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
Curriculum Equation Key Balanced Photosynthesis EquationFast Exam Solution
6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂
Word Equation & Conditions: Carbon Dioxide + Water (+ Light & Chlorophyll) -> Glucose + Oxygen
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
- Most triose phosphate is used, with ATP, to regenerate RuBP so the cycle can continue.
Net output
- For every six turns, two triose phosphate molecules leave to form one hexose such as glucose.
Other products
- Triose phosphate can also be used to make lipids, amino acids, and nucleotides.
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
- Supplies energy for the reduction of GP and the regeneration of RuBP.
Reduced NADP
- Provides the hydrogen that reduces GP to triose phosphate.
Link
- If light is low, ATP and reduced NADP run out and GP accumulates.
RuBP level
- A lack of ATP also causes RuBP to fall, since it cannot be regenerated.
| Compound | Carbons | Role |
|---|---|---|
| RuBP | 5 | CO₂ acceptor |
| GP | 3 | First stable product |
| TP | 3 | Carbohydrate precursor |
Part 4 of 6Using Numbers
Per CO₂ fixed
- Three ATP and two reduced NADP are used for each CO₂ fixed.
Per glucose
- Six turns fix six CO₂, using 18 ATP and 12 reduced NADP.
Carbon count
- Twelve TP form, and ten are needed to regenerate six RuBP, leaving two for one glucose.
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?
- Each CO₂ fixed uses 2 ATP + 1 ATP for regeneration = 3 ATP in total, and 2 reduced NADP.
- One glucose has 6 carbons, so 6 CO₂ must be fixed.
- ATP = 6 x 3 = 18.
- 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.
Last updated:
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.