Follow the Krebs cycle and coenzyme reduction
Explain citrate formation, regeneration of oxaloacetate, decarboxylation, dehydrogenation and reduction of NAD and FAD.
Section 12.2 · Respiration
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- ATP transfers energy for cellular work, and enzymes control metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which 4C molecule accepts the acetyl group at the start of the Krebs cycle?
What you will learn
- Trace acetyl entry into citrate and regeneration of oxaloacetate
- Relate decarboxylation and dehydrogenation to reduced coenzymes
Build the explanation
In the Krebs cycle, the 2C acetyl group carried by acetyl coenzyme A combines with 4C oxaloacetate to form 6C citrate. Citrate is converted back to oxaloacetate through a series of small steps, allowing the cycle to continue. Carbon dioxide is released in decarboxylation reactions. Hydrogen is removed in dehydrogenation reactions and accepted by NAD and FAD, reducing them to reduced NAD and reduced FAD. ATP is also formed in the cycle.
Apply it carefully
Oxaloacetate is regenerated rather than used up permanently. Do not claim that every step is a single reaction or that the cycle produces glucose. Keep decarboxylation (CO₂ loss) distinct from dehydrogenation (hydrogen removal); the reduced coenzymes carry hydrogen onward to the inner mitochondrial membrane.
Pause and explain
Which coenzymes are reduced during Krebs-cycle dehydrogenation?
Worked example
How does a 2C acetyl group contribute to formation of citrate?
Show the worked solution
- Combine the acetyl group with the 4C acceptor.
- 2C + 4C forms 6C citrate.
Answer Acetyl-CoA transfers a 2C acetyl group to 4C oxaloacetate, forming 6C citrate.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Outline how the Krebs cycle begins and how its acceptor is regenerated.
Give me a hint
Track 2C and 4C into 6C, then name the regenerated molecule.
Compare my reasoning
- A 2C acetyl group combines with 4C oxaloacetate to form 6C citrate.
- Citrate is converted through small steps back to oxaloacetate, while CO₂ and reduced coenzymes are produced.
Acetyl-CoA transfers a 2C acetyl group to 4C oxaloacetate, forming 6C citrate. A series of small reactions converts citrate back to oxaloacetate. Decarboxylation releases CO₂ and dehydrogenation reduces NAD and FAD.
Look for these in your work
- Carbon numbers correct
- Citrate named
- Oxaloacetate regeneration stated
- CO₂ and reduced coenzymes included
A student says decarboxylation reduces NAD. Separate the two processes.
Give me a hint
One removes carbon dioxide; the other removes hydrogen.
Compare my reasoning
- Decarboxylation removes CO₂.
- Dehydrogenation removes hydrogen, which can reduce NAD or FAD.
Decarboxylation removes carbon dioxide from an intermediate. Dehydrogenation removes hydrogen; NAD and FAD accept it and become reduced NAD and reduced FAD.
Look for these in your work
- Decarboxylation defined
- Dehydrogenation defined
- Coenzyme reduction linked
A tracer follows a two-carbon acetyl group into a Krebs-cycle intermediate. Explain why the cycle can continue after that acetyl group is processed.
Give me a hint
Identify the acceptor's fate.
Compare my reasoning
- The acetyl group combines with oxaloacetate to form citrate.
- Oxaloacetate is regenerated through a series of reactions and can accept another acetyl group.
The acetyl group combines with oxaloacetate to form citrate. The cycle processes citrate through several reactions and regenerates oxaloacetate, so the acceptor can enter another turn.
Look for these in your work
- Acetyl entry stated
- Citrate named
- Acceptor regeneration explains cycle
Common mistakes
- Treating oxaloacetate as permanently consumed
- Confusing decarboxylation and dehydrogenation
- Calling a cycle a straight pathway with no regenerated acceptor
What can you explain now?
Which 4C molecule accepts the acetyl group at the start of the Krebs cycle?
Compare with the explanation
Oxaloacetate
The combination forms 6C citrate.
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Make it stick
How do acetyl-CoA, oxaloacetate, carbon dioxide and reduced coenzymes feature in the Krebs cycle?
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