In Cellular Respiration glucose and oxygen became CO₂, water and ATP. The first stages take glucose apart and capture its energy, mostly not as ATP but as high-energy electrons loaded onto carriers. Follow the carbon and the electrons, and the stages make sense.
- Glycolysis (cytoplasm, no O₂ needed): glucose (6 C) → 2 pyruvate (3 C each). It spends 2 ATP and makes 4: a net 2 ATP, plus 2 NADH.
- Pyruvate oxidation (mitochondrial matrix): each pyruvate loses one carbon as CO₂ and becomes acetyl-CoA (2 C), making 1 NADH. Per glucose: 2 CO₂, 2 NADH.
- Krebs cycle (citric acid cycle, matrix): each acetyl-CoA joins a 4-carbon molecule, and the cycle releases 2 CO₂ per turn. Per turn: 3 NADH, 1 FADH₂, 1 ATP. The cycle turns twice per glucose.
| Per glucose | CO₂ | ATP | NADH | FADH₂ |
|---|---|---|---|---|
| Glycolysis | 0 | 2 (net) | 2 | 0 |
| Pyruvate oxidation | 2 | 0 | 2 | 0 |
| Krebs cycle (2 turns) | 4 | 2 | 6 | 2 |
| Total so far | 6 | 4 | 10 | 2 |
Key idea
Glycolysis splits glucose into 2 pyruvate (net 2 ATP, 2 NADH). Pyruvate oxidation and the Krebs cycle release all the carbon as CO₂ and load electrons onto NADH and FADH₂. Those carriers, not ATP, are the main product; they power oxidative phosphorylation.
Why it works
Why does breaking glucose apart give energy at all? The electrons in the C–H bonds of glucose are held loosely. In CO₂, carbon has given up those electrons to oxygen. Each time an enzyme strips electrons and hydrogen off a carbon (an oxidation), it hands them to NAD⁺ or FAD, saving much of the free energy for later. That is why ΔG for burning glucose is so negative, and why cells release it in small steps instead of in one flame.
Worked examples
Worked example: Tracking carbon
A glucose molecule labeled with radioactive carbon is broken down completely. Where are its six carbons after the Krebs cycle?
- Glycolysis keeps all 6 carbons, in 2 pyruvate.
- Pyruvate oxidation releases 2 as CO₂.
- The Krebs cycle releases the other 4 as CO₂ (2 per turn).
Answer: All six are in CO₂: 2 from pyruvate oxidation and 4 from the Krebs cycle.
Worked example: Counting carriers
A cell breaks down 5 glucose molecules completely. How many NADH and FADH₂ does it make before the electron transport chain?
- Per glucose: 2 + 2 + 6 = 10 NADH and 2 FADH₂.
- For 5 glucose: $5 \times 10 = 50$ NADH and $5 \times 2 = 10$ FADH₂.
Answer: 50 NADH and 10 FADH₂.
Common mistake
Thinking the Krebs cycle makes most of the ATP. It makes only 2 ATP per glucose, the same as glycolysis. Its big product is 6 NADH and 2 FADH₂, whose electrons drive the electron transport chain, where most ATP is made. Check the table: ATP from these stages totals only 4.