A gardener notices that a plant kept in a dark cupboard for two weeks grows pale and weak, while an identical plant on a sunny windowsill stays green and healthy. Which raw ingredient is missing for the plant in the cupboard, stopping it from making its own food?
- Carbon dioxide, because a closed cupboard traps stale air that contains none of this gas at all
- Water, because the soil in a dark cupboard dries out far faster than soil in direct sunlight
- Light energy, which the green pigment chlorophyll needs to absorb before it can drive the reaction that turns carbon dioxide and water into glucose and oxygen
- Warmth, because a cupboard is always colder than a sunny windowsill, and the chemical reaction cannot start below room temperature
Why C? And why not the others?
Correct answer: C. Light energy, which the green pigment chlorophyll needs to absorb before it can drive the reaction that turns carbon dioxide and water into glucose and oxygen
Photosynthesis needs light energy, which the green pigment chlorophyll absorbs and uses to drive the reaction that combines carbon dioxide and water into glucose and oxygen; without light, a plant kept in a dark cupboard cannot carry out this reaction, so it grows pale and weak. The option about carbon dioxide is wrong because a cupboard's air still contains carbon dioxide, just like any other room; sealing a plant away from the sun does not remove this gas from the air around it. The option about water is wrong because a dark, enclosed cupboard would if anything slow down water loss compared to a sunny windowsill, not speed it up. The option about warmth is wrong because photosynthesis can proceed at ordinary room temperatures found in a cupboard; it is the absence of light, not a lack of warmth, that stops the reaction.
Source: BBC Bitesize GCSE Biology: Photosynthesis