UPSC Prelims 2014 · Question 45 of 100

UPSC Prelims 2014 question on Photosynthesis Energy Conversion

Which one of the following is the process involved in photosynthesis?

  1. Potential energy is released to form free energy.
  2. Free energy is converted into potential energy and stored.
  3. Food is oxidized to release carbon dioxide and water.
  4. Oxygen is taken, and carbon dioxide and water vapour are given out.
Show answer

Answer: B. Free energy is converted into potential energy and stored.

Verdict

The answer is that free energy is converted into potential energy and stored. Photosynthesis captures light energy and locks it into the chemical bonds of carbohydrate.

Analysis

Light energy is freely available and is trapped by chlorophyll, then used to build glucose, where it sits as chemical potential energy until respiration releases it. The first option reverses the direction and describes respiration rather than photosynthesis. The third option, food oxidised to release carbon dioxide and water, is a definition of respiration outright. The fourth option reverses the gas exchange, since photosynthesis takes in carbon dioxide and releases oxygen, not the other way around.

Source

National Center for Biotechnology Information reference material on photosynthesis.

How to crack it

Three of the four options describe respiration in different words, so the fastest route is to identify the respiration options and discard them as a block rather than to evaluate each one separately. Photosynthesis and respiration are mirror images in energy direction, in gas direction and in carbon direction, and UPSC exploits that symmetry constantly. Fix the direction of all three arrows once and this whole family of questions resolves in seconds.

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