Question
Describe the four complexes of the electron transport chain in mitochondria. How does the proton gradient created by the ETC drive ATP synthesis through chemiosmosis? Calculate the total ATP yield from one molecule of NADH and one molecule of FADH₂.
(NEET 2022, similar pattern)
Solution — Step by Step
The ETC is located on the inner mitochondrial membrane:
| Complex | Name | What it does | Protons pumped |
|---|---|---|---|
| I | NADH dehydrogenase | Accepts electrons from NADH, passes to CoQ | 4 H⁺ |
| II | Succinate dehydrogenase | Accepts electrons from FADH₂, passes to CoQ | 0 H⁺ |
| III | Cytochrome bc1 | Transfers electrons from CoQ to Cyt c | 4 H⁺ |
| IV | Cytochrome c oxidase | Passes electrons to O₂ (final acceptor) | 2 H⁺ |
Mobile carriers: Coenzyme Q (ubiquinone) shuttles electrons from Complex I/II to III. Cytochrome c shuttles electrons from III to IV.
At Complex IV, electrons are finally transferred to molecular oxygen ():
This is why we breathe — oxygen is needed as the terminal electron acceptor. Without it, the entire chain backs up and ATP production halts.
As electrons flow through Complexes I, III, and IV, protons () are pumped from the matrix to the intermembrane space, creating a concentration gradient.
This gradient (called the proton motive force) drives protons back through ATP synthase (Complex V) — a molecular turbine that converts ADP + Pi to ATP.
For each NADH: 10 H⁺ are pumped (4 + 4 + 2). Approximately 4 H⁺ are needed per ATP, giving about 2.5 ATP per NADH.
For each FADH₂: electrons enter at Complex II (skipping Complex I), so only 6 H⁺ are pumped (0 + 4 + 2), giving about 1.5 ATP per FADH₂.
Why This Works
The ETC is essentially a controlled “burn” of electrons. Instead of releasing all the energy at once (like fire), the chain breaks the process into small steps. At each complex, a little energy is released and used to pump protons across the membrane.
The genius of chemiosmosis (proposed by Peter Mitchell, Nobel Prize 1978) is that it converts chemical energy into a physical gradient, then back into chemical energy (ATP). The proton gradient is like water behind a dam — ATP synthase is the turbine.
Complex II doesn’t pump protons because the energy drop from FADH₂ to CoQ is too small. That’s why FADH₂ gives fewer ATP than NADH.
Alternative Method — Quick ATP Count
For NEET, the simplified count (NCERT values):
| Source | ATP per molecule |
|---|---|
| 1 NADH (mitochondrial) | 2.5 (or 3 in older books) |
| 1 FADH₂ | 1.5 (or 2 in older books) |
Total from 1 glucose: 10 NADH (= 25 ATP) + 2 FADH₂ (= 3 ATP) + 2 GTP from Krebs cycle (= 2 ATP) + 2 ATP from glycolysis + 2 ATP from Krebs = 30-32 ATP (net).
NEET uses the newer values (2.5 per NADH, 1.5 per FADH₂), giving a net of about 30-32 ATP per glucose. Some older NCERT editions show 36-38 ATP (using 3 per NADH, 2 per FADH₂). Check which value your textbook uses, but for NEET 2023 onwards, the 30-32 figure is standard.
Common Mistake
Students often forget that the 2 NADH from glycolysis are produced in the cytoplasm, not the mitochondrial matrix. To enter the ETC, they must cross the inner mitochondrial membrane via shuttle systems. The glycerol-3-phosphate shuttle converts them to FADH₂ (1.5 ATP each), while the malate-aspartate shuttle keeps them as NADH (2.5 ATP each). NEET questions sometimes specifically test this — the 2 cytoplasmic NADH may yield only 3 ATP (not 5) depending on the shuttle used.