UPSC CDS 2 2022 GK Paper

Concept:Photosynthesis uses light energy absorbed by chlorophyll to split water and reduce carbon dioxide into carbohydrates.Explanation:Light energy is not directly converted to kinetic energy; it is converted to chemical energy, but not to directly oxidize carbon dioxide. Hence option A is incorrect.Option B is incorrect because light energy is converted to chemical energy, but it causes oxidation of water (splitting) not reduction of water.Chlorophyll absorbs light energy, not chemical energy of light. Option C is wrong because chlorophyll's absorption leads to splitting of water and reduction of carbon dioxide, not oxidation.Option D is correct: Chlorophyll absorbs light energy, which causes splitting of water molecules ($\mathrm{H_2O} \rightarrow 2\mathrm{H}^+ + 2e^- + \frac{1}{2}\mathrm{O_2}$) and provides electrons and protons for the reduction of carbon dioxide ($\mathrm{CO_2}$) to carbohydrates.Thus, photosynthesis involves both light-dependent and light-independent reactions, where light energy drives the reduction of $\mathrm{CO_2}$ using electrons from water.Answer:Chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide.