentation—The conversion of pyruvate to either ethanol and carbon dioxide (yeast) or lactic acid (animal cells); does not require oxygen.

Fibrous protein—A protein composed of long sheets or strands, such as collagen.

Fischer projection—A method of drawing organic molecules in which horizontal lines are coming out of the page (wedges) and vertical lines are going into the page (dashes).

Flavin adenine dinucleotide(FAD)—An energy carrier that accepts electrons and feeds them into the electron transport chain.

Flavoprotein—A protein bonded to FAD.

Fluid mosaic model—The representation of the plasma membrane as a dynamic phospholipid bilayer with interactions of cholesterol, proteins, and carbohydrates.

Frameshift mutation—A change in DNA in which the reading frame of the codons in mRNA is shifted due to the insertion or deletion of nucleotides (other than in multiples of three).

Fructose—A monosaccharide found predominantly in fruit and honey.

Furanose—A five-membered ring sugar.

G protein-coupled receptors—A special class of membrane receptors with an associated GTP-binding protein; activation of a G protein-coupled receptor involves dissociation and GTP hydrolysis.

Galactose—A monosaccharide found predominantly in dairy.

Ganglioside—A sphingolipid with a head group containing an oligosaccharide and one or more N-acetylneuraminic acid (NANA) molecules.

Gap junctions—Cell–cell junctions that allow for the passage of small molecules between adjacent cells.

Gene—A unit of DNA that encodes a specific protein or RNA molecule.

Globoside—A sphingolipid with multiple carbohydrate groups attached as a head group.

Globular protein—A protein with a roughly spherical structure, such as myoglobin.

Glucagon—A mediator of glucose release that is secreted by pancreatic α-cells; rises in response to low blood glucose.

Glucogenic—Describes amino acids that can be converted into intermediates that feed into gluconeogenesis; all except leucine and lysine.

Gluconeogenesis—The production of glucose from other biomolecules; carried out by the liver and kidney.

Glucose—The primary monosaccharide used for fuel by all cells of the body, with the formula C6H12O6.

Glycerol—A three-carbon alcohol that serves as the backbone for glycerophospholipids, sphingolipids, and triacylglycerols.

Glycerophospholipid—A lipid containing a glycerol backbone with a phosphate group, bound by ester linkages to two fatty acids.

Glycogen—A branched polymer of glucose that represents a storage form of glucose.

Glycogenesis—The synthesis of glycogen granules.

Glycogenolysis—The breakdown of glycogen granules.

Glycolysis—The breakdown of glucose to two molecules of pyruvate, with the formation of energy carriers (NADH); occurs under both aerobic and anaerobic conditions.

Glycosidic linkage—The bond between the anomeric carbon of a sugar and another molecule.

Glycosphingolipid—A sphingolipid with a head group composed of sugars; includes cerebrosides and globosides.

Glycosylation—The addition of sugars to a molecule.

Haworth projection—A method for depicting cyclic sugars as planar rings with –OH groups sticking up or down from the plane of the sugar.

Hemiacetal—A carbon atom bonded to an alkyl group, an –OR group, an –OH group, and a hydrogen.

Hemiketal—A carbon atom bonded to two alkyl groups, an –OR group, and an –OH group.

Hess's law—A relationship that states that the total change in any state function is the same regardless of the path taken or the number of steps, and is equal to the difference between initial and final values of that state function.

Heterochromatin—The tightly coiled DNA that appears dark-colored under a microscope; transcriptionally inactive.

Heterogeneous nuclear RNA (hnRNA)—The precursor to processed mRNA; converted to mRNA by adding a poly-A tail and 5′ cap, and splicing out introns.

Histone—A structural protein about which DNA is coiled in eukaryotic cells.

Holoenzyme—An enzyme that has already bound a required prosthetic group, coenzyme, or cofactor.

Homeostasis—The stable internal state of an organism; homeostasis is not synonymous with equilibrium.

Homogenization—The process of breaking cell membranes and creating a uniform mixture of cell components for further separation; may be accomplished chemically or physically.

Hormone-sensitive lipase—The enzyme responsible for the mobilization of fatty acids from adipocytes; responds to a decrease in insulin levels.

Hydrolase—An enzyme that catalyzes the cleavage of a molecule with the addition of water.

Hydrolysis—Breaking a covalent bond with the assistance of a water molecule.

