• It is the final stage of aerobic respiration.

• Occurs in the inner mitochondrial membrane (cristae).

• It uses the electrons from reduced NAD (NADH) and reduced FAD (FADH₂) to generate ATP.

• Involves two key processes:

1. Electron Transport Chain (ETC)

2. Chemiosmosis

1. Electron Transport Chain (ETC)

• NADH (and FADH) are oxidised. The hydrogen splits into a proton and an electron.

• The electrons pass to the electron transport chain and pass along electron carriers in a series of redox reactions. This transfer releases energy.

• This energy is used to pump protons into the intermembrane space of the mitochondria.

2. Chemiosmosis

• A proton gradient is established (high H⁺ in intermembrane space).

• Protons diffuse back into the matrix via the enzyme ATP synthase.

• This process provides energy for the synthesis of ATP from ADP + Pi.

• This is called oxidative phosphorylation because the production of ATP used energy from the oxidation of reduced coenzymes.

3. Formation of Water

• At the end of the ETC, electrons combine with:

• Protons (H⁺)

• Oxygen (½ O₂)

• This forms water (H₂O):

4e− + 4H+ + O₂ → 2H₂O

• We say that oxygen acts as the final electron acceptor in the electron transport chain.

ATP Yield (Theoretical)

Note: Actual ATP yield may vary due to mitochondrial membrane leakage and energy used for transporting ADP/Pi.

1. Electron Transport Chain (ETC) & Oxidative Phosphorylation

Oxygen is the final electron acceptor in the ETC. Without it:

• Electrons cannot be passed along the ETC because there's nowhere for them to go at the end.

• Electron carriers remain reduced (full of electrons) — NADH and FADH₂ cannot unload their electrons.

• This means NAD and FAD are not regenerated.

• No chemiosmosis occurs → no ATP is made by oxidative phosphorylation.

• Therefore, ATP production drops dramatically (from ~32 to just 2 per glucose!).

2. Link Reaction

The link reaction needs NAD to accept hydrogen when pyruvate is oxidised.

Without oxygen:

• NAD is not regenerated (because NADH isn't oxidised in the ETC).

• So, pyruvate cannot be converted to acetyl-CoA.

• Link reaction stops.

3. Krebs Cycle

The Krebs cycle also requires oxidised NAD and FAD to accept hydrogen atoms.

Without oxygen:

• NAD and FAD are not regenerated, so no further reduction of these coenzymes can occur.

• Therefore, the Krebs cycle stops almost immediately after oxygen is lost.