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5.2.5Topic 5 · On the Wild Side

Light-independent Reactions — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 17 of 34 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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5.2.5 Light-independent Reactions — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the stages of the Calvin cycle and explain how carbon dioxide is fixed and glucose and other products are made.The light-independent reactions, also called the Calvin cycle, take place in th…

  • reduced NADP
  • Carbon fixation
  • ribulose bisphosphate (RuBP)
  • glycerate 3-phosphate (GP)
  • triose phosphate (TP)
  • Regeneration of RuBP

Part 1 of 6Where and why

The light-independent reactions, also called the Calvin cycle, take place in the stroma of the chloroplast.

ATP
They do not use light directly, but they use the ATP and reduced NADP made in the light-dependent reactions.
The cycle fixes carbon dioxide into organic compounds, making the carbon available to the plant and, eventually, to all consumers.
Because it needs the products of the light-dependent reactions, it stops soon after dark.

Part 2 of 6Stages of the cycle

1
Carbon fixation
Carbon fixation carbon dioxide diffuses into the stroma and combines with the five-carbon compound ribulose bisphosphate (RuBP).
  • The reaction is catalysed by the enzyme rubisco and produces an unstable six-carbon intermediate that immediately splits into two molecules of the three-carbon compound glycerate 3-phosphate (GP).
2
Reduction
Reduction each GP is converted into triose phosphate (TP).
  • This needs energy from ATP, which is converted to ADP and Pi, and hydrogen from reduced NADP, which is converted back to NADP.
  • The ADP and NADP return to the thylakoids for re-use.
3
Regeneration of RuBP
Regeneration of RuBP most of the triose phosphate (five out of every six molecules) is used to regenerate RuBP, a process that needs ATP. This allows the cycle to continue.
4
Product formation
Product formation one in six molecules of TP is taken out of the cycle. Two molecules of TP can join to form hexose sugar such as glucose, and TP is also the starting point for other organic molecules.

To make one molecule of glucose requires the cycle to turn six times, fixing six molecules of CO2, and uses 18 ATP and 12 reduced NADP.

Part 3 of 6Fate of triose phosphate

Triose phosphate is a key intermediate. Its carbon skeletons are used for:

Carbohydrates
Carbohydrates hexose sugars, sucrose for transport, starch for storage and cellulose for cell walls.
Lipids
Lipids glycerol from TP, and fatty acids from GP / acetyl units.
Amino acids
Amino acids carbon skeletons from GP combined with nitrate from the soil.
Nucleotides
Nucleotides and other organic molecules, requiring nitrogen and phosphorus.

Plants therefore need minerals as well as light, water and CO2. The cycle is regulated by the supply of ATP and reduced NADP and by the activity of enzymes, including rubisco, which is activated in the light.

Part 4 of 6Limiting factors and experimental evidence

If CO2 concentration falls, less RuBP is carboxylated, so RuBP accumulates and GP falls.

1
If light intensity falls, less ATP and reduced NADP are produced, so GP accumulates (because it cannot be reduced), and RuBP falls (because it cannot be regenerated).
2
These predictions can be checked with graphs of concentration of GP and RuBP against time after a change in conditions, which are a typical data question.
3
Temperature affects all enzymes in the cycle.
4
Calvin used radioactive 14CO2 supplied to the alga Chlorella in an apparatus called the lollipop, stopped the reaction by killing the cells in hot ethanol at different times and identified labelled compounds by two-way chromatography and autoradiography; the first compound labelled was GP, showing it to be the first product of fixation.

Part 5 of 6Photorespiration and efficiency

Rubisco is not entirely specificit can also combine RuBP with oxygen, particularly at high temperature and low carbon dioxide concentration, in a wasteful process called photorespiration that uses energy and releases carbon dioxide without making useful sugar.

1
Plants such as maize and sugar cane (C4 plants) have evolved a mechanism that concentrates CO2 around rubisco, so they perform better in hot, bright conditions; this is beyond the core specification but helps explain why enzyme specificity matters.
2
For crop improvement, scientists try to increase the efficiency of rubisco or the supply of CO2.
3
Greenhouse growers raise the CO2 concentration, provide supplementary light and control temperature to maximise the rate of the Calvin cycle and so the yield.
4
Be ready to interpret a graph of the effect of CO2 concentration and light intensity on rate, naming the limiting factor at each point, for example light intensity at a plateau that increases when the lamp is brightened.

Part 6 of 6Exam-style questions

  1. Describe the role of rubisco in the Calvin cycle. [2]2 marks

  2. A plant in bright light and a steady CO2 supply is put into darkness. Predict and explain the change in the concentration of GP and RuBP. [4]4 marks

  3. Calculate how many molecules of CO2 are fixed and how many ATP molecules used to make one molecule of glucose (the cycle uses 3 ATP per CO2 fixed). [2]2 marks

Key terms

TermMeaning
Calvin cycleCycle of reactions in the stroma that fixes CO2 into carbohydrates.
Carbon fixationCombining CO2 with RuBP to produce organic compounds.
RuBPRibulose bisphosphate, five-carbon CO2 acceptor.
GPGlycerate 3-phosphate, three-carbon first product of fixation.
Triose phosphateThree-carbon sugar made by reducing GP.
RubiscoEnzyme catalysing the fixation of CO2 by RuBP.
ReductionGain of electrons or hydrogen.
RegenerationRe-forming RuBP so the cycle continues.

Avoid these mistakes

1
Saying that CO2 is fixed by combining with GP: it combines with RuBP.
2
Thinking that the first stable product is glucose: it is GP.
3
Forgetting that most triose phosphate is recycled into RuBP.
4
Mixing up the rolesATP supplies energy and reduced NADP supplies hydrogen for reduction.
5
Stating that the cycle happens in the thylakoids: it takes place in the stroma.

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