krebs cycle

Describe the exact steps that occur when Acetyl CoA enters the Krebs cycle. Include carbon counts.

• Coenzyme A detaches from the acetyl group and is recycled.

• The 2C acetyl group combines with 4C oxaloacetate.

• This forms a 6C citrate molecule.

Describe what happens chemically when 6C citrate is converted into a 5C intermediate. (3 marks)

• Decarboxylation: Citrate loses a carbon atom as CO2

• Dehydrogenation: Citrate is oxidised (loses hydrogen atoms).

• Coenzyme Reduction: The hydrogen atoms are accepted by NAD to form reduced NAD.

Describe the changes that occur when the 5C intermediate is converted into a 4C intermediate. (3 marks)

• Decarboxylation: The 5C molecule loses a carbon atom as CO2.

• Dehydrogenation: It is oxidised/loses hydrogen atoms.

• Coenzyme Reduction: Another molecule of NAD is turned into reduced NAD.

How is ATP produced during the conversion of the 4C intermediate back into 4C oxaloacetate? State the exact mechanism term.

• ATP is directly synthesised from ADP and inorganic phosphate

• Via substrate-level phosphorylation

During the final regeneration phase (4C to 4C), which specific coenzymes are reduced and how many of each?

• One FAD is reduced to reduced FAD.

• One NAD is reduced to reduced NAD.

State the net yield of all products from the Krebs cycle per molecule of glucose.

• 6 reduced NAD [1]

• 2 reduced FAD [1]

• 2 ATP [1]

• 4 CO2 [1]

• (Note: The cycle turns twice per glucose because glycolysis produces two pyruvates!)

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The Krebs cycle does not use oxygen. Explain why the cycle completely stops if oxygen is absent.

• Without oxygen, oxidative phosphorylation (the electron transport chain) stops.

• Reduced NAD and reduced FAD cannot drop off their hydrogen atoms / cannot be reoxidised.

• The matrix runs out of empty NAD and FAD, leaving none available to accept hydrogens in the Krebs cycle.