Cell Biology and Metabolism: Enzyme Regulation, Membrane Transport, Cell Signaling, and Cellular Respiration

Vocabulary practice flashcards covering enzyme regulation, membrane transport, cell signaling pathways, glycolysis, pyruvate oxidation, and the citric acid cycle.

Competitive Inhibitor

An inhibitor that competes with the substrate for binding at the enzyme active site; its inhibitory effect can be overcome by adding enough substrate because substrate binding and releasing occurs at a characteristic diffusion rate.

Uncompetitive Inhibitor

An allosteric inhibitor that binds specifically to the enzyme-substrate ($ES$) complex, preventing product release and restricting enzyme movement so it cannot function.

Noncompetitive Inhibitor

An allosteric inhibitor that binds to a site outside the active site on either the free enzyme or the $ES$ complex, producing less predictable inhibitory effects.

Allosteric Regulation

Regulation occurring at quaternary structure sites separate from the active site, causing the enzyme to oscillate between an active conformation that binds substrate and an inactive conformation that prevents product formation.

Feedback Inhibition

A cellular control mechanism in which the end product of a metabolic pathway binds to an allosteric site on an upstream enzyme to inhibit its own further production.

Denaturation and Irreversible Inhibition

The least likely natural mechanisms for cellular enzyme control because cells cannot abruptly alter internal pH or temperature, and cells are conservative with energy expenditure.

Integral Membrane Proteins

Proteins embedded within the hydrophobic core of the lipid bilayer that possess amphipathic characteristics to interact with both interior lipids and aqueous surroundings.

Peripheral Membrane Proteins

Membrane-associated proteins located entirely on the outside surfaces of the lipid bilayer.

Carrier Proteins

Membrane transport proteins that bind specific solutes, change conformation, and release them on the other side; they facilitate diffusion at slower rates than channels and are susceptible to saturation.

Aquaporins

Channel proteins that specialize in the facilitated diffusion of water across the cell membrane.

Hypotonic Solution

An external solution with a lower solute concentration than the cytoplasm, causing water to enter via osmosis and resulting in cell swelling.

Hypertonic Solution

An external solution with a higher solute concentration than the cytoplasm, causing water to exit via osmosis and resulting in cell shrinking.

Primary Active Transport (PAT)

Active transport that directly utilizes the energy from $ATP$ hydrolysis to move solutes against their concentration gradient, establishing potential energy.

Secondary Active Transport (SAT)

Transport that utilizes the potential energy gradient generated by primary active transport to move another solute against its concentration gradient.

Sodium-Potassium ($Na^+/K^+$) Pump

A primary active transport pump that hydrolyzes one $ATP$ into $ADP$ to phosphorylate an amino acid residue, altering its conformation to transport $3\,Na^+$ out of the cell and $2\,K^+$ into the cell.

Agonist

A molecule that mimics a natural ligand to bind and activate a specific cellular receptor.

Antagonist

A molecule that binds to a receptor to block or reduce downstream signaling.

G-Protein Coupled Receptor (GPCR)

A transmembrane receptor linked to a heterotrimeric G-protein ($\alpha$, $\beta$, $\gamma$ subunits) that, upon ligand binding, prompts the $\alpha$ subunit to release $GDP$, bind $GTP$, and dissociate to propagate signaling.

Kinase Cascade

A signaling relay where an activated receptor-ligand complex triggers sequential phosphorylation of downstream proteins, amplifying the initial signal.

Second Messengers

Non-protein chemical signals within the cytoplasm—such as $cAMP$, $DAG$, and $IP_3$—that interact noncovalently to transduce signals within the cell.

Hexokinase

The Step 1 glycolysis enzyme that transfers a phosphate from $ATP$ to glucose, trapping it in the cell; it is regulated by both product feedback inhibition and competitive inhibition by glucose-6-phosphate.

Phosphofructokinase-1 (PFK-1)

The key regulatory enzyme of Step 3 in glycolysis that is subject to feedback inhibition when high levels of $ATP$ bind to its allosteric site.

Pyruvate Kinase

The enzyme catalyzing Step 10 of glycolysis that converts phosphoenolpyruvate to pyruvate while generating $ATP$, regulated by feedback inhibition from $ATP$ and acetyl-CoA.

Glycolysis Net Yield

For each molecule of glucose processed through the 10 steps of glycolysis, the net outcome is $2\,\text{pyruvate}$, $2\,\text{Net ATP}$ (investing 2, generating 4), and $2\,NADH$.

Pyruvate Dehydrogenase (PDH) Complex

A multienzyme complex in the mitochondrial matrix that links glycolysis to the citric acid cycle by oxidizing pyruvate into acetyl-CoA, $CO_2$, and $NADH$; regulated by covalent modification and feedback inhibition.

Substrate-Level Phosphorylation

The direct synthesis of $ATP$ or $GTP$ powered by breaking chemical groups off a metabolic substrate, as seen in Step 5 of the citric acid cycle and steps 7 and 10 of glycolysis.

Citric Acid Cycle Per-Turn Yield

The direct products generated per acetyl-CoA molecule: $3\,NADH$, $1\,FADH_2$, $1\,ATP/GTP$, and $2\,CO_2$ (doubled to $6\,NADH$, $2\,FADH_2$, $2\,ATP/GTP$, and $4\,CO_2$ per original glucose).

Oxidative Phosphorylation

The metabolic pathway where reduced electron carriers ($NADH$ and $FADH_2$) pass electrons to inner mitochondrial membrane complexes to synthesize the bulk of cellular $ATP$.