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Cellular Respiration Deep Dive

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The full pre-ETC ledger

Before the electron transport chain, respiration harvests carriers stage by stage, per glucose. Glycolysis (cytosol): invests 2 ATP, produces 4, for a net 2 ATP, plus 2 NADH and 2 pyruvate. Pyruvate oxidation (matrix, runs twice): 2 NADH, 2 CO₂, 2 acetyl-CoA — but 0 ATP. Krebs cycle (matrix, runs twice): 2 ATP (via GTP, substrate-level), 6 NADH, 2 FADH₂, and 4 CO₂. Add it up: 4 ATP made directly, 10 NADH, 2 FADH₂, and all 6 carbons of glucose exhaled as 6 CO₂. The direct ATP is small — the value is in those 12 loaded carriers.

Carriers delivered to the ETC (per glucose)
10 NADH + 2 FADH₂ → ETC → ~26–28 ATP
Approx. 2.5 ATP per NADH and 1.5 ATP per FADH₂: (10 × 2.5) + (2 × 1.5) = 25 + 3 = 28, trimmed by shuttle costs to ~26–28.

The electron transport chain builds the proton-motive force

NADH and FADH₂ do not make ATP themselves — they deliver electrons to protein complexes in the inner mitochondrial membrane. As electrons pass from complex to complex toward oxygen, the released energy pumps H⁺ from the matrix into the intermembrane space. This creates a steep proton-motive force: a combined concentration and charge gradient (the intermembrane space becomes acidic and positive). FADH₂ yields slightly less ATP than NADH because it enters the chain later, past the first pumping complex, so it drives fewer protons across.

Chemiosmosis and ATP synthase

The trapped protons can return to the matrix only through ATP synthase, a molecular turbine. H⁺ flowing down the proton-motive force spins the enzyme, which phosphorylates ADP → ATP — this coupling of a gradient to ATP synthesis is chemiosmosis. Oxygen’s sole job is to sit at the end of the chain as the final electron acceptor, combining with spent electrons and H⁺ to form H₂O. Remove O₂ and electrons have nowhere to go: the chain backs up, H⁺ pumping stops, the gradient collapses, and ATP synthase falls silent — which is why the ~28 ATP of oxidative phosphorylation is entirely oxygen-dependent.

Aerobic vs. anaerobic: 32 vs. 2

Total aerobic yield ≈ 4 direct ATP + ~26–28 from oxidative phosphorylation = ~30–32 ATP per glucose (the range reflects the metabolic cost of shuttling cytosolic NADH into the mitochondrion, and non-integer proton stoichiometry). Anaerobically there is no final electron acceptor, so pyruvate oxidation, Krebs, and the ETC all shut down. Fermentation does not add ATP — it merely reoxidizes NADH back to NAD⁺ (to lactate in muscle, or ethanol + CO₂ in yeast) so that glycolysis alone can keep turning out its net 2 ATP. That is a roughly 15-fold energy penalty for living without oxygen.

Worked example

Account for the approximate total ATP yield from the complete aerobic respiration of one glucose, showing where each contribution originates.

  1. 1.Direct (substrate-level) ATP: glycolysis nets 2, the Krebs cycle makes 2 → 4 ATP total made without the ETC.
  2. 2.Electron carriers harvested: 2 NADH (glycolysis) + 2 NADH (pyruvate oxidation) + 6 NADH (Krebs) = 10 NADH, plus 2 FADH₂ (Krebs).
  3. 3.Oxidative phosphorylation: about 2.5 ATP per NADH and 1.5 per FADH₂ → (10 × 2.5) + (2 × 1.5) = 25 + 3 = 28, reduced to ~26–28 after NADH-shuttle costs.
  4. 4.Sum the direct and oxidative contributions: 4 + (~26–28).
Answer: About 30–32 ATP per glucose: 4 made directly (2 in glycolysis, 2 in the Krebs cycle) and roughly 26–28 from oxidative phosphorylation driven by 10 NADH and 2 FADH₂.
Checkpoint

After glycolysis, pyruvate oxidation, and the Krebs cycle — but before the electron transport chain — what has one glucose produced in direct ATP and electron carriers?

Watch out

NADH and FADH₂ are not ATP. They are electron carriers that feed the ETC; their energy becomes ATP only after chemiosmosis. Also resist the shortcut that ATP synthase "uses oxygen" — it is driven by the proton-motive force, and oxygen’s only role is to be the final electron acceptor that keeps the chain flowing.

Checkpoint

What directly powers ATP synthase during oxidative phosphorylation?

Checkpoint

Anaerobic fermentation yields only about 2 ATP per glucose versus ~30–32 for aerobic respiration. Which explanation is correct?

On the exam

Lock in the ledger: glycolysis → 2 ATP + 2 NADH (cytosol); pyruvate oxidation → 2 NADH; Krebs → 2 ATP + 6 NADH + 2 FADH₂ (matrix); oxidative phosphorylation → ~26–28 ATP from 10 NADH + 2 FADH₂ via the proton-motive force, with O₂ as final electron acceptor → H₂O. Total ~30–32; strip away O₂ and you are left with glycolysis’s 2.

Answer the 3 checkpoints as you read.

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