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UPSC Prelims 1979 · Environment · Nature’s Machinery
Green plants in the sea are useful for the respiration of fish because
- (a) They give out oxygen
- (b) They give out carbon dioxide
- (c) They give out oxygen and carbon dioxide simultaneously
- (d) They take oxygen and give out carbon dioxide
Show the answer and explanation
Answer: (a) They give out oxygen
Sea plants and phytoplankton release oxygen that dissolves in water — fish take it in through their gills.
The idea
Fish breathe oxygen that is dissolved in water, and water holds very little of it. It is refilled from two sources: mixing with air at the surface, and photosynthesis by water plants and algae.
Sunlight reaches water plants and algae → They photosynthesise → Oxygen is released straight into the water → Fish take it in through their gills
Why the others are wrong
(b) Fish need oxygen, not CO₂.
(c) Plants release CO₂ at night by respiration, but that doesn’t help fish.
(d) That is respiration — it uses up oxygen.
Also know
At night the same plants only respire and use oxygen. In a pond choked with algae, oxygen can fall so low before dawn that fish die. This is why too much plant growth (eutrophication) is harmful.
🔒 Phytoplankton produce about half of Earth’s oxygen.