AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The oxidised NAD produced in this way can be used in further glycolysis.
Full specification wording
Respiration (A-level only): The oxidised NAD produced in this way can be used in further glycolysis.
NAD is a coenzyme, and a cell holds only a small pool of it.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 5 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
NAD is a coenzyme, and a cell holds only a small pool of it. Every triose phosphate oxidised in glycolysis loads one NAD with hydrogen, so unless that reduced NAD is unloaded again the pool is quickly exhausted and glycolysis stops, taking with it the cell's only anaerobic source of ATP. Aerobically, the unloading happens at the inner mitochondrial membrane, where reduced NAD is oxidised and passes its electrons to the electron transfer chain. Anaerobically, it happens when reduced NAD donates its hydrogen to pyruvate or to ethanal, forming lactate or ethanol. Either way, oxidised NAD returns to the cytoplasm and accepts hydrogen from the next triose phosphate, so glycolysis continues and the net two ATP per glucose keep being made.
What earns the marks
- Stating that NAD is regenerated in its oxidised form when reduced NAD gives up its hydrogen.
- The oxidised NAD accepting hydrogen from triose phosphate in further glycolysis.
- Explaining that glycolysis, and so ATP production, would stop if NAD were not regenerated.
- Identifying the small size of the cell's NAD pool as the reason recycling is necessary.
Common mistakes and how to put them right
Saying NAD is used up in respiration rather than continuously recycled.
Confusing the direction of the change, so that reduced NAD is described as being reduced again.
Crediting the anaerobic pathway with producing ATP, when its purpose is to regenerate NAD.
Assuming a fresh supply of NAD is synthesised for every glucose molecule respired.
Exam tips
- When asked why lactate or ethanol is made at all, answer in terms of regenerating NAD, not of disposing of pyruvate.
- Use the words oxidised and reduced precisely, because examiners look for the correct direction of hydrogen transfer.
- Finish the chain of reasoning: NAD regenerated, glycolysis continues, ATP still produced.
Try the lesson
The start of the Respiration (A-level only): The oxidised NAD produced in this way can be used in further glycolysis. lesson, made from checked teaching notes for this specification point.
Have a go first
The oxidised NAD produced during anaerobic respiration is reused in glycolysis. What is the role of NAD in glycolysis?
Listen
NAD is a coenzyme, and a cell holds only a small pool of it. Every triose phosphate oxidised in glycolysis loads one NAD with hydrogen, so unless that reduced NAD is unloaded again the pool is quickly exhausted and glycolysis stops, taking with it the cell's only anaerobic source of ATP. Aerobically, the unloading happens at the inner mitochondrial membrane, where reduced NAD is oxidised and passes its electrons to the electron transfer chain. Anaerobically, it happens when reduced NAD donates its hydrogen to pyruvate or to ethanal, forming lactate or ethanol. Either way, oxidised NAD returns to the cytoplasm and accepts hydrogen from the next triose phosphate, so glycolysis continues and the net two ATP per glucose keep being made. A strong answer earns marks for these points. stating that NAD is regenerated in its oxidised form when reduced NAD gives up its hydrogen. the oxidised NAD accepting hydrogen from triose phosphate in further glycolysis. explaining that glycolysis, and so ATP production, would stop if NAD were not regenerated. identifying the small size of the cell's NAD pool as the reason recycling is necessary. Exam tip: When asked why lactate or ethanol is made at all, answer in terms of regenerating NAD, not of disposing of pyruvate.
Key words
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oxidised NAD
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limited NAD pool
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continuation of glycolysis
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anaerobic ATP supply
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See every definition
- oxidised NAD
- The form produced when reduced NAD gives up its hydrogen, which can be used in further glycolysis.
- limited NAD pool
- A cell holds only a small pool of this coenzyme, so unless reduced NAD is unloaded again the pool is quickly exhausted and glycolysis stops.
- continuation of glycolysis
- Oxidised NAD returns to the cytoplasm and accepts hydrogen from the next triose phosphate, so the net two ATP per glucose keep being made.
- anaerobic ATP supply
- Glycolysis is the cell's only anaerobic source of ATP, and it stops if the pool is exhausted.
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