3.3.11Theme C — Interaction and interdependence
Interdependence of Photosynthetic Reactions — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 11 of 11 Unit quiz
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.
Section 1 of 4The Two Stages Linked
The two stages form a loop in which each supplies what the other needs.
Products passed on
- Products passed on The light-dependent reactions supply ATP and NADPH, which are the energy and hydrogen needed in the Calvin cycle.
Substances returned
- Substances returned The Calvin cycle regenerates ADP, inorganic phosphate and NADP⁺, which are the reactants needed for the light-dependent reactions.
Different sites
- Different sites The first stage happens at the thylakoid membranes and the second in the stroma, but substances diffuse quickly between them.
Water and carbon dioxide
- Water and carbon dioxide Water is split in the first stage and carbon dioxide is fixed in the second, so each stage needs a different raw material.
Overall
- Overall Neither stage can continue for long without the other, which is why photosynthesis is described as interdependent.
Section 2 of 4Predicting Changes
Light removed
- Light removed ATP and NADPH fall, so GP cannot be reduced;
- GP builds up and the amount of RuBP falls because it is not being regenerated.
Carbon dioxide removed
- Carbon dioxide removed RuBP cannot be carboxylated, so RuBP builds up and the amount of GP falls.
Light increased when limiting
- Light increased when limiting More ATP and NADPH are made, so reduction speeds up and more triose phosphate and RuBP are produced.
Carbon dioxide increased when limiting
- Carbon dioxide increased when limiting More RuBP is carboxylated, which increases GP and the rate of fixation until another factor limits.
Temperature
- Temperature High temperature slows the Calvin cycle first, because it relies on enzymes such as rubisco that denature.
| Change | GP level | RuBP level |
|---|---|---|
| Light turned off | Increases | Decreases |
| Carbon dioxide removed | Decreases | Increases |
| Both restored | Return to steady values | Return to steady values |
Section 3 of 4Explaining the Interdependence
1
Light is absorbed by photosystems, which excite electrons and drive photolysis, the electron transport chain and chemiosmosis.
2
ATP and NADPH made in these reactions move into the stroma for use in the Calvin cycle.
3
The Calvin cycle uses ATP and NADPH to reduce GP, and returns ADP, phosphate and NADP⁺.
4
Because these returned substances are needed, the light-dependent reactions slow if the Calvin cycle is blocked.
5
The linked reactions together convert light energy and carbon dioxide into stable chemical energy in carbohydrate.
Section 4 of 4Exam Checklist & Tips
3.3.11 – Key Points for the Exam
- Interdependence A relationship in which each part relies on the other.
- NADP⁺ The oxidised coenzyme that accepts electrons in the light-dependent reactions.
- Carboxylation The addition of carbon dioxide to RuBP.
- Steady state A condition in which concentrations stay constant because formation equals use.
Common Mistakes
- Saying that the light-independent reactions need no products of the light-dependent stage.
- Mixing up the effectswith no light GP accumulates, whereas with no carbon dioxide RuBP accumulates.
- Forgetting that ADP and NADP⁺ are returned to the first stage and are needed there.
- Describing the Calvin cycle as a stage that occurs only at night.
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