Biology · Introductory biology · Worked example
Count the ATP made from one glucose
Follow one glucose molecule through glycolysis, pyruvate oxidation and the citric acid cycle. How many ATP, NADH, FADH₂ and CO₂ does it make? About how many ATP does it give in all once NADH and FADH₂ pass their electrons to the electron transport chain? Use 2.5 ATP per NADH and 1.5 per FADH₂.
Glycolysis
One glucose gives 2 pyruvate, a net 2 ATP and 2 NADH. No carbon has been lost yet.
Pyruvate oxidation
Each pyruvate loses one carbon as CO₂ and makes 1 NADH as it becomes acetyl-CoA. For the 2 pyruvate: 2 CO₂ and 2 NADH.
Two turns of the citric acid cycle
Each turn makes 3 NADH, 1 FADH₂, 1 ATP and 2 CO₂, and the 2 acetyl-CoA drive two turns: 6 NADH, 2 FADH₂, 2 ATP and 4 CO₂.
Add up each stage
ATP: 2 + 2 = 4. NADH: 2 + 2 + 6 = 10. FADH₂: 2. CO₂: 2 + 4 = 6, all six of glucose’s carbons.
Estimate the total ATP
Oxidative phosphorylation adds about 28 ATP to the 4. The real yield varies, and it is nearer 30 when the NADH made in glycolysis enters the mitochondrion through a shuttle that passes its electrons to FAD.
Result
Before the electron transport chain, one glucose gives 4 ATP, 10 NADH, 2 FADH₂ and 6 CO₂. With 2.5 ATP per NADH and 1.5 per FADH₂ that is about 32 ATP in all, or nearer 30 depending on how glycolysis’s NADH enters the mitochondrion.
Your turn
Without oxygen, yeast ferments glucose. How many ATP does it make per glucose, and what fraction is that of the 32 from aerobic respiration?
Show the answer and explanation
2 ATP, one-sixteenth of the aerobic yield.
Fermentation only regenerates NAD⁺ so that glycolysis can continue; glycolysis nets 2 ATP per glucose, and nothing else makes any. That is about 6% of the aerobic yield.
Keep exploring
The citric acid cycle opens in Biology with each product of one turn beside the step that makes it; step through it to see where each NADH, FADH₂, ATP and CO₂ comes from.
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