3.3.10Inquiry Process
Light Independent Reactions — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 125 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Stages of the Calvin Cycle
The Calvin cycle takes place in the stroma and does not need light directly, but depends on ATP and NADPH from the light-dependent reactions.
1
Carbon fixation: rubisco catalyses the addition of carbon dioxide to the five-carbon compound ribulose bisphosphate (RuBP), forming an unstable six-carbon compound.
2
The unstable six-carbon compound splits immediately into two molecules of glycerate 3-phosphate (GP), each with three carbon atoms.
3
Reduction: GP is converted to triose phosphate (TP) using hydrogen from NADPH and energy from ATP.
4
Regeneration: most of the triose phosphate is used, with more ATP, to make RuBP again, so that the cycle can continue.
5
The remaining triose phosphate leaves the cycle and is used to make carbohydrates and other organic compounds.
Part 2 of 5What the Cycle Needs and Makes
Rubisco
- Rubisco The enzyme that fixes carbon dioxide;
- it is very abundant in leaves because the reaction it catalyses is relatively slow.
ATP
- ATP Supplies energy for the reduction of GP and for the regeneration of RuBP.
NADPH
- NADPH Supplies hydrogen and electrons for reducing GP to triose phosphate.
Carbon dioxide
- Carbon dioxide The source of carbon atoms;
- it enters the leaf through the stomata and diffuses into the chloroplast.
Products
- Products Triose phosphate is used to make glucose, starch, sucrose, cellulose, amino acids and lipids.
| Molecule | Carbon atoms | Role |
|---|---|---|
| RuBP | 5 | Acceptor of carbon dioxide |
| GP | 3 | First stable product |
| Triose phosphate | 3 | Reduced product; used for syntheses and to regenerate RuBP |
Part 3 of 5Energy and Carbon Accounting
Energy and Carbon Accounting
Three turns
- Three turns Three molecules of carbon dioxide are fixed in three turns, giving six triose phosphate, of which only one is a net gain.
Regeneration share
- Regeneration share Five of every six triose phosphate molecules are needed to regenerate RuBP, and one-sixth leaves the cycle.
Per carbon dioxide fixed
- Per carbon dioxide fixed About three ATP and two NADPH are used for each molecule of carbon dioxide that is fixed.
Making glucose
- Making glucose Two triose phosphates are needed to make one six-carbon glucose, which takes six turns of the cycle.
Part 4 of 5Worked Example
Question Calculate the ATP and NADPH needed to make one glucose molecule in the Calvin cycle, given that six carbon dioxide molecules are fixed.
- Six turns fix six CO₂ and give twelve GP, which are reduced to twelve triose phosphate.
- Reduction of 12 GP uses 12 ATP and 12 NADPH.
- Ten of the twelve triose phosphate regenerate six RuBP using 6 ATP.
- The remaining two triose phosphate form one glucose. Total ATP = 12 + 6 = 18.
Answer One glucose needs 18 ATP and 12 NADPH.
Part 5 of 5Exam Checklist & Tips
3.3.10 – Key Points for the Exam
- Calvin cycle The light-independent pathway that converts carbon dioxide into organic compounds.
- Rubisco The enzyme that attaches carbon dioxide to RuBP.
- RuBP Ribulose bisphosphate, the five-carbon carbon dioxide acceptor.
- Glycerate 3-phosphate (GP) The three-carbon compound formed when the unstable six-carbon compound splits.
- Triose phosphate A three-carbon sugar phosphate made by reducing GP.
Common Mistakes
- Writing that the reactions can occur in the dark indefinitely; they stop soon after ATP and NADPH run out.
- Placing the cycle in the thylakoid instead of the stroma.
- Saying that carbon dioxide combines with GP rather than with RuBP.
- Forgetting that most triose phosphate is recycled to regenerate RuBP.
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