MCAT Bio/Biochem flashcards

31 cards. Read the term and the definition here. The deck at the top of this page is optional.

Zwitterion

A molecule containing both positive and negative charges but no net charge, common for amino acids at physiological pH where the carboxyl is deprotonated and the amino group protonated.

Peptide bond

A covalent bond formed by condensation between the carboxyl group of one amino acid and the amino group of another, releasing water and creating a planar, rigid linkage.

Isoelectric point

The pH at which a molecule carries zero net charge. At the pI, amino acids and proteins do not migrate in an electric field during electrophoresis.

Phosphodiester bond

A covalent linkage connecting the 3' hydroxyl of one nucleotide sugar to the 5' phosphate of the next, forming the sugar-phosphate backbone of nucleic acids.

Michaelis constant Km

The substrate concentration at which reaction velocity is half of Vmax. A lower Km indicates higher enzyme affinity for substrate; competitive inhibitors raise apparent Km.

Competitive inhibition

An inhibitor binds the enzyme active site, competing with substrate. This raises apparent Km while leaving Vmax unchanged because saturating substrate can outcompete the inhibitor.

Noncompetitive inhibition

An inhibitor binds a site other than the active site and lowers Vmax without changing Km, because substrate binding is unaffected but the fraction of active enzyme decreases.

Fluid mosaic model

A description of membranes as a phospholipid bilayer with embedded proteins, cholesterol, and glycoproteins that move laterally, giving the membrane dynamic fluidity.

Endosymbiotic theory

The proposal that mitochondria and chloroplasts originated from engulfed bacteria, supported by their own circular DNA, ribosomes, and double membranes.

Na+/K+ ATPase

A membrane pump that uses ATP to move 3 sodium ions out and 2 potassium ions into the cell, maintaining the negative resting membrane potential.

Meiosis I

The reductional division of meiosis in which homologous chromosomes separate, producing two haploid cells with chromosomes that still have two sister chromatids.

Meiosis II

The equational division of meiosis in which sister chromatids separate, producing four haploid gametes from the two cells generated in meiosis I.

Crossing over

The exchange of genetic material between homologous chromosomes during prophase I of meiosis, which increases genetic variation in resulting gametes.

Apoptosis

Programmed cell death executed by caspases, characterized by controlled cell shrinkage and fragmentation without inflammation, in contrast to necrosis.

G1/S checkpoint

A cell cycle checkpoint that verifies cell size and nutrient availability and checks for DNA damage before the cell commits to DNA replication.

Primary structure

The linear sequence of amino acids in a polypeptide chain, held together by peptide bonds and determined by the gene encoding the protein.

Alpha helix

A secondary structure in which the polypeptide coils with hydrogen bonds between backbone carbonyl and amide groups spaced about four residues apart along the same chain.

Beta sheet

A secondary structure of extended polypeptide strands linked by hydrogen bonds between backbone groups of adjacent strands, forming a pleated sheet arrangement.

Disulfide bridge

A covalent bond between the thiol groups of two cysteine residues that stabilizes tertiary and quaternary protein structure and can be broken by reducing agents.

Denaturation

Loss of a protein's secondary, tertiary, or quaternary structure due to heat, pH change, or chemicals, without breaking peptide bonds of the primary sequence.

Allosteric enzyme

An enzyme regulated by effectors binding at a site distinct from the active site, producing a sigmoidal velocity curve reflecting cooperative changes in activity.

Lineweaver-Burk plot

A double-reciprocal graph of 1/v versus 1/[S] whose y-intercept equals 1/Vmax and x-intercept equals -1/Km, used to determine enzyme kinetic parameters.

Cofactor

A nonprotein molecule or ion required for enzyme activity, such as metal ions or coenzymes; tightly bound cofactors are called prosthetic groups.

Glycolysis

The cytosolic pathway converting glucose to two pyruvate molecules, netting 2 ATP and 2 NADH without requiring oxygen.

Pyruvate dehydrogenase

The enzyme complex converting pyruvate to acetyl-CoA, producing one NADH and one CO2 per pyruvate and linking glycolysis to the citric acid cycle.

Citric acid cycle

A mitochondrial matrix pathway that oxidizes acetyl-CoA, yielding per turn 3 NADH, 1 FADH2, 1 GTP/ATP, and 2 CO2.

Oxidative phosphorylation

The process in which electron transport chain carriers pass electrons to oxygen, pumping protons to build a gradient that drives ATP synthase to make ATP.

Fermentation

An anaerobic pathway that regenerates NAD+ from NADH by reducing pyruvate to lactate or ethanol, allowing glycolysis to continue without oxygen.

Gluconeogenesis

The synthesis of glucose from lactate, glycerol, and amino acids, occurring mainly in the liver and using distinct enzymes at the irreversible steps of glycolysis.

Phosphofructokinase-1

A key regulated enzyme of glycolysis that is inhibited by ATP and citrate and activated by AMP, controlling flux through the pathway.

ATP yield per glucose

The approximate number of ATP molecules generated from full oxidation of one glucose, commonly cited as about 30-32 in eukaryotes including glycolysis, the citric acid cycle, and oxidative phosphorylation.