Study guide
Cellular Respiration: A Study Guide That Actually Explains It
Four stages, one net equation, 30–32 ATP. Read the guide, then drop it into Ellie for a review deck.
Cellular respiration converts glucose to ATP through four connected stages: glycolysis, pyruvate oxidation, the Krebs cycle (citric acid cycle), and the electron transport chain (ETC) coupled to chemiosmosis. The net equation is famous: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP. What trips students up is where each stage happens, what each stage produces, and how the net ATP yield adds up.
The four stages
Each stage happens in a specific cellular location and produces specific molecules.
- Glycolysis — cytoplasm. Glucose → 2 pyruvate. Net: 2 ATP, 2 NADH.
- Pyruvate oxidation — mitochondrial matrix. Pyruvate → acetyl-CoA. Per glucose: 2 NADH, 2 CO₂.
- Krebs cycle — matrix. Acetyl-CoA → CO₂. Per glucose: 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂.
- Electron transport chain — inner membrane. NADH/FADH₂ → electrons → H⁺ gradient → ATP synthase → ATP. Per glucose: ~26–28 ATP.
Chemiosmosis — the elegant mechanism
The ETC pumps H⁺ from the matrix into the intermembrane space, creating a proton gradient. H⁺ flows back through ATP synthase (which spins like a turbine), driving ADP + Pi → ATP. This is the same mechanism as photophosphorylation in chloroplasts — evolution reuses the pattern.
Total ATP yield
The classic textbook total is 36–38 ATP per glucose. Modern estimates are 30–32 because NADH and FADH₂ shuttling into the mitochondrion costs some ATP.
Anaerobic respiration
Without oxygen, the ETC halts. Cells rely on glycolysis alone (2 ATP). Pyruvate is fermented — to lactate in animals (Cori cycle), to ethanol + CO₂ in yeast.
Frequently asked questions
Why is O₂ the final electron acceptor?
O₂ has high electronegativity — it pulls electrons strongly, making the ETC energetically favorable. Without O₂ to accept electrons, the chain stalls.
How much ATP does each NADH produce?
About 2.5 ATP for matrix NADH; less for cytoplasmic NADH (shuttled in). FADH₂ produces about 1.5 ATP because it enters the chain later.
Where does CO₂ come from?
Pyruvate oxidation releases 2 CO₂ per glucose; the Krebs cycle releases 4 more. Total: 6 CO₂ — matching the net equation.
Is cellular respiration the reverse of photosynthesis?
In net inputs and outputs, yes. In mechanism, no — different enzymes and pathways.
Why lactate in muscle?
During intense exercise, oxygen supply lags demand. Muscle switches to lactate fermentation to regenerate NAD⁺ so glycolysis can continue.
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