Q1. [3] medium thorough-understanding
A student sets up two identical potted plants under bell jars sealed airtight to glass plates. A watch-glass containing potassium hydroxide solution is placed inside one of the bell jars. After two hours in sunlight, a starch test is performed on a leaf from each plant. The plant without potassium hydroxide shows a strong positive starch test, while the plant with potassium hydroxide shows little or no starch. Explain why this difference occurs, and identify which specific step of photosynthesis is being disrupted in the second plant.
Generated by claude-sonnet-4-6 · 2026-06-26 01:00 · grounding rag
Model Answer
Potassium hydroxide (KOH) absorbs carbon dioxide from the air inside the bell jar. The plant in that setup therefore has no CO₂ available, even though sunlight and chlorophyll are present. Without CO₂, the plant cannot carry out photosynthesis and so produces no glucose, which means no starch accumulates — giving a negative starch test. The plant without KOH uses the CO₂ present in the air and photosynthesises normally, giving a positive starch test.
Step disrupted: The third step of photosynthesis — reduction of carbon dioxide to carbohydrates — is disrupted in the second plant, because the raw material (CO₂) has been removed by KOH.
Source: Chapter 5, Section 5.2.1 — Autotrophic Nutrition (Activity 5.2)
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Explanation
- The question is directly based on Activity 5.2 from the textbook. Examiners expect you to name KOH's role (absorbing CO₂) and link it to the absence of starch.
- Clearly identify the specific step disrupted — "reduction of CO₂ to carbohydrates" — using the exact language from the three steps listed in Section 5.2.1. Vague answers like "photosynthesis stops" will lose marks.
- Note that light absorption and water-splitting steps are not disrupted — only the carbon fixation step is affected. Mentioning this distinction shows deeper understanding and can earn full marks.