UPSC Prelims 2011 Q48: Environment & Ecology – Biogeochemical Cycles & Earth Systems
Consider the following :
1. Photosynthesis
2. Respiration
3. Decay of organic matter
4. Volcanic action
Which of the above add carbon dioxide to the carbon cycle on Earth ?
- (a) 1 and 4 only
- (b) 2 and 3 only
- (c) 2, 3 and 4 only
- (d) 1, 2, 3 and 4
Answer: (c) 2, 3 and 4 only
Explanation
What adds CO2 to the carbon cycle
The carbon cycle has processes that remove CO2 (fixation) and processes that release/add it back.
| Process | Verdict | Reason |
|---|---|---|
| 1. Photosynthesis | Incorrect | Photosynthesis removes/fixes CO2 from the atmosphere into organic matter; it does not add CO2 |
| 2. Respiration | Correct | Living organisms release CO2 as a byproduct of cellular respiration |
| 3. Decay of organic matter | Correct | Decomposition by microbes breaks down organic matter and releases CO2 |
| 4. Volcanic action | Correct | Volcanic eruptions release CO2 stored deep in the Earth into the atmosphere |
Carbon cycle: sources vs sinks:
| Role | Process |
|---|---|
| CO2 sink (removes) | Photosynthesis |
| CO2 sources (add) | Respiration, decay, volcanic action |
Answer: (c) 2, 3 and 4 only.
Why each option is right or wrong
- (a) Incorrect. Photosynthesis removes CO2, it does not add it; this pairing wrongly includes process 1.
- (b) Incorrect. Volcanic action (4) also adds CO2, so leaving it out of the list is wrong.
- (c) CORRECT. Respiration, decay, and volcanic action all release CO2 into the cycle; photosynthesis is a sink, not a source.
- (d) Incorrect. Including photosynthesis (1) is wrong, since it is a CO2 sink, not a source.
Key point
CO2 is ADDED to the carbon cycle by respiration, decay of organic matter, and volcanic action — NOT by photosynthesis, which is a CO2 sink (removes it).
Remember
CO2 added to carbon cycle by: respiration + decay + volcanic action. Photosynthesis REMOVES CO2, not adds it.
Common mistakes
- Confusing photosynthesis (a sink) with the sources that add CO2.
- Forgetting volcanic action is a natural geological source of atmospheric CO2.
- Not clearly separating sink processes from source processes in the carbon cycle.
Exam tip
Photosynthesis = sink (removes CO2). Respiration, decay, volcanoes = sources (add CO2). Never group photosynthesis with the adders.
Sources
- NCERT Class XII Biology — Ecosystem (carbon cycle)
- Intergovernmental Panel on Climate Change — Climate Change 2007: The Physical Science Basis, Chapter 7 (Couplings Between Changes in the Climate System and Biogeochemistry)