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5.1.4Unit 5: Respiration, Internal Environment, Coordination and Gene Technology

Photosynthesis: Light-Independent Stage — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 6 parts Lesson 11 of 36 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key معادلة البناء الضوئي الموزونة - Balanced Photosynthesis EquationFast Exam Solution
6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂
Word Equation & Conditions: Carbon Dioxide + Water (+ Light & Chlorophyll) -> Glucose + Oxygen | ثاني أكسيد الكربون + ماء (في وجود الضوء والكلوروفيل) ← جلوكوز + أكسجين
Location & Specifications: البلاستيدات الخضراء (Chloroplast): التفاعلات الضوئية في الثايلاكويد، ودورة كالفن في الستروما.
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Part 1 of 6Fixing Carbon Dioxide

The cycle takes place in the stroma and does not directly need light.

1
Carbon dioxide diffuses into the stroma and combines with the five-carbon compound ribulose bisphosphate (RuBP).
2
The enzyme rubisco catalyses this carbon fixation, forming an unstable six-carbon compound.
3
The unstable compound breaks down at once into two molecules of glycerate 3-phosphate (GP), which has three carbons.
4
GP is reduced to triose phosphate (TP) using ATP and reduced NADP from the light-dependent stage.

Part 2 of 6Regenerating RuBP and Making Products

Most triose phosphate is recycled, and a small amount leaves the cycle.

Regeneration
  • Regeneration Most triose phosphate is used, with ATP, to regenerate RuBP so the cycle can continue.
Net output
  • Net output For every six turns, two triose phosphate molecules leave to form one hexose such as glucose.
Other products
  • Other products Triose phosphate can also be used to make lipids, amino acids, and nucleotides.
Starch and cellulose
  • Starch and cellulose Glucose is converted to starch for storage or to cellulose for cell walls.

Part 3 of 6Role of Inputs from Stage One

Role of Inputs from Stage One
ATP
  • ATP Supplies energy for the reduction of GP and the regeneration of RuBP.
Reduced NADP
  • Reduced NADP Provides the hydrogen that reduces GP to triose phosphate.
Link
  • Link If light is low, ATP and reduced NADP run out and GP accumulates.
RuBP level
  • RuBP level A lack of ATP also causes RuBP to fall, since it cannot be regenerated.
CompoundCarbonsRole
RuBP5CO₂ acceptor
GP3First stable product
TP3Carbohydrate precursor

Part 4 of 6Using Numbers

Per CO₂ fixed
  • Per CO₂ fixed Three ATP and two reduced NADP are used for each CO₂ fixed.
Per glucose
  • Per glucose Six turns fix six CO₂, using 18 ATP and 12 reduced NADP.
Carbon count
  • Carbon count Twelve TP form, and ten are needed to regenerate six RuBP, leaving two for one glucose.
Site
  • Site The enzymes of the cycle float in the stroma, close to the thylakoids.

Part 5 of 6Worked Example

Question How many ATP and reduced NADP are needed to make one molecule of glucose in the Calvin cycle?

  1. Each CO₂ fixed uses 2 ATP + 1 ATP for regeneration = 3 ATP in total, and 2 reduced NADP.
  2. One glucose has 6 carbons, so 6 CO₂ must be fixed.
  3. ATP = 6 x 3 = 18.
  4. Reduced NADP = 6 x 2 = 12.

Answer 18 ATP and 12 reduced NADP per glucose.

Part 6 of 6Exam Checklist & Tips

5.1.4 – Key Points for the Exam

  • Carbon fixation Joining carbon dioxide to an organic acceptor molecule, here RuBP.
  • Rubisco Ribulose bisphosphate carboxylase, the enzyme that fixes carbon dioxide.
  • GP Glycerate 3-phosphate, the three-carbon compound formed after fixation.
  • Triose phosphate A three-carbon sugar phosphate formed by reducing GP.
  • RuBP Ribulose bisphosphate, the five-carbon acceptor regenerated in the cycle.

Common Mistakes

  • Saying that the light-independent stage does not need the light stage; it needs its ATP and reduced NADP.
  • Writing that GP is oxidised; it is reduced to triose phosphate.
  • Giving the wrong carbon numbers for RuBP, GP and TP.
  • Leaving out that RuBP is regenerated to keep the cycle running.

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Biology 0610 & 9700 Exam Model Limiting Factors Master

Photosynthesis: Limiting Factors & Graph Plateau

6CO2 + 6H2O → C6H12O6 + 6O2 (Light & Chlorophyll required)

  • Limiting Factor Definition: Something present in the environment in such short supply that it restricts life processes.
  • Graph Plateau: When rate levels off as light intensity increases, light is no longer the limiting factor; CO2 concentration or temperature has become limiting.
  • High Temperature Drop: Beyond optimum temp (~40°C), enzymes (e.g. RuBisCO) denature, permanently decreasing rate.
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