UPSC Prelims 2014 Q19: Environment & Ecology – Biogeochemical Cycles & Earth Systems
Which of the following adds/add carbon dioxide to the carbon cycle on the planet Earth?
1. Volcanic action
2. Respiration
3. Photosynthesis
4. Decay of organic matter
Select the correct answer
using the code given below.
- (a) 1 and 3 only
- (b) 2 only
- (c) 1, 2 and 4 only
- (d) 1, 2, 3 and 4
Answer: (c) 1, 2 and 4 only
Explanation
What adds carbon dioxide to the carbon cycle?
| Process | Adds carbon dioxide? | How |
|---|---|---|
| 1. Volcanic action | Yes | Eruptions release carbon dioxide from inside the Earth |
| 2. Respiration | Yes | Living things break down food and breathe out carbon dioxide |
| 3. Photosynthesis | No | Plants take in carbon dioxide to make food |
| 4. Decay of organic matter | Yes | Decomposers release carbon dioxide from dead matter |
The carbon cycle in brief:
| Direction | Processes |
|---|---|
| Into the air (sources) | Respiration, decay, burning of fuels, volcanoes, forest fires |
| Out of the air (sinks) | Photosynthesis, dissolving in oceans, formation of carbonate rocks |
Answer: (c) 1, 2 and 4 only.
Why each option is right or wrong
- (a) Incorrect. Photosynthesis removes carbon dioxide; respiration and decay are left out.
- (b) Incorrect. Volcanic action and decay also add carbon dioxide.
- (c) CORRECT. Volcanic action, respiration and decay add carbon dioxide to the cycle.
- (d) Incorrect. Photosynthesis takes carbon dioxide out of the air.
Key point
Carbon dioxide sources: respiration, decay, combustion, volcanoes. Sinks: photosynthesis, oceans, carbonate rocks.
Remember
Photosynthesis takes CO2 in; the rest give it out.
Common mistakes
- Counting photosynthesis as a source of carbon dioxide.
- Forgetting volcanoes as a natural source.
- Confusing decay (releases carbon) with fossil formation (stores carbon).
Exam tip
Photosynthesis is the only listed process that removes carbon dioxide.
Sources
- NCERT Class XII — Biology (Ecosystem: nutrient cycling, carbon cycle)
- NCERT Class XI — Fundamentals of Physical Geography, Ch 15 (Life on the Earth: biogeochemical cycles)