In Cellular Respiration you saw the overall trade: sugar and oxygen in, carbon dioxide, water and energy out. Here we follow the sugar step by step to see where each product is made and where most of the ATP comes from.
Key idea
$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$$ Glycolysis (in the cytoplasm) splits glucose. The Krebs cycle (in the mitochondrial matrix) releases its carbon as CO₂, and the electron transport chain (on the inner mitochondrial membrane) makes most of the ATP: about 30 per glucose in all.
| Stage | Where | Products per glucose |
|---|---|---|
| Glycolysis | Cytoplasm | 2 pyruvate, 2 ATP (net), 2 NADH |
| Pyruvate oxidation | Mitochondrial matrix | 2 acetyl-CoA, 2 CO₂, 2 NADH |
| Krebs (citric acid) cycle | Mitochondrial matrix | 4 CO₂, 2 ATP, 6 NADH, 2 FADH₂ |
| Electron transport chain and chemiosmosis | Inner mitochondrial membrane | About 26–28 ATP; water |
Why it works
Burning sugar in a flame releases its energy all at once as heat. Cells release it in small steps so they can catch it. Glycolysis and the Krebs cycle strip high-energy electrons from the sugar and load them onto the carriers NADH and FADH₂. Those carriers drop the electrons at the electron transport chain, where the electrons' energy pumps H⁺ across the inner membrane. The H⁺ flows back through the enzyme ATP synthase, which turns like a tiny mill and makes ATP. At the very end, oxygen accepts the spent electrons and picks up H⁺, becoming water. Without oxygen to clear them away, the chain backs up and stops.
Worked example: Count the carbon
Where do the 6 CO₂ molecules made from one glucose come from?
- Glucose has 6 carbons.
- Pyruvate oxidation releases 2 of them as 2 CO₂.
- The Krebs cycle releases the other 4 as 4 CO₂.
- So every carbon of the CO₂ came from glucose.
Answer: 2 from pyruvate oxidation and 4 from the Krebs cycle, all from glucose's carbon
Worked example: Scale it up
A cell completely breaks down 5 glucose molecules by aerobic respiration. About how many ATP does it make, and how many O₂ does it use?
- About 30 ATP per glucose: $5 \times 30 = 150$ ATP.
- 6 O₂ per glucose: $5 \times 6 = 30$ O₂.
Answer: About $150$ ATP; $30$ O₂
Common mistake
Thinking the oxygen we breathe in becomes the carbon dioxide we breathe out. The carbon dioxide is made from glucose's carbon, in pyruvate oxidation and the Krebs cycle. The oxygen we breathe accepts electrons at the end of the electron transport chain and becomes water. Check by following the carbon: O₂ has none, so it cannot be the source of CO₂.