The Krebs Cycle

Where does the Krebs cycle take place?

• In the mitochondrial matrix.

What molecule combines with Acetyl CoA to start the cycle?

• Oxaloacetate (4C).

What is formed when Acetyl CoA combines with Oxaloacetate?

• Citrate (6C).

What happens to Coenzyme A after the acetyl group is dropped off?

• It is recycled/reused to go back to the Link Reaction and transport another acetyl group.

Which coenzyme is only involved in the Krebs cycle?

• FAD.

How is ATP produced in the Krebs cycle?

• By substrate-level phosphorylation (direct transfer of phosphate to ADP).

How many molecules of Reduced NAD are produced per turn of the cycle?

• 3.

How many molecules of ATP are produced per glucose molecule in the Krebs cycle?

• : 2. (1 per turn × 2 turns).

How many molecules of CO₂ are produced per glucose molecule in the Krebs cycle?

• 4. (2 per turn × 2 turns).

Why is this pathway called a "cycle"?

• Because the starting molecule, Oxaloacetate (4C), is regenerated at the end of the process so it can accept another acetyl group.

What is the main function of the Krebs cycle?

• To release hydrogen atoms (carried by Reduced NAD and FAD) to be used in oxidative phosphorylation (the Electron Transport Chain).

What happens to the CO₂ produced?

• It diffuses out of the mitochondrion, out of the cell, and into the blood to be excreted by the lungs.

(6 marks) Describe the sequence of events in the Krebs cycle that leads to the production of Reduced NAD and ATP.

• Acetyl CoA (2C) combines with Oxaloacetate (4C) to form Citrate (6C).

• Citrate undergoes decarboxylation (loses CO₂) and dehydrogenation (loses H) to form a 5C compound; this reduces NAD.

• The 5C compound undergoes further decarboxylation and dehydrogenation to form a 4C compound; this reduces more NAD.

• The 4C compound is converted back to Oxaloacetate.

• During this regeneration, substrate-level phosphorylation produces ATP.

• More dehydrogenation occurs, reducing FAD and another NAD.

3 marks) Explain the significance of the formation of Reduced NAD and Reduced FAD in the Krebs cycle.

• They act as carriers of hydrogen atoms (or protons and electrons) (1).

• They transport these hydrogens to the cristae (inner mitochondrial membrane) for the Electron Transport Chain (1).

• Here, the hydrogens are used to generate ATP via oxidative phosphorylation (1).

(2 marks) Why does the Krebs cycle stop if there is no oxygen available?

• The Krebs cycle does not use oxygen directly, but it relies on NAD and FAD (1).

• Oxygen is the final electron acceptor in the Electron Transport Chain. Without it, the ETC stops, and Reduced NAD/FAD cannot be re-oxidised back into NAD/FAD (1).

• Without NAD/FAD, the Krebs cycle cannot continue.

(2 marks) Fatty acids can be broken down into 2-carbon acetyl groups. Explain how fatty acids can be used as a fuel for respiration.

• The acetyl groups formed from fatty acids can combine with Coenzyme A to form Acetyl CoA (1).

• This Acetyl CoA can enter the Krebs cycle directly, bypassing glycolysis and the link reaction, to generate Reduced NAD/FAD and ATP (1).

(3 marks) A student calculates that 1 molecule of glucose produces 38 ATP. However, the actual yield is often lower (around 30-32 ATP). Suggest reasons why.

• Some ATP is used to actively transport pyruvate into the mitochondria.

• Some ATP is used to transport Reduced NAD (from glycolysis) into the mitochondria.

• Some protons may leak across the mitochondrial membrane (bypassing ATP synthase). (Any 3 points)