Hydrophilic—Being attracted to water; describes polar and charged compounds, and those that can participate in hydrogen bonding.

Hydrophobic—Being repelled by water; describes nonpolar, uncharged compounds (usually lipids or certain R groups of amino acids).

Hypertonic—A solution that has a greater concentration than the one to which it is being compared.

Hypotonic—A solution that has a lower concentration than the one to which it is being compared.

Induced fit model—The best-supported of the most prominent theories of enzyme specificity; states that the enzyme and substrate experience a change in conformation during binding to increase complementarity. Usually contrasted with the lock and key theory.

Inducible system—An operon that requires an inducer to remove a repressor protein from the operator site to begin transcription of the relevant gene; also called a positive control system.

Initiation—The start of translation, in which the small subunit of the ribosome binds to the mRNA molecule, and the first tRNA (methionine or N-formylmethionine) is bound to the start codon (AUG).

Insulin—The primary mediator of carbohydrate metabolism that is secreted by pancreatic β-cells; rises in response to high blood glucose.

Internal energy—The sum of all of the different interactions between and within atoms in a system; vibration, rotation, linear motion, and stored chemical energies all contribute.

Intron—A portion of hnRNA that is spliced out to form mRNA; remains in the nucleus during processing.

Irreversible inhibition—A decrease in enzyme activity that results from the interaction of an inhibitor that binds permanently at either the active site or an allosteric site; in laboratory settings, irreversible inhibitors are sometimes called suicide substrates.

Isoelectric focusing—A specialized method of separating proteins by their isoelectric point using electrophoresis; the gel is modified to possess a pH gradient.

Isoelectric point (pI)—The pH at which an amino acid is predominantly in zwitterionic form.

Isoform—A slightly different version of the same protein, often specific to a given tissue.

Isomerase—An enzyme that catalyzes the constitutional or stereochemical rearrangement of a molecule.

Isotonic—A solution that has the same concentration as the one to which it is being compared.

Jacob–Monod model—The description of the structure and function of operons in prokaryotes, in which operons have structural genes, an operator site, a promoter site, and a regulator gene.

Ketal—A carbon atom bonded to two alkyl groups and two –OR groups.

Ketogenesis—The synthesis of ketone bodies from the metabolic products of β-oxidation or amino acid metabolism; occurs under conditions of starvation.

Ketogenic—Describes amino acids that can be converted into ketone bodies.

Ketolysis—The breakdown of ketone bodies for use as acetyl-CoA.

Ketose—A sugar in which the highest-order functional group is a ketone; can be categorized by number of carbons (triose, tetrose, pentose, hexose, and so on).

Kinase—A specific transferase protein that catalyzes the movement of a phosphate group, generally from ATP, to a molecule of interest.

KmThe concentration of substrate at which an enzyme runs at half its maximal velocity; a measure of enzyme affinity (the higher the Km, the lower the affinity).

Lactose—A disaccharide composed of glucose and galactose.

Lariat—The lasso-shaped structure formed during the removal of introns in mRNA processing.

Ligase—An enzyme that catalyzes the synthesis of large polymeric biomolecules, most commonly nucleic acids.

Lipid—A molecule that is insoluble in water and soluble in nonpolar organic solvents.

Lipoprotein—The transport mechanism for lipids within the circulatory and lymphatic systems; includes chylomicrons and VLDL, which transport triacylglycerols; and HDL, IDL, and LDL, which transport cholesterol and cholesteryl esters.

Lock and key theory—One of the two most prominent theories of enzyme specificity; states that the enzyme and the substrate have a static but complementary state. Less-supported than the induced fit model.

Lyase—An enzyme that catalyzes the cleavage or synthesis of a molecule without the addition or loss of water.

Maltose—A disaccharide composed of two glucose molecules.

Matrix—The contents of the inner mitochondrial membrane; includes soluble enzymes of the electron transport chain and mitochondrial DNA.

Membrane receptors—Transmembrane protein molecules that act enzymatically or as ion channels to participate in signal transduction.

Messenger RNA (mRNA)—The strand of RNA formed after transcription of DNA; transfers to the cytoplasm to be translated.

Micelle—A collection of fatty acid or phospholipid molecules oriented to minimize free energy through hydrophobic and hydrophilic interactions; generally a sphere with a hydrophobic core and hydrophilic exterior.

Missense mutation—A mutation in which one amino acid is substituted for by a different amino acid.

