Oxidative Phosphorylation

This lesson covers: 

1. Where oxidative phosphorylation occurs
2. The reactants and products of the oxidative phosphorylation
3. The main steps of oxidative phosphorylation
4. Calculating the total ATP yield from aerobic respiration

Introduction to oxidative phosphorylation

In the stages of aerobic respiration so far, only substrate-level phosphorylation has produced ATP, when a phosphate group is transferred from an intermediate to ADP.

Oxidative phosphorylation, the final stage of aerobic respiration, generates ATP by using the flow of protons through ATP synthase.

Where oxidative phosphorylation occurs:

• Oxidative phosphorylation occurs across the inner mitochondrial membrane of the cristae.
• The cristae are the inner folded membranes in mitochondria where the enzymes and proteins involved in oxidative phosphorylation are located.
• Some of these proteins are involved in the electron transfer chain: a series of electron carrier molecules in the inner mitochondrial membrane that release energy in stages.

Releasing energy in stages allows more energy to be harvested for the benefit of the organism, rather than being released as heat.

Reactants and products of oxidative phosphorylation

Reactants of oxidative phosphorylation:

• Reduced NAD
• Reduced FAD
• Oxygen
• ADP and inorganic phosphate

Products of oxidative phosphorylation:

• NAD
• FAD
• Water
• ATP

The main steps of oxidative phosphorylation

Oxidative phosphorylation has several key steps:

1. Reduced NAD and reduced FAD release hydrogen, transferring protons (H⁺) and electrons (e^-) into the mitochondrial matrix.
2. High-energy electrons are passed to an electron carrier from reduced NAD and reduced FAD.
3. The electrons are passed along a series of electron carrier molecules in the electron transport chain embedded in the inner mitochondrial membrane, releasing energy as they are transferred.
4. The energy is used to actively transport protons across the inner mitochondrial membrane from the mitochondrial matrix into the intermembrane space.
5. The accumulation of protons in the intermembrane space sets up a steep electrochemical gradient of protons across the inner membrane.
6. Protons diffuse back into the mitochondrial matrix down their electrochemical gradient through ATP synthase.
7. This releases energy and catalyses the synthesis of ATP from ADP and inorganic phosphate (P_i).
8. Oxygen is the final electron acceptor, and combines with electrons and protons to form water, helping to maintain the proton gradient.

Chemiosmosis

In aerobic respiration, chemiosmosis is the diffusion of protons across the partially permeable inner mitochondrial membrane, down their electrochemical gradient through ATP synthase channels.

The movement of the protons releases energy that is used to synthesise ATP. It converts the energy from the electrochemical gradient into chemical energy stored in ATP molecules.

Total ATP yield per glucose molecule

The table below shows the total number of ATP molecules produced from one glucose molecule during different stages of aerobic respiration.

Each stage (except the link reaction) contributes to the overall yield of ATP, with oxidative phosphorylation being the most productive.

In conclusion, aerobic respiration in eukaryotic cells approximately yields 34 ATP molecules from the complete oxidation of one glucose molecule.