ATP Yield per Glucose: Electron Transport Chain and Oxidative Phosphorylation

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

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

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.