Linear and cyclical pathways: Higher Level only: This statement is Higher Level only. Use the guide's examples: glycolysis, the Krebs cycle and the Calvin cycle.
Practise this as you read
- Distinguish between linear and cyclical pathways.
- Identify each named pathway as linear or cyclical.
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The big idea: In a linear pathway the end product is different from the starting substance and leaves the pathway. In a cyclical pathway the end product is the starting substance, so it is regenerated and used again.
The points to remember
- Glycolysis is linear: glucose is changed step by step into pyruvate.
- The Krebs cycle is cyclical: oxaloacetate accepts an acetyl group and is regenerated at the end.
- The Calvin cycle is cyclical: RuBP fixes carbon dioxide and is regenerated.
- Both kinds are chains of steps, each catalysed by its own enzyme.
Remember it as: Linear: in at one end, out at the other. Cyclical: back to the start.
Real example: one acetyl group per turn. Each turn of the Krebs cycle takes in one acetyl group and releases two carbon dioxide molecules; the oxaloacetate is left over to start the next turn.
Trap: What is recycled in a cycle is a substrate (such as oxaloacetate), not the enzymes. Enzymes are reused in every pathway.
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How it is asked: Expect a short Distinguish question on cyclical and linear pathways, or which named pathway is which.
IB-style questionDistinguish[2 marks]
Distinguish between linear and cyclical metabolic pathways, using named examples.
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