AP Biology: Cellular Respiration Overview (Glycolysis, Krebs Cycle, Electron Transport Chain, Chemiosmosis)

Cellular Respiration Overview (Glycolysis, Krebs Cycle, Electron Transport Chain, Chemiosmosis)

Concept Summary

• Cellular Respiration: Catabolic pathway that oxidizes glucose to produce ATP, NADH, and FADH?, releasing CO? and H?O as waste.
• Glycolysis: Anaerobic 10-step pathway in the cytosol that splits glucose (6C) into two pyruvate (3C), yielding 2 net ATP and 2 NADH.
• Krebs Cycle (Citric Acid Cycle): Aerobic 8-step cycle in the mitochondrial matrix that fully oxidizes acetyl-CoA (2C) to CO?, generating 3 NADH, 1 FADH?, and 1 ATP per turn.
• Electron Transport Chain (ETC): Series of protein complexes (I-IV) in the inner mitochondrial membrane that transfer electrons from NADH/FADH? to O?, creating a proton gradient for chemiosmosis.
• Chemiosmosis: Process where ATP synthase uses the proton-motive force (H? gradient) to phosphorylate ADP-ATP, yielding ~28–34 ATP per glucose.

Core Questions

WHAT (definitional)

Q: What is substrate-level phosphorylation? A: Direct transfer of a phosphate group from a substrate to ADP, producing ATP (occurs in glycolysis and Krebs cycle). Trap/Clarification: Not the same as oxidative phosphorylation (ETC/chemiosmosis); substrate-level is enzyme-mediated and yields fewer ATP.

Q: What is oxidative phosphorylation? A: ATP synthesis driven by the ETC and chemiosmosis, coupling electron transfer to proton pumping and ATP synthase activity. Trap/Clarification: Requires O? as the final electron acceptor; without O?, the ETC stalls, halting ATP production.

WHY (causal/explanatory)

Q: Why is the Krebs cycle considered amphibolic? A: It both degrades acetyl-CoA (catabolic) and provides intermediates (e.g., ?-ketoglutarate, oxaloacetate) for anabolic pathways (e.g., amino acid synthesis). Trap/Clarification: Not just for ATP production; its intermediates are critical for biosynthesis (e.g., citrate-fatty acids).

Q: Why is oxygen essential for the ETC? A: O? is the final electron acceptor, combining with H? to form H?O; without it, electrons back up, halting NADH/FADH? oxidation and proton pumping. Trap/Clarification: O? is not directly used in ATP synthesis but is required to maintain the proton gradient.

HOW (process/application)

Q: How is the proton gradient established in the ETC? A: Electrons from NADH/FADH? flow through complexes I-IV, pumping H? from the matrix into the intermembrane space (via complexes I, III, IV). Trap/Clarification: FADH? enters at complex II, bypassing complex I, so it pumps fewer H? (yields less ATP).

Q: How is ATP yield calculated per glucose? A: Glycolysis (2 ATP) + Krebs (2 ATP) + ETC/chemiosmosis (~28–34 ATP) = ~30–38 ATP total (varies by cell type and shuttle mechanisms). Trap/Clarification: The malate-aspartate shuttle (liver/heart) yields 3 ATP/NADH, while the glycerol-3-phosphate shuttle (muscle) yields 2 ATP/NADH.

CAN (conditions/possibilities)

Q: Can glycolysis occur without oxygen? A: Yes; glycolysis is anaerobic and proceeds in the cytosol regardless of O? presence, but NADH must be recycled (e.g., via fermentation). Trap/Clarification: Without O?, pyruvate is reduced to lactate/ethanol (fermentation) to regenerate NAD?, not oxidized to acetyl-CoA.

Q: Under what conditions does the ETC produce reactive oxygen species (ROS)? A: When electrons leak prematurely (e.g., at complex I/III) and react with O?, forming superoxide (O), often due to hypoxia or mitochondrial damage. Trap/Clarification: ROS are harmful byproducts, not part of normal ETC function.

Quick Facts & Traps

• Fact: Glycolysis is the only stage that occurs in the cytosol; all others (Krebs, ETC) are mitochondrial.
• Trap: "Glycolysis requires oxygen"-Reality: Glycolysis is anaerobic; O? is only needed for pyruvate oxidation and the ETC.
• Fact: NADH yields ~2.5–3 ATP in the ETC; FADH? yields ~1.5–2 ATP (enters at complex II).
• Trap: "The Krebs cycle produces CO? from glucose"-Reality: CO? comes from the oxidation of acetyl-CoA (2C), not directly from glucose (6C).
• Fact: ATP synthase rotates as H? flow through it, with the F? subunit catalyzing ADP + P?-ATP.
• Trap: "All cells produce 38 ATP/glucose"-Reality: Yield varies (e.g., 30–32 ATP in brain cells due to NADH shuttle differences).

Rapid-Fire True/False

• Statement: The Krebs cycle directly consumes O?. Answer: FALSE Why the common mistake happens: Confusion with the ETC, which uses O? as the final electron acceptor.

• Statement: Fermentation produces ATP. Answer: FALSE Why the common mistake happens: Fermentation regenerates NAD? for glycolysis but does not itself generate ATP (only glycolysis does).

• Statement: The proton gradient in the ETC is established by active transport. Answer: TRUE Why the common mistake happens: Misattributing the gradient to passive diffusion; it’s driven by redox reactions in the ETC.