A2 1 / A2 2 respiration structured question · 6 marks
Describe the role of the Krebs cycle in the production of ATP.
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A2 · A2 2 Biochemistry, Genetics and Evolutionary Trends
The Krebs cycle: what to remember and what to leave out
The short answer
The Krebs cycle runs in the mitochondrial matrix. Acetyl coenzyme A (2C) joins oxaloacetate (4C) to form citrate (6C), which is decarboxylated and dehydrogenated back to oxaloacetate. Per turn: 2 CO₂, 3 reduced NAD, 1 reduced FAD and 1 ATP by substrate-level phosphorylation.
The carbon bookkeeping
- Glycolysis (cytoplasm): glucose (6C) → 2 pyruvate (3C), net 2 ATP and 2 reduced NAD.
- Link reaction (matrix): pyruvate (3C) → acetyl CoA (2C), releasing CO₂ and making reduced NAD.
- Krebs (matrix): acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C) → 5C → back to oxaloacetate (4C).
- One glucose gives two turns of the cycle, because it gave two pyruvates.
Why the cycle matters
The cycle itself makes very little ATP directly. Its real job is to strip hydrogen from intermediates and load it onto NAD and FAD. Those reduced coenzymes carry electrons to the electron transport chain on the inner mitochondrial membrane, where the great majority of ATP is made by chemiosmosis.
Rough yields per reduced coenzyme: about 2.5–3 ATP from reduced NAD and about 1.5–2 ATP from reduced FAD, giving roughly 30–32 ATP per glucose overall. Quote the value your specification uses and state that it is a theoretical maximum.
Anaerobic conditions
Without oxygen the final electron acceptor is missing, the electron transport chain backs up, NAD is not regenerated and both the link reaction and the Krebs cycle stop. Only glycolysis continues, with pyruvate converted to lactate (animals) or ethanol and CO₂ (yeast) to reoxidise NAD.
Phrases that earn the marks
- decarboxylation releases carbon dioxide
- dehydrogenation / hydrogen removed and accepted by NAD
- reduced NAD and reduced FAD carry electrons to the electron transport chain
- oxaloacetate is regenerated
- substrate-level phosphorylation produces one ATP per turn
- oxygen is the final electron acceptor
Where marks get lost
- Saying the Krebs cycle 'makes ATP' as its main function — it mainly makes reduced coenzymes.
- Forgetting that one glucose drives two turns.
- Writing NADH when the specification wants 'reduced NAD'.
- Placing the cycle on the cristae; it happens in the matrix.
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Exam questions on krebs cycle a level, marked
Two A2-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
A2 1 / A2 2 respiration structured question · 6 marks
Describe the role of the Krebs cycle in the production of ATP.
Command word: Describe — how to answer itMark scheme — 6 creditworthy points
- Takes place in the matrix of the mitochondrion
- Acetyl coenzyme A (2C) combines with oxaloacetate (4C) to form citrate (6C)
- Citrate is decarboxylated, releasing carbon dioxide
- Citrate is dehydrogenated / hydrogen removed and accepted by NAD and FAD
- Reduced NAD and reduced FAD pass electrons to the electron transport chain
- One ATP produced per turn by substrate-level phosphorylation; oxaloacetate is regenerated
Full-mark answer
The Krebs cycle takes place in the mitochondrial matrix. Acetyl coenzyme A (2C) combines with oxaloacetate (4C) to form citrate (6C). Citrate is then decarboxylated, releasing carbon dioxide, and dehydrogenated, with the hydrogen accepted by NAD and FAD to form reduced NAD and reduced FAD. One ATP is produced per turn by substrate-level phosphorylation, and oxaloacetate is regenerated so the cycle can continue. The main contribution to ATP yield is indirect: reduced NAD and reduced FAD deliver electrons to the electron transport chain, where oxidative phosphorylation produces most of the cell's ATP.
Examiner insight: The question says 'in the production of ATP', so the final marking point — that the real yield comes from the reduced coenzymes feeding the electron transport chain — is essential. A pure description of the cycle typically caps at 4 marks.
A2 'explain' question — recurring anaerobic respiration link · 3 marks
Explain why, in the absence of oxygen, the Krebs cycle stops even though oxygen is not used directly in the cycle.
Command word: Explain — how to answer itMark scheme — 3 creditworthy points
- Oxygen is the final electron acceptor at the end of the electron transport chain
- Without oxygen the electron transport chain stops, so reduced NAD and reduced FAD cannot be reoxidised
- No NAD or FAD available to accept hydrogen in the Krebs cycle, so the dehydrogenation reactions stop
Full-mark answer
Oxygen acts as the final electron acceptor at the end of the electron transport chain. Without it, electrons cannot pass along the chain, so reduced NAD and reduced FAD cannot be reoxidised back to NAD and FAD. Because there is no oxidised NAD or FAD to accept hydrogen from the dehydrogenation reactions, the Krebs cycle stops.
Examiner insight: The key idea examiners look for is the word 'reoxidised'. Answers that simply say 'no oxygen so no respiration' score zero.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemeCommon questions
- Where does the Krebs cycle take place?
- In the mitochondrial matrix. The electron transport chain that follows it is on the inner mitochondrial membrane (the cristae).
- How many ATP does the Krebs cycle produce?
- One ATP directly per turn by substrate-level phosphorylation, so two per glucose. Most ATP comes later, from the reduced NAD and FAD it produces.
- Do I need to name every intermediate?
- No. A-level questions want acetyl CoA, oxaloacetate, citrate and the carbon counts, plus the products of each turn.
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