ATP per Glucose: ETC and Oxidative Phosphorylation Count

ATP Yield per Glucose: Electron Transport Chain and Oxidative Phosphorylation

By StudyStore · Published 2026-11-04 · 2 min read · 401 words

Oxidative phosphorylation turns NADH and FADH₂ into ATP. Electrons pass down the chain in the inner mitochondrial membrane, protons are pumped into the intermembrane space, and ATP synthase lets them flow back to make ATP (chemiosmosis).

The chain

Complex	Takes electrons from	Pumps H⁺
I (NADH dehydrogenase)	NADH	Yes
II (succinate dehydrogenase)	FADH₂	No
III (cytochrome bc1) via ubiquinone	I and II	Yes
IV (cytochrome c oxidase) via cytochrome c	III	Yes

O₂ is the final acceptor, forming H₂O. Complex IV needs O₂, so no oxygen stops the whole chain.

P/O ratios

NADH: about 2.5 ATP (older texts: 3).
FADH₂: about 1.5 ATP (older texts: 2). It enters at complex II and skips complex I.

Counting one glucose

Stage	Direct ATP/GTP	NADH	FADH₂
Glycolysis (net)	2	2	0
Pyruvate to acetyl-CoA (x2)	0	2	0
Citric acid cycle (x2)	2	6	2
Total	4	10	2

Using 2.5 and 1.5: 10 x 2.5 + 2 x 1.5 = 28 from the chain, plus 4 substrate-level (2 net from glycolysis plus 2 from the cycle) gives about 30. If the glycolysis NADH enters by the malate-aspartate shuttle, it keeps its full value and the total is 32. With the older ratios (3 and 2) the total is 36 to 38.

Shuttles

Glycerol-3-phosphate shuttle (muscle, brain): delivers electrons as FADH₂, so cytosolic NADH is worth about 1.5 ATP.
Malate-aspartate shuttle (heart, liver): delivers NADH, worth about 2.5 ATP.

Uncouplers and inhibitors

2,4-dinitrophenol and thermogenin (UCP1): let protons leak back, so the chain runs but little ATP is made and heat is released.
Cyanide, carbon monoxide: block complex IV.
Oligomycin: blocks ATP synthase; the gradient builds up and the chain slows.

Common traps

Giving FADH₂ the same value as NADH. It skips complex I.
Forgetting to say which number the question wants. Use the ratio the question gives.
Saying the chain makes ATP directly. ATP synthase makes it from the proton gradient.

One-minute recap

NADH about 2.5 ATP, FADH₂ about 1.5.
One glucose gives about 30 to 32 ATP (36 to 38 with older ratios).
Uncouplers dissipate the gradient and make heat.

Check yourself: 5-question mini-quiz

Try each question first, then click it to reveal the answer.

1. Why does FADH2 yield less ATP than NADH?

It enters at complex II and bypasses complex I, so fewer protons are pumped.

2. What is the final electron acceptor?

O2, which becomes H2O at complex IV.

3. What does an uncoupler do?

It lets protons leak across the membrane, so O2 use continues but less ATP is made and heat is released.

4. Which shuttle gives cytosolic NADH full value?

The malate-aspartate shuttle.

5. What does oligomycin block?

ATP synthase.

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Written by StudyStore from standard scientific facts. Study aid only. Always check values against the reference your exam provides.