All subjects · Biology · WBI15 (Unit 5)

Oxidative phosphorylation and chemiosmosis

Most ATP is made on the inner mitochondrial membrane, using the hydrogen collected by NAD and FAD and oxygen as the final electron acceptor.

The electron transport chain

1. Reduced NAD and reduced FAD deliver hydrogen atoms to the electron transport chain on the inner membrane (cristae).

2. Each hydrogen atom splits into a proton (H+) and an electron.

3. Electrons pass along a series of electron carriers in redox reactions, releasing energy at each step.

4. This energy is used to pump H+ from the matrix into the intermembrane space.

5. At the end, oxygen is the final electron acceptor: it combines with electrons and H+ to form water.

Chemiosmosis

H+ builds up in the intermembrane space, forming an electrochemical gradient. The inner membrane is impermeable to H+ except through ATP synthase. As H+ diffuses back into the matrix through ATP synthase, the energy drives the phosphorylation of ADP to ATP. This is chemiosmosis, and it makes most of the ATP from aerobic respiration.

Reduced FAD gives its hydrogen further along the chain than reduced NAD, so it leads to fewer ATP.

Why oxygen matters, and ATP yield

Without oxygen, electrons cannot leave the chain, so it stops. Reduced NAD and FAD are not reoxidised, so the Krebs cycle and link reaction also stop. Only glycolysis continues, using anaerobic pathways.

Older textbooks assume 3 ATP per reduced NAD and 2 per reduced FAD, giving a theoretical maximum of about 38 ATP per glucose. Modern estimates are lower, about 30 to 32, because more H+ must flow through ATP synthase per ATP than was once thought, energy is used to transport pyruvate, ADP and ATP across the mitochondrial membranes, and some H+ leaks back across the inner membrane. The actual yield varies between cells and is often lower still.

Worked example

Assume each reduced NAD gives 2.5 ATP and each reduced FAD gives 1.5 ATP. Per glucose there are 10 reduced NAD and 2 reduced FAD, plus 4 ATP made directly. What is the total yield?

  1. Reduced NAD: 10 x 2.5 = 25
  2. Reduced FAD: 2 x 1.5 = 3
  3. Total = 25 + 3 + 4 = 32

Answer: 32 ATP per glucose.

Key idea: Reduced NAD and FAD give H to the electron transport chain; electrons pass along carriers, energy pumps H+ into the intermembrane space; H+ flows back through ATP synthase making ATP (chemiosmosis). Oxygen is the final electron acceptor, forming water.

Check yourself

  1. If each reduced NAD gives 3 ATP and each reduced FAD gives 2 ATP, how many ATP come from oxidative phosphorylation of 10 reduced NAD and 2 reduced FAD?
    Show answer

    34. 30 + 4 = 34.

  2. What is the final electron acceptor in the electron transport chain?
    • Oxygen
    • NAD
    • Carbon dioxide
    • Pyruvate
    Show answer

    Oxygen. Oxygen accepts electrons and H+ to make water.

  3. What is the energy released by electrons in the electron transport chain used for?
    • Splitting water
    • Making glucose
    • Pumping H+ into the intermembrane space
    • Decarboxylating pyruvate
    Show answer

    Pumping H+ into the intermembrane space. Proton pumping builds a gradient.

  4. What is chemiosmosis?
    • The movement of water across a membrane
    • The flow of H+ down its gradient through ATP synthase, driving ATP synthesis
    • The active transport of glucose
    • The splitting of hydrogen atoms by oxygen
    Show answer

    The flow of H+ down its gradient through ATP synthase, driving ATP synthesis. Proton flow drives ATP synthase.

  5. Why does the Krebs cycle stop when oxygen is absent?
    • Glucose cannot enter cells
    • Oxygen is a substrate of every Krebs enzyme
    • The mitochondria burst
    • Reduced NAD and FAD cannot be reoxidised, so no NAD or FAD is available
    Show answer

    Reduced NAD and FAD cannot be reoxidised, so no NAD or FAD is available. Without the chain running, NAD and FAD stay reduced.

  6. Why does reduced FAD lead to fewer ATP than reduced NAD?
    • It carries more hydrogen
    • It passes its hydrogen to the chain at a later point, so fewer H+ are pumped
    • It is used in glycolysis
    • It cannot enter mitochondria
    Show answer

    It passes its hydrogen to the chain at a later point, so fewer H+ are pumped. Entering later means fewer pumping steps.

  7. Why is the actual ATP yield per glucose lower than the theoretical maximum?
    • Glycolysis makes no ATP
    • Oxygen is not needed
    • ATP synthase makes too much ATP
    • Some H+ leaks across the membrane and energy is used for transport
    Show answer

    Some H+ leaks across the membrane and energy is used for transport. Leakage and transport costs reduce the yield.

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Last updated 7 November 2026. Spotted a mistake? Tell us.