UPSC Prelims 2014 · Question 79 of 100

UPSC Prelims 2014 question on Carbon Dioxide Sources Cycle

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.

  1. 1 and 3 only
  2. 2 only
  3. 1, 2 and 4 only
  4. 1, 2, 3 and 4
Show answer

Answer: C. 1, 2 and 4 only

Verdict

The answer is 1, 2 and 4 only. Volcanic action, respiration and decay all add carbon dioxide, while photosynthesis removes it.

Analysis

Statement 1 is CORRECT. Volcanic activity releases carbon dioxide from the earth's interior and is the main geological source over long timescales. Statement 2 is CORRECT. Respiration in plants, animals and microbes oxidises organic carbon and releases carbon dioxide. Statement 4 is CORRECT. Decay of organic matter by decomposers returns carbon to the atmosphere. Statement 3 is INCORRECT. Photosynthesis is the sink of the carbon cycle, fixing atmospheric carbon dioxide into carbohydrate.

Source

NCERT Class XI Geography, Chapter 15.

How to crack it

The carbon cycle has exactly one large biological sink, photosynthesis, and everything else on a short list is a source. So in any adds carbon dioxide question, find photosynthesis and exclude it, and the answer usually follows without evaluating the remaining items. The mirror question, which removes carbon dioxide, is answered by photosynthesis plus ocean dissolution plus rock weathering. Learning the cycle as sources against sinks, rather than as a list of processes, answers both directions.

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