How does cellular respiration work?
Cellular respiration breaks glucose down to carbon dioxide and water and stores the released energy as ATP. It runs in stages, glycolysis, pyruvate oxidation, the Krebs cycle and oxidative phosphorylation, and the last stage, which makes most of the ATP, needs oxygen.
The stages in order
The overall equation is C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy. The cell gets there in steps, so the energy is captured a little at a time instead of released as heat all at once.
- Glycolysis (cytoplasm): one glucose is split into two pyruvate, for a net gain of 2 ATP and 2 NADH. No oxygen is needed.
- Pyruvate oxidation (mitochondrial matrix): each pyruvate becomes acetyl CoA, releasing CO2 and making NADH.
- Krebs cycle, also called the citric acid cycle (matrix): the acetyl groups are oxidized to CO2. Per glucose, the cycle yields 2 ATP, 6 NADH and 2 FADH2.
- Oxidative phosphorylation (inner mitochondrial membrane): NADH and FADH2 hand electrons to the electron transport chain, which pumps protons into the intermembrane space. The protons flow back through ATP synthase, which makes ATP. Oxygen accepts the electrons at the end and becomes water.
Most textbooks now put the total at about 30 to 32 ATP per glucose; older ones say 36 to 38. Either way, the large majority comes from the last stage.
Why oxygen matters, and what happens without it
Oxygen is the final electron acceptor. Without it, electrons back up in the chain, proton pumping stops, and NADH cannot be turned back into NAD+. Since glycolysis needs NAD+, the cell regenerates it by fermentation: lactic acid fermentation in your muscles, or alcohol fermentation (ethanol and CO2) in yeast. Fermentation makes no extra ATP; it only keeps glycolysis and its 2 ATP going.
Common exam mistakes
- Thinking the CO2 you breathe out contains the oxygen you breathed in. The O2 ends up in water; the carbon and oxygen in CO2 come from the glucose.
- Saying only animals respire. Plants carry out cellular respiration too, alongside photosynthesis.
- Saying ATP synthase is powered directly by electrons. It is powered by protons flowing down their gradient (chemiosmosis).
Seeing it move
The hard part is usually the inner membrane: which way the protons go and why that makes ATP. Ask Isotoon "Why does the electron transport chain need oxygen?" and you get a short animated video that follows electrons down the chain and protons back through ATP synthase. For the rest of the course, see biology and AP Biology.
Questions
- Is glycolysis aerobic or anaerobic?
- Glycolysis itself does not use oxygen, so it runs either way. Whether pyruvate then enters the mitochondria or goes to fermentation depends on whether oxygen is available.
- Why do NADH and FADH2 give different amounts of ATP?
- FADH2 passes its electrons into the chain at a later point than NADH, so fewer protons are pumped for each pair of its electrons, and less ATP results.
- Is cellular respiration the same as breathing?
- No. Breathing moves air in and out of the lungs. Cellular respiration is the chemistry inside cells that uses the oxygen breathing delivers and produces the carbon dioxide you exhale.