Questions: Cellular Respiration Overview

Question 1

Of the ~30–32 ATP produced by complete aerobic oxidation of one glucose molecule, where does the majority come from?

• A. Glycolysis, via substrate-level phosphorylation
• B. The Krebs cycle, via substrate-level phosphorylation
• C. The electron transport chain, via oxidative phosphorylation
• D. Pyruvate oxidation, via direct ATP synthesis

Answer: (C)

Glycolysis and the Krebs cycle together produce only about 4 ATP directly (2 each via substrate-level phosphorylation). They also produce NADH and FADH₂, which carry electrons to the electron transport chain. The ETC uses the energy from those electrons to pump protons across the inner mitochondrial membrane, and ATP synthase harnesses that gradient to synthesize ~26–28 ATP. The ETC is the main ATP factory.

Question 2

Cellular respiration and breathing (pulmonary ventilation) are two names for the same biological process.

• T. True
• F. False

Answer: False

Breathing is the mechanical process of moving air in and out of the lungs to exchange O₂ and CO₂ with the environment. Cellular respiration is the intracellular enzymatic process — occurring in the cytoplasm and mitochondria — by which cells oxidize glucose and capture the energy as ATP. They are related (breathing supplies O₂ for respiration) but are distinct processes.

Question 3

Why is oxygen essential for aerobic cellular respiration, even though oxygen does not directly participate in glycolysis or the Krebs cycle?

Model answer: Oxygen is the terminal electron acceptor in the electron transport chain. NADH and FADH₂ produced in glycolysis and the Krebs cycle donate electrons to the ETC, which pumps protons to drive ATP synthesis. Those electrons must ultimately be passed to a final acceptor — oxygen — to form water. Without oxygen, the ETC stalls, NADH and FADH₂ cannot be reoxidized to NAD⁺ and FAD, and the upstream reactions (Krebs cycle, pyruvate oxidation) can no longer proceed.

This question targets a common conceptual gap: students learn 'aerobic respiration requires oxygen' without understanding why. The key is that O₂ is needed to regenerate the electron carriers (NAD⁺, FAD) that are essential for earlier stages. Without that regeneration, the entire pathway backs up. This is also why anaerobic pathways (fermentation) evolved: they regenerate NAD⁺ without oxygen, at the cost of far less ATP.