OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
Respiration: (e) the process and site of the Krebs cycle
The Krebs cycle is a sequence of enzyme-controlled reactions that oxidise acetyl groups, releasing carbon dioxide and reducing coenzymes.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 5 of 13
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The Krebs cycle is a sequence of enzyme-controlled reactions that oxidise acetyl groups, releasing carbon dioxide and reducing coenzymes. It occurs in the mitochondrial matrix, where the enzymes for the cycle are located. Acetyl coenzyme A delivers a two-carbon acetyl group, which joins a four-carbon intermediate to form a six-carbon compound; this is decarboxylated and dehydrogenated in stages, regenerating the four-carbon intermediate so the cycle can repeat. Carbon dioxide is released, and the reduced coenzymes NADH and FADH₂ are produced, along with a small yield of ATP by substrate-level phosphorylation. The reduced coenzymes later supply electrons to the electron transport chain.
What earns the marks
- States that the Krebs cycle occurs in the mitochondrial matrix.
- Explains that acetyl coenzyme A donates a two-carbon acetyl group that combines with a four-carbon intermediate to form a six-carbon compound.
- Describes decarboxylation releasing carbon dioxide and dehydrogenation producing reduced NAD and reduced FAD.
- Identifies that the four-carbon intermediate is regenerated so the cycle continues.
- Recognises that a small amount of ATP is produced by substrate-level phosphorylation and that reduced coenzymes pass electrons to the electron transport chain.
Common mistakes and how to put them right
Placing the Krebs cycle on the cristae or inner membrane.
Put it right by stating it occurs in the mitochondrial matrix, while the electron transport chain is on the inner membrane.
Treating the cycle as producing large amounts of ATP directly.
Put it right by noting only a small ATP yield, with most ATP arising later from oxidative phosphorylation.
Forgetting that the four-carbon intermediate is regenerated.
Put it right by describing the cycle as returning to its starting four-carbon compound after each turn.
Exam tips
- Read each option carefully and eliminate those naming the wrong site, such as cytoplasm or cristae.
- Link each named product to the step that forms it: carbon dioxide from decarboxylation, reduced coenzymes from dehydrogenation.
- Check whether an option confuses the Krebs cycle with glycolysis or the electron transport chain before selecting.
Try the lesson
The start of the Respiration: (e) the process and site of the Krebs cycle lesson, made from checked teaching notes for this specification point.
Have a go first
In which part of a eukaryotic cell does the Krebs cycle take place?
Listen
The Krebs cycle is a sequence of enzyme-controlled reactions that oxidise acetyl groups, releasing carbon dioxide and reducing coenzymes. It occurs in the mitochondrial matrix, where the enzymes for the cycle are located. Acetyl coenzyme A delivers a two-carbon acetyl group, which joins a four-carbon intermediate to form a six-carbon compound; this is decarboxylated and dehydrogenated in stages, regenerating the four-carbon intermediate so the cycle can repeat. Carbon dioxide is released, and the reduced coenzymes NADH and FADH₂ are produced, along with a small yield of ATP by substrate-level phosphorylation. The reduced coenzymes later supply electrons to the electron transport chain. A strong answer earns marks for these points. States that the Krebs cycle occurs in the mitochondrial matrix. Explains that acetyl coenzyme A donates a two-carbon acetyl group that combines with a four-carbon intermediate to form a six-carbon compound. Describes decarboxylation releasing carbon dioxide and dehydrogenation producing reduced NAD and reduced FAD. Identifies that the four-carbon intermediate is regenerated so the cycle continues. A common slip: Placing the Krebs cycle on the cristae or inner membrane. Instead, stating it occurs in the mitochondrial matrix, while the electron transport chain is on the inner membrane. Exam tip: Read each option carefully and eliminate those naming the wrong site, such as cytoplasm or cristae.
Key words
Say what each word means before you turn the card.
Mitochondrial matrix
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Decarboxylation
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Dehydrogenation
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Four-carbon intermediate
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See every definition
- Mitochondrial matrix
- The site of the Krebs cycle, where the enzymes for the cycle are located.
- Decarboxylation
- A stage in which carbon dioxide is released as the six-carbon compound is broken down.
- Dehydrogenation
- A stage in which the reduced coenzymes NADH and FADH₂ are produced.
- Four-carbon intermediate
- The compound that a two-carbon acetyl group joins to form a six-carbon compound, and which is regenerated so the cycle can repeat.
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