Oxidative Phosphorylation

Oxidative phosphorylation is the final stage of aerobic respiration and the stage that produces the majority of ATP. It takes place on the inner mitochondrial membrane and involves the electron transport chain (ETC) and the enzyme ATP synthase. Reduced coenzymes (NADH and FADH₂) from glycolysis, the link reaction, and the Krebs cycle donate their electrons to the ETC, and the energy released as electrons pass along the chain is used to generate a proton gradient that drives ATP synthesis.

Key Definition: Oxidative phosphorylation is the synthesis of ATP from ADP and inorganic phosphate, driven by the transfer of electrons along the electron transport chain and the resulting chemiosmotic gradient across the inner mitochondrial membrane.

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Mitochondrial Structure and Oxidative Phosphorylation

The inner mitochondrial membrane is highly folded into cristae, which greatly increase the surface area available for:

• Electron transport chain carrier proteins.
• ATP synthase enzymes.

The intermembrane space (between the inner and outer membranes) is a small, confined volume, which allows a steep proton (H⁺) gradient to build up rapidly.

The matrix contains the enzymes of the Krebs cycle and the link reaction, producing the reduced coenzymes close to the inner membrane where they are needed.

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The Electron Transport Chain (ETC)

The ETC consists of a series of electron carrier proteins (also called cytochromes and iron-sulphur proteins) embedded in the inner mitochondrial membrane. These carriers accept and donate electrons in a specific sequence, based on their increasing electronegativity (affinity for electrons).

Key Electron Carriers

• Complex I (NADH dehydrogenase) — accepts electrons from NADH.
• Complex II (Succinate dehydrogenase) — accepts electrons from FADH₂ (this complex is also part of the Krebs cycle).
• Ubiquinone (Coenzyme Q) — a mobile carrier that transfers electrons from Complexes I and II to Complex III.
• Complex III (Cytochrome bc₁ complex) — passes electrons to cytochrome c.
• Cytochrome c — a small mobile protein that shuttles electrons from Complex III to Complex IV.
• Complex IV (Cytochrome c oxidase) — the final electron carrier; passes electrons to oxygen.

How the ETC Works

1. NADH donates its two electrons to Complex I. As the electrons pass through Complex I, energy is released and used to pump H⁺ ions (protons) from the mitochondrial matrix into the intermembrane space.

2. FADH₂ donates its two electrons to Complex II. Complex II does not pump protons (this is why FADH₂ generates fewer ATP than NADH).

3. Electrons from both complexes are passed to ubiquinone, then to Complex III, which pumps more H⁺ ions into the intermembrane space.

4. Electrons pass via cytochrome c to Complex IV, which pumps additional H⁺ ions into the intermembrane space.

5. At Complex IV, electrons are finally accepted by oxygen (O₂), the terminal electron acceptor. Oxygen combines with the electrons and H⁺ ions to form water:

   ½O₂ + 2H⁺ + 2e⁻ → H₂O

Key Definition: Oxygen is the terminal (final) electron acceptor in the electron transport chain. Without oxygen, electrons cannot be passed along the chain, the proton gradient collapses, and oxidative phosphorylation stops.

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Chemiosmosis and ATP Synthesis

The Proton (Electrochemical) Gradient

As electrons pass along the ETC, the energy released is used to actively pump H⁺ ions from the matrix into the intermembrane space. This creates:

• A concentration gradient — higher [H⁺] in the intermembrane space than in the matrix.
• An electrical gradient (charge difference) — the intermembrane space becomes more positively charged.
• Together, these form a proton motive force (electrochemical gradient).

ATP Synthase

The inner mitochondrial membrane is impermeable to H⁺ ions (they cannot diffuse freely across the phospholipid bilayer). The only route for H⁺ ions to return to the matrix is through the channel protein ATP synthase.