Metabolism
The electron transport chain explained
This is where the overwhelming majority of ATP is produced, and where the NADH and FADH2 from earlier pathways finally get spent.
Location and components
The chain sits in the inner mitochondrial membrane, arranged in four complexes plus ATP synthase.
| Complex | Role | Pumps H+? |
|---|---|---|
| I (NADH dehydrogenase) | Accepts electrons from NADH | Yes |
| II (succinate dehydrogenase) | Accepts electrons from FADH2 | No |
| III (cytochrome bc1) | Passes electrons to cytochrome c | Yes |
| IV (cytochrome c oxidase) | Transfers electrons to O2, forming water | Yes |
Because FADH2 enters at complex II and bypasses complex I, it drives fewer protons across the membrane — which is exactly why it yields less ATP than NADH (roughly 1.5 vs 2.5).
Chemiosmosis
As electrons move down the chain, complexes I, III, and IV pump protons from the matrix into the intermembrane space. This builds an electrochemical gradient — the proton-motive force. Protons then flow back into the matrix through ATP synthase, and that flow drives the rotation that phosphorylates ADP into ATP. Peter Mitchell's chemiosmotic theory describing this won the 1978 Nobel Prize in Chemistry.
Why oxygen matters
Oxygen is the final electron acceptor at complex IV. Without it, electrons back up, the carriers stay reduced, NAD+ is not regenerated, and the Krebs cycle halts. This is the precise reason aerobic respiration requires oxygen — not because oxygen is consumed to "burn" fuel directly.
Total aerobic yield
| Stage | Direct ATP | Carriers |
|---|---|---|
| Glycolysis | 2 | 2 NADH |
| Pyruvate → acetyl-CoA | 0 | 2 NADH |
| Krebs cycle (×2) | 2 | 6 NADH, 2 FADH2 |
Using the modern estimates of ~2.5 ATP per NADH and ~1.5 per FADH2, one glucose yields roughly 30–32 ATP. Older textbooks state 36–38; the figure was revised downward once the cost of transporting cytosolic NADH into the mitochondrion was accounted for. If an exam expects the older number, it will usually say so.
Frequently asked questions
How much ATP does one glucose molecule produce?
Modern estimates give roughly 30-32 ATP per glucose, using about 2.5 ATP per NADH and 1.5 per FADH2. Older textbooks cite 36-38, a figure revised downward once the cost of shuttling cytosolic NADH into the mitochondrion was included.
Why does FADH2 yield less ATP than NADH?
FADH2 donates its electrons at complex II, bypassing complex I. Because one fewer proton-pumping complex is involved, it contributes less to the proton gradient and therefore drives less ATP synthesis.
What is the role of oxygen in the electron transport chain?
Oxygen is the final electron acceptor at complex IV, where it combines with electrons and protons to form water. Without it, electrons back up, NAD+ is not regenerated, and the Krebs cycle stops.
Keep reading: The Krebs cycle explained · Glycolysis explained · What is ATP?