Mitochondria—The organelle responsible for aerobic respiration, generating ATP from the breakdown products of other biomolecules and energy carriers reduced in various metabolic pathways; contains an inner and outer membrane.

Mixed inhibition—A decrease in enzyme activity that results from the interaction of an inhibitor with an allosteric site; mixed inhibitors bind to free enzyme and to substrate-bound enzyme with different affinities. They cannot be overcome by addition of substrate and impact both Km andvmax.

Molten globules—Intermediate states in the folding of a protein.

Monocistronic—The coding pattern of eukaryotes in which one mRNA molecule codes for only one protein.

Monoprotic—Containing only one hydrogen (acid), or being able to pick up only one hydrogen (base).

Monosaccharide—A single sugar monomer; common examples are glucose, galactose, and fructose.

Motor proteins—Proteins that are involved in cell motility through interactions with structural proteins; motor proteins have ATPase activity and include myosin, kinesin, and dynein.

Mutarotation—The rapid interconversion between different anomers of a sugar.

Nicotinamide adenine dinucleotide (NAD+)—An energy carrier that accepts electrons and feeds them into the electron transport chain.

Nicotinamide adenine dinucleotide phosphate (NADP+)—An electron donor produced in the pentose phosphate pathway that is involved in biosynthesis, oxidative stress, and immune function.

NMR spectroscopy—A method of determining molecular structure that uses the relative position of carbons and hydrogens determined by the relative shielding and spins of electrons observed when molecule is exposed to a magnetic field.

Noncompetitive inhibition—A decrease in enzyme activity that results from the interaction of an inhibitor with an allosteric site; noncompetitive inhibitors bind equally well to free enzyme and to substrate-bound enzyme. They cannot be overcome by addition of substrate.

Nonsense mutation—A mutation in which a coding codon is changed to a stop codon. Also called a truncation mutation.

Nontemplate synthesis—The method of de novo synthesis of lipids and carbohydrates that relies on gene expression and enzyme specificity rather than the genetic template of DNA or RNA.

N-terminus—The free amino end of a polypeptide.

Nuclear pore—A hole in the nuclear envelope that permits the entrance and exit of substrates.

Open system—A system capable of exchanging both matter and energy with the environment.

Operator site—A component of the operon in prokaryotes; a nontranscribable region of DNA that is capable of binding a repressor protein.

Operon—In prokaryotes, a cluster of genes transcribed as a single mRNA that can be regulated by repressors or inducers, depending on the system.

Osmosis—The simple diffusion of water.

Osmotic pressure—The pressure necessary to counteract the effect of an osmotic gradient against pure water; one of the colligative properties.

Oxidative phosphorylation—The transfer of a phosphate group, generally to ATP that is powered by a gradient formed by oxidation–reduction reactions; occurs in the mitochondria.

Oxidoreductase—An enzyme that catalyzes an oxidation–reduction reaction, often using an electron carrier as a cofactor.

Pancreatic lipase—The primary enzyme involved in the digestion of lipids.

Pancreatic proteases—The enzymes that are primarily responsible for the digestion of proteins in the small intestine; they include trypsin, chymotrypsin, and carboxypeptidases A and B, and are secreted as zymogens.

Paracellular transport—The transport of materials through the interstitial space without interactions with the cytoplasm or cell membrane.

Passive transport—The movement of a molecule down its concentration gradient without energy investment; includes simple and facilitated diffusion and osmosis.

Pentose phosphate pathway—A metabolic process that produces NADPH and ribose 5-phosphate for nucleotide synthesis.

Peptide—A molecule composed of more than one amino acid; can be subdivided into dipeptides (two amino acids), tripeptides (3), oligopeptides (up to 20), and polypeptides (more than 20).

Peptide bond—An amide bond between the carboxyl group of one amino acid and the amino group of another amino acid.

Phospholipid—A lipid containing a phosphate and alcohol (glycerol or sphingosine) joined to hydrophobic fatty acid tails.

Phosphorylation—The placement of a phosphate group onto a compound.

pKaThe pH at which half of the of the molecules of a given acid are deprotonated; [HA] = [A].

Point mutation—The substitution of one nucleotide for another in DNA.

Polarity—An uneven sharing of electrons in a molecule, creating a slightly positive side and a slightly negative side.

Polycistronic—The coding pattern of prokaryotes, in which one mRNA may code for multiple proteins.

