Light independent reaction
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of the light dependent reaction and chloroplast structure. You can test your knowledge on these below.
What are the three main products of the light-dependent reaction?
ATP, reduced NADP (NADPH), and oxygen.
Which enzyme is responsible for ATP production during the light-dependent stage?
ATP synthase.
Where does the light independent reaction take place?
The stroma
Topic Explainer Video
Check out this @JoeDoesBiology video that explains light independent reaction or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Overview of the Light-Independent Reaction (LIR)
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Also known as the Calvin Cycle.
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Takes place in the stroma of the chloroplast.
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Does not require light directly, but depends on products of the light-dependent reaction (ATP and NADPH).
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Involves the fixation of carbon dioxide (CO₂) to ultimately form triose phosphate, which can be used to build glucose and other organic molecules.
Step-by-Step Breakdown of the Calvin Cycle
1. Carbon Fixation
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CO₂ diffuses into the stroma from the atmosphere.
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It combines with a 5-carbon compound called ribulose bisphosphate (RuBP).
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This reaction is catalysed by the enzyme RuBisCO.
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It forms an unstable 6-carbon compound, which immediately splits into two 3-carbon molecules of GP (glycerate 3-phosphate).
2. Reduction of GP to Triose Phosphate
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ATP (from the light-dependent reaction) provides energy.
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Reduced NADP donates hydrogen atoms to reduce 2 x GP into 2 x triose phosphate.
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For every 6 molecules of Triose Phosphate:
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1 molecule may be used to make glucose or other organic molecules.
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5 molecules are used in the regeneration of RuBP.
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3. Regeneration of RuBP
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Triose Phosphate molecules are converted back into RuBP using ATP.
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This ensures the cycle can continue.
Summary of the Light Dependent and Light Independent reaction
The light dependent reaction provides the ATP and NADPH for the light independent reaction.
ADP and Pi, as well as NADP, can then return to the thylakoids to be used again in the light dependent reaction.
Key Outputs of the Light-Independent Reaction
|
Molecule |
Role |
|---|---|
|
Triose phosphate |
Used to synthesise glucose, starch, amino acids, lipids |
|
Regenerated RuBP |
Ensures continuous fixation of CO₂ |
|
NADP |
Returns to the thylakoids for re-reduction in the light-dependent reaction |
Key Terms
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Stroma: The fluid-filled matrix of the chloroplast where the Light independent reaction occurs.
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RuBP (Ribulose bisphosphate): A 5-carbon compound that reacts with CO₂.
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RuBisCO: The enzyme that catalyses the fixation of CO₂ to RuBP.
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GP (Glycerate 3-phosphate): A 3-carbon intermediate formed from CO₂ and RuBP.
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Triose phosphate: A 3-carbon sugar that can form glucose and other organic molecules.
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Regeneration: The process of reforming RuBP from TP using ATP.
No answer provided.
Exam Tip
When answering questions about the Calvin cycle, you can use abbreviations for RuBP, RuBisCO and GP but use the full name for Triose phosphate.
Be specific about the roles of ATP and NADPH. ATP provides the energy and NADPH provides the hydrogen to reduce 2x GP to 2x triose phosphate.
No answer provided.
Describe how the light-independent reaction of photosynthesis results in the production of triose phosphate and the regeneration of RuBP. (6 marks)
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CO₂ diffuses into the stroma and combines with RuBP (a 5-carbon molecule).
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This reaction is catalysed by the enzyme RuBisCO.
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Two molecules of GP are formed.
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GP is reduced to two molecules of triose phosphate
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Using hydrogen from reduced NADP and energy from ATP.
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Some triose phosphate is used to synthesise organic molecules (e.g., glucose).
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The remaining triose phosphate is used to regenerate RuBP.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!