(a) List the three main events that occur during photosynthesis.
(b) Explain the role of each of the following in photosynthesis: (i) chlorophyll, (ii) stomata, (iii) water.
(c) A plant kept in a dark room for 72 hours is then placed in bright sunlight. After a few hours, a leaf is tested for starch using iodine solution. Predict the result and explain.
Generated by claude-sonnet-4-6 · 2026-06-26 01:00 · grounding rag
Model Answer
(a) Three main events during photosynthesis:
- Absorption of light energy by chlorophyll.
- Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen.
- Reduction of carbon dioxide to carbohydrates.
(b) Role of each component:
- (i) Chlorophyll: Absorbs light energy and converts it into chemical energy needed for photosynthesis.
- (ii) Stomata: Tiny pores on leaf surface through which massive amounts of CO₂ enter the leaf. Guard cells regulate opening and closing of stomata.
- (iii) Water: Taken up from soil by roots; it is split during photosynthesis to release hydrogen (used to reduce CO₂) and oxygen (released as byproduct).
(c) Prediction and Explanation:
The leaf will turn blue-black with iodine solution, confirming the presence of starch.
After 72 hours in darkness, all starch in the leaf gets used up. Once placed in bright sunlight, chlorophyll absorbs light energy, CO₂ enters through stomata, and photosynthesis resumes. Carbohydrates (starch) are produced and stored in the leaf. Hence, iodine turns blue-black.
Source: Life Processes, Section 5.2.1
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Explanation
- Part (a) is directly lifted from the textbook — memorise the exact three points in order.
- Part (b): Each sub-point is worth ~½ mark; keep answers to one line each.
- Part (c): Examiners expect two things — the correct result (blue-black) AND the reasoning (dark room depletes starch → sunlight restores photosynthesis → starch accumulates). Missing either loses marks. The 72-hour dark period is a standard destarching technique used in Activity 5.1.