Polyprotic—Containing more than one hydrogen (acid), or being able to pick up more than one hydrogen (base).

Polysaccharide—A long chain of monosaccharides linked by glycosidic bonds; can be divided into homopolysaccharides (only one type of monosaccharide is used) and heteropolysaccharides (more than one type of monosaccharide is used).

Prenylation—The addition of lipid groups to a molecule.

Primary structure—The linear sequence of amino acids in a polypeptide.

Promoter region—The portion of DNA upstream from a gene; contains the TATA box, which is the site where RNA polymerase II binds to start transcription.

Prostaglandin—A group of 20-carbon molecules that are unsaturated carboxylic acids derived from arachidonic acid; act as paracrine or autocrine hormones.

Prosthetic group—A cofactor or coenzyme that is covalently bonded to a protein to permit its function.

Proteinogenic—The ability of certain (20) amino acids to be integrated into proteins.

Proton-motive force—The proton concentration gradient across the inner mitochondrial membrane that is created in the electron transport chain and used in oxidative phosphorylation.

Pyranose—A six-membered ring sugar.

Pyruvate—An important metabolic intermediate that can feed into the citric acid cycle, fermentation, or gluconeogenesis.

Q cycle—The shuttling of electrons between ubiquinol and ubiquinone in the inner mitochondrial membrane as a part of Complex III's function.

Quaternary structure—The interaction between different subunits of a multi-subunit protein; stabilized by R group interactions.

Reaction coupling—The tendency of unfavorable biological reactions to occur concurrently with favorable reactions, often catalyzed by a single enzyme.

Reducing sugar—A sugar that can reduce other compounds and can be detected by Tollen's or Benedict's reagent.

Release factor—The protein that binds to the stop codon during termination of translation.

Renaturation—The regaining of the correct secondary, tertiary, and quaternary structure after denaturation of a protein.

Repressible system—An operon that requires a repressor to bind to a corepressor before binding to the operator site to stop transcription of the relevant gene; also called a negative control system.

Repressor—For enzymes, an inhibitor of enzyme action; for operons, a species that binds to the operator region to stop transcription of the relevant gene.

Respiratory control—The coordinated regulation of the different aerobic metabolic processes.

Respiratory quotient—A numerical representation that can be used to determine the most prevalent type of biomolecule being used in metabolism; the ratio of carbon dioxide produced to oxygen consumed.

Respirometry—A method of measuring metabolism through the consumption of oxygen.

Resting membrane potential—The electrical potential that results from the unequal distribution of charge around the cell membrane; resting membrane potential characterizes a cell that has not been stimulated.

Ribonucleic acid (RNA)—A nucleic acid found in both the nucleus and cytoplasm and most closely linked with transcription and translation, as well as some gene regulation.

Ribosomal RNA (rRNA)—The structural and enzymatic RNA found in ribosomes that takes part in translation.

Ribozyme—An RNA molecule with enzymatic activity.

Saponification—The reaction between a fatty acid and a strong base, resulting in a negatively charged fatty acid anion bound to a metal ion; creates soap.

Saturation—The presence or absence of double bonds in a fatty acid; saturated fatty acids have only single bonds, whereas unsaturated fatty acids have at least one double bond.

Secondary structure—The local structure of neighboring amino acids; most common are α-helices and β-pleated sheets.

Sequencing—Determining the order of amino acids in a polypeptide, or of nucleotides in a nucleic acid.

Shine–Dalgarno sequence—The site of initiation of translation in prokaryotes.

Shuttle mechanism—A method of functionally transferring a compound across a membrane without the actual molecule crossing; common examples are the glycerol 3-phosphate shuttle and malate–aspartate shuttle.

Sialic acid—The common name of N-acetylneuraminic acid (NANA), which is the terminal portion of the head group in a ganglioside.

Side chain—The variable component of an amino acid that gives the amino acid its identity and chemical properties; also called an R group.

Silent mutation—A mutation in the wobble position of a codon or noncoding DNA that leads to no change in the protein produced during translation.

Simple diffusion—The movement of solute molecules through the cell membrane down their concentration gradients without a transport protein; used for small, nonpolar, lipophilic molecules and water.

Small nuclear ribonucleoproteins (snRNPs)—The protein portion of the spliceosome complex.

Small nuclear RNA (snRNA)—The RNA portion of the spliceosome complex.

