AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The light-independent reaction in such detail as to show that: carbon…
Full specification wording
Photosynthesis (A-level only): The light-independent reaction in such detail as to show that: carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This reaction is catalysed by the enzyme rubisco ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate some of the triose phosphate is used to regenerate RuBP in the Calvin cycle some of the triose phosphate is converted to useful organic substances.
In the stroma, carbon dioxide combines with the five-carbon ribulose bisphosphate (RuBP) in a reaction catalysed by rubisco.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 4 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In the stroma, carbon dioxide combines with the five-carbon ribulose bisphosphate (RuBP) in a reaction catalysed by rubisco. The six-carbon product is unstable and immediately splits into two molecules of the three-carbon glycerate 3-phosphate (GP), so one RuBP yields two GP. GP is then reduced to triose phosphate, using reduced NADP as the source of hydrogen and energy from the hydrolysis of ATP, both supplied by the light-dependent reaction. Some of the triose phosphate is used to regenerate RuBP so that the cycle can continue; this regeneration also requires ATP. The remaining triose phosphate leaves the cycle and is converted into useful organic substances, first hexose sugars such as glucose, then starch, cellulose, amino acids, lipids and nucleotides.
What earns the marks
- States that carbon dioxide combines with ribulose bisphosphate, described as carbon fixation.
- States that the reaction is catalysed by rubisco and produces two molecules of glycerate 3-phosphate.
- States that GP is reduced to triose phosphate.
- States that reduced NADP is used in the reduction of GP.
- States that ATP is used in the reduction of GP (or in the regeneration of RuBP).
- States that some triose phosphate is used to regenerate ribulose bisphosphate.
- States that some triose phosphate is converted to a named useful organic substance (e.g. glucose, starch, cellulose, amino acids, lipids, nucleotides).
Common mistakes and how to put them right
Naming GP wrongly, for example as glucose 3-phosphate.
Put it right: The correct name is glycerate 3-phosphate (GP).
Using reduced NAD in place of reduced NADP for the reduction of GP.
Put it right: Photosynthesis uses reduced NADP.
Saying one RuBP gives one GP, missing that the six-carbon intermediate splits into two three-carbon molecules.
Put it right: One RuBP yields two GP.
Forgetting that most triose phosphate regenerates RuBP, and claiming all of it becomes glucose.
Put it right: Some triose phosphate regenerates RuBP and some is converted to organic substances.
Confusing the roles of ATP and reduced NADP.
Put it right: Reduced NADP supplies hydrogen/reducing power, while ATP supplies energy.
Exam tips
- For a 'describe the light-independent reaction' question, write one short sentence for each stage: carbon fixation, reduction, regeneration, and product formation.
- Name rubisco explicitly, because the enzyme is credited alongside the production of two GP.
- An annotated cycle diagram can earn marks, provided every molecule on it is named in full.
Try the lesson
The start of the Photosynthesis (A-level only): The light-independent reaction in such detail as to show that: carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This reaction is catalysed by the enzyme rubisco ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate some of the triose phosphate is used to regenerate RuBP in the Calvin cycle some of the triose phosphate is converted to useful organic substances. lesson, made from checked teaching notes for this specification point.
Have a go first
Which enzyme catalyses the reaction between carbon dioxide and ribulose bisphosphate in the light-independent reaction?
Listen
In the stroma, carbon dioxide combines with the five-carbon ribulose bisphosphate (RuBP) in a reaction catalysed by rubisco. The six-carbon product is unstable and immediately splits into two molecules of the three-carbon glycerate 3-phosphate (GP), so one RuBP yields two GP. GP is then reduced to triose phosphate, using reduced NADP as the source of hydrogen and energy from the hydrolysis of ATP, both supplied by the light-dependent reaction. Some of the triose phosphate is used to regenerate RuBP so that the cycle can continue; this regeneration also requires ATP. The remaining triose phosphate leaves the cycle and is converted into useful organic substances, first hexose sugars such as glucose, then starch, cellulose, amino acids, lipids and nucleotides. A strong answer earns marks for these points. States that carbon dioxide combines with ribulose bisphosphate, described as carbon fixation. States that the reaction is catalysed by rubisco and produces two molecules of glycerate 3-phosphate. States that GP is reduced to triose phosphate. States that reduced NADP is used in the reduction of GP. A common slip: Naming GP wrongly, for example as glucose 3-phosphate. Instead, the correct name is glycerate 3-phosphate (GP). Exam tip: For a 'describe the light-independent reaction' question, write one short sentence for each stage: carbon fixation, reduction, regeneration, and product formation.
See it
Key words
Say what each word means before you turn the card.
carbon fixation
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rubisco
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glycerate 3-phosphate
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triose phosphate
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See every definition
- carbon fixation
- Carbon dioxide combines with ribulose bisphosphate, a reaction catalysed by rubisco.
- rubisco
- The enzyme that catalyses the reaction in which carbon dioxide combines with ribulose bisphosphate.
- glycerate 3-phosphate
- Two molecules of this three-carbon compound are produced when carbon dioxide reacts with ribulose bisphosphate.
- triose phosphate
- GP is reduced to this using reduced NADP and ATP, and some of it is used to regenerate RuBP.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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Test your understanding of Photosynthesis (A-level only): The light-independent reaction in such detail as to show that: carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This reaction is catalysed by the enzyme rubisco ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate some of the triose phosphate is used to regenerate RuBP in the Calvin cycle some of the triose phosphate is converted to useful organic substances.
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