5.1.3Unit 5: Respiration, Internal Environment, Coordination and Gene Technology
Photosynthesis: Light-Dependent Stage — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 10 of 36 Unit quiz
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.
Section 1 of 4Photosystems and Photolysis
Pigments are arranged in photosystems in the thylakoid membranes.
Photosystem II
- Photosystem II Absorbs light, with its reaction centre chlorophyll a absorbing best at 680 nm (P680).
Photoionisation
- Photoionisation Light excites electrons in chlorophyll so they leave and are passed to an electron carrier.
Photolysis
- Photolysis Light splits water: 2H₂O → 4H⁺ + 4e⁻ + O₂, replacing electrons lost from photosystem II.
Photosystem I
- Photosystem I Absorbs light with the reaction centre absorbing best at 700 nm (P700).
Products of photolysis
- Products of photolysis Electrons replace those lost, protons help to form the gradient, and oxygen is released.
Section 2 of 4Non-Cyclic Photophosphorylation
Electrons move in a one-way flow from water to NADP.
1
Photosystem II absorbs light and excited electrons leave its chlorophyll, which are replaced by photolysis of water.
2
The electrons pass along carriers of the electron transport chain, releasing energy at each step.
3
This energy pumps protons into the thylakoid space, and the protons return through ATP synthase to form ATP.
4
Electrons reach photosystem I, are re-excited by light, and are accepted by NADP along with protons to form reduced NADP.
Section 3 of 4Cyclic Photophosphorylation
A shorter route uses only photosystem I.
Cyclic Photophosphorylation
Electron path
- Electron path Excited electrons from photosystem I return to it through the carriers instead of reaching NADP.
Product
- Product Only ATP is made, because the electrons do not reduce NADP.
No photolysis
- No photolysis No water is split and no oxygen is released in the cyclic pathway.
Purpose
- Purpose It supplies additional ATP, which the Calvin cycle needs more of than reduced NADP.
| Feature | Non-cyclic | Cyclic |
|---|---|---|
| Photosystems | I and II | I only |
| Water split? | Yes | No |
| ATP made? | Yes | Yes |
| Reduced NADP made? | Yes | No |
| Oxygen released? | Yes | No |
Section 4 of 4Exam Checklist & Tips
5.1.3 – Key Points for the Exam
- Photolysis The splitting of water using light energy.
- Photosystem A cluster of pigments and proteins in the thylakoid membrane that absorbs light.
- Electron transport chain A series of carriers in a membrane passing electrons and releasing energy.
- Reduced NADP NADP that has accepted electrons and protons.
- Non-cyclic A pathway in which electrons pass from water through both photosystems to NADP.
Common Mistakes
- Saying that carbon dioxide is split to release oxygen; water is split.
- Writing that ATP is made in the stroma during this stage; it is made across the thylakoid membrane.
- Confusing which photosystem receives electrons from water; this is photosystem II.
- Stating that the cyclic pathway makes reduced NADP.
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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.