Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Cellular respiration releases energy from food molecules to make ATP. Respiration and fermentation occur in all forms of life.
- Oxidation = loss of electrons; reduction = gain. NAD⁺ → NADH and FAD → FADH₂ carry electrons to the ETC.
- The ETC is a redox series in the inner mitochondrial membrane (plasma membrane in prokaryotes). Electron flow pumps H⁺ into the intermembrane space.
- Chemiosmosis: H⁺ flows back through ATP synthase, driving ADP + Pᵢ → ATP (oxidative phosphorylation). Cristae increase the surface area for this.
- Uncoupling: if H⁺ leaks back without passing through ATP synthase, the energy becomes heat (brown fat in some endotherms).
- Fermentation: without O₂, NADH reduces pyruvate to lactic acid or ethanol (+ CO₂), regenerating NAD⁺ so glycolysis continues. Only 2 ATP per glucose.
Key relationships
| Stage | Location | In | Out |
|---|---|---|---|
| Glycolysis | cytosol | glucose, NAD⁺, ADP + Pᵢ | 2 pyruvate, net 2 ATP, 2 NADH |
| Pyruvate oxidation | matrix | pyruvate, NAD⁺ | CO₂, NADH, two-carbon unit |
| Krebs cycle | matrix | two-carbon unit, NAD⁺, FAD | CO₂, NADH, FADH₂, a little ATP |
| ETC + ATP synthase | inner membrane | NADH, FADH₂, O₂, ADP + Pᵢ | NAD⁺, FAD, H₂O, most ATP |
| Compartment | H⁺ concentration | pH |
|---|---|---|
| Intermembrane space | high | lower |
| Matrix | low | higher |
Assumptions behind the numbers
- ATP per glucose is a model value: about 30–32 aerobically (older books: 36–38) and 2 by fermentation. Use the figure a question gives you.
- A rate from total change ÷ time assumes the graph is a straight line.
- Respirometer readings need a control tube to correct for temperature and pressure changes.
Mistakes to avoid
- Saying O₂ is used in the Krebs cycle.
- Saying the ETC makes ATP directly.
- Pumping protons into the matrix, or giving the matrix the lower pH.
- Saying fermentation makes ATP.
- Treating anaerobic respiration (non-O₂ acceptor, ETC) as fermentation (no ETC).
- Forgetting that plants respire.
- Skipping links in a cause-and-effect chain (block ETC → no pumping → no gradient → no ATP synthesis).
Quick self-check
- Why does glycolysis stop without fermentation when O₂ is absent? (NADH cannot unload to the ETC, so NAD⁺ runs out.)
- Matrix pH 7.8, intermembrane space pH 7.0. How many times higher is [H⁺] outside? (10^0.8 ≈ 6.3 times)
- A drug lets H⁺ leak across the inner membrane. Effect on O₂ use and ATP? (O₂ use rises, ATP falls, more heat.)
Next: practice questions.