AP Biology · Unit 3 · Topic 3.5
Four stages, one glucose, and a running ATP count. Follow the carbon and the electrons all the way to oxygen.
Learning objective 3.5.A Describe the processes of aerobic cellular respiration, and explain the role of oxygen in them.
Step through the four stages and watch two things: which compartment the stage happens in, and where the carbon goes. Six carbon atoms come in as glucose and six leave as carbon dioxide, but not from the stages most students guess.
The diagram is wider than your screen — scroll it sideways to see the whole mitochondrion.
Not in glycolysis, which releases none, and not in oxidative phosphorylation, which involves no carbon at all. Two carbon dioxide come from pyruvate oxidation and four from the citric acid cycle, and both of those happen inside a mitochondrion.
Two ATP are spent before any are made. An answer that says glycolysis produces four ATP is not wrong about the gross figure and is wrong about the one that matters.
Four ATP per glucose are made directly, by attaching a phosphate to ADP during a reaction. Everything else comes from the proton gradient. The bars below are drawn from the same numbers the stages produced.
Switch between the two chips above. The carbon does not change, the electrons do not change, the oxygen does not change. Only the ATP assigned per electron carrier changes, and with it the total. If you are asked for a number, either is defensible provided you say which you are using.
This is why losing the oxygen supply is catastrophic rather than merely inconvenient. It is not that a cell loses a stage; it is that it loses the stage that was doing almost all of the work.
Oxygen is used at the very end of the process, and nowhere else. Predict which stages that should stop before you switch it off.
This is the single most common error on this topic. Fermentation is not an alternative way of making ATP; it is a way of recycling NAD⁺ so that the one stage which can still make ATP is able to keep going.
The damage travels backwards from the end. The chain stops because it has nothing to pass its electrons to; NADH then cannot be reoxidised, so every stage that needs NAD⁺ stops as well. Glycolysis needs NAD⁺ too — which is exactly why fermentation exists.
A sprinting muscle running on glycolysis alone is getting a sixteenth of the ATP it would get from the same glucose with oxygen available. That is why anaerobic effort cannot be sustained, and why it burns through glucose so fast.
Answer these in your own words before you open the answers.
1.
2.
3.
4.
5.
6.
7.
8.
| Stage | Where | CO₂ | NADH | FADH₂ | Net ATP |
|---|
Read the CO₂ column down: it is the answer to “where does the carbon go”, and it is the column students most often fill in wrongly.