Sodium–potassium pump—An ATPase that exchanges three sodium cations for two potassium cations; responsible for maintaining cell volume and the resting membrane potential.

Solvation layer—The layer of solvent particles that interacts directly with the surface of a dissolved species.

Sphingolipid—A lipid containing a sphingosine or sphingoid backbone, bound to fatty acid tails; include ceramide, sphingomyelins, glycosphingolipids, and gangliosides.

Sphingomyelin—A sphingophospholipid containing a sphingosine backbone and a phosphate head group.

Spliceosome—The apparatus used for splicing out introns and bringing exons together during mRNA processing.

Starch—A branched polymer of glucose, used for energy storage in plants; common examples are amylose and amylopectin.

Start codon—The first codon in an mRNA molecule that codes for an amino acid (AUG for methionine or N-formylmethionine).

Stereoisomers—Compounds that have the same chemical formula and backbone, differing only in their spatial orientation; also called optical isomers.

Steroid—A derivative of cholesterol.

Stop codon—The last codon of translation (UAA, UGA, or UAG); release factor binds here, terminating translation.

Structural proteins—Proteins that are involved in the cytoskeleton and extracellular matrix; they are generally fibrous in nature and include collagen, elastin, keratin, actin, and tubulin.

Substrate—The molecule upon which an enzyme acts.

Substrate-level phosphorylation—The transfer of a phosphate group from a high-energy compound to ATP or another compound; occurs in glycolysis.

Sucrose—A disaccharide composed of glucose and fructose.

Surfactant—A compound that lowers the surface tension between two solutions, acting as a detergent or emulsifier.

TATA box—The site of binding for RNA polymerase II during transcription; named for its high concentration of thymine and adenine bases.

Tautomerization—The rearrangement of bonds within a compound, usually by moving a hydrogen and forming a double bond.

Template strand—The strand of DNA that is transcribed to form mRNA; also called the antisense strand.

Termination—The end of translation, in which the ribosome finds a stop codon and release factor binds it, allowing the peptide to be freed from the ribosome.

Terpene—A class of lipids built from isoprene moieties; have carbon groups in multiples of five.

Terpenoid—A terpene derivative that has undergone oxygenation or rearrangement of the carbon skeleton.

Tertiary structure—The three-dimensional shape of a polypeptide, stabilized by numerous interactions between R groups.

Thyroid hormones—The primary permissive metabolic hormones involved in the regulation of the basal metabolic rate.

Tight junctions—Cell–cell junctions that prevent the paracellular transport of materials; tight junctions form a collar around cells and link cells within a single layer.

Titration—A laboratory technique in which a solution of unknown concentration is mixed with a solution of known concentration to determine the unknown concentration.

Transcellular transport—The transport of materials through the cell; requires interaction with the cytoplasm and may require transport proteins.

Transcription—The production of an mRNA molecule from a strand of DNA.

Transcription factors—Proteins that help RNA polymerase II locate and bind to the promoter region of DNA.

Transfer RNA (tRNA)—A folded strand of RNA that contains a three-nucleotide anticodon that pairs with an appropriate mRNA codon during translation, and is charged with the corresponding amino acid.

Transferase—An enzyme that catalyzes the transfer of a functional group.

Translation—The production of a protein from an mRNA molecule.

Triacylglycerol—A glycerol molecule esterified to three fatty acid molecules; the most common form of fat storage within the body.

Uncompetitive inhibition—A decrease in enzyme activity that results from the interaction with an inhibitor at the allosteric site; uncompetitive inhibitors bind only to the substrate-bound enzyme and cannot be overcome by addition of substrate.

UV spectroscopy—A method of determining the concentration of protein in an isolate by comparison against a protein standard; relies on the presence of aromatic amino acids. It can also be used with nucleic acids and other compounds.

Vitamin—An organic essential coenzyme that assists an enzyme in carrying out its action.

Wax—A high-melting point lipid composed of a very long chain alcohol and a very long chain fatty acid.

Wobble—The third nucleotide of a codon that often plays no role in specifying an amino acid; an evolutionary development designed to protect against mutations.

X-ray crystallography—A method of determining molecular structure using apparent bond angles and diffraction and refraction of X-rays.

Zwitterion—A molecule that contains charges, but is neutral overall. Most often used to describe amino acids.

Zymogen—An enzyme that is secreted in an inactive form and must be activated by cleavage; common examples are digestive enzymes.