CDS (II) 2022 General Knowledge · Question 19 of 120

Correct — D, Chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide. This is NCERT's three-step account of photosynthesis compressed into a single sentence. The Class 10 chapter lists the events as absorption of light energy by chlorophyll; conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen; and reduction of carbon dioxide to carbohydrates. Option D names all three in the right order and, crucially, gets the chemistry the right way round — water is the substance that is split and oxidised, releasing the oxygen we breathe, while carbon dioxide is the substance that is reduced, gaining hydrogen to become carbohydrate. Every wrong option in this question inverts one of those two.

• (a)Light energy is directly converted to kinetic energy which oxidizes carbon dioxide. — Two errors in one line. Light energy is stored as chemical energy, not turned into kinetic energy, and carbon dioxide is reduced rather than oxidised — it is already in the most oxidised state carbon reaches.
• (b)Light energy is directly converted to chemical energy which causes reduction of water. — The energy half is right and the chemistry half is backwards. Water is oxidised and split, which is where the oxygen comes from; reducing water would be the reverse of what photosynthesis does to it.
• (c)Chlorophyll absorbs the chemical energy of light which causes oxidation of carbon dioxide. — Light carries radiant energy, not chemical energy, and again carbon dioxide is reduced, not oxidised. The phrase 'chemical energy of light' is the giveaway.

Photosynthesis is an oxidation-reduction pair running in one place. Chlorophyll in the thylakoid membranes absorbs light and uses it to pull electrons off water, which splits into hydrogen and oxygen; the oxygen escapes as a by-product. Those electrons and the energy stored with them in ATP and NADPH are then used in the stroma to add hydrogen to carbon dioxide, building it into carbohydrate. Nothing is created out of light — light supplies the energy that drives electrons uphill from water to carbon.

Four options that all use the same vocabulary can be sorted with two questions. First, what happens to water, and what happens to carbon dioxide? Water is split, carbon dioxide is reduced. Any option that says the opposite about either substance is out, which removes A, B and C between them. Second, what does light do? It is absorbed by chlorophyll and converted into chemical energy; calling it kinetic energy or calling light itself chemical is a category error. A memory aid that survives the harder version of this question: the oxygen released comes from water, not from carbon dioxide — a fact established by isotope-labelling experiments and asked directly in many papers.

• The events of photosynthesis are absorption of light by chlorophyll, conversion of light energy to chemical energy with splitting of water into hydrogen and oxygen, and reduction of carbon dioxide to carbohydrate.
• The oxygen released in photosynthesis comes from water, not from carbon dioxide.
• The light reactions occur on the thylakoid membranes; the carbon reactions occur in the stroma of the chloroplast.
• Desert plants take in carbon dioxide at night and form an intermediate that the light energy acts on during the day, so the steps need not follow one another immediately.
• Carbohydrate not used at once is stored by the plant as starch.

• Saying carbon dioxide is split and water is reduced; it is the other way round.
• Believing the oxygen given off comes from carbon dioxide.
• Describing light as chemical energy, or its conversion as kinetic energy.

As a which-statement-is-correct item on the sequence, as a question on the source of the released oxygen, or as an energy-conversion question.

• practice — not a real PYQ

  The oxygen released during photosynthesis comes from

  • (a)carbon dioxide
  • (b)water
  • (c)chlorophyll
  • (d)glucose

  Answer(b) water — water is split into hydrogen and oxygen, and it is that oxygen which is released.

• practice — not a real PYQ

  In a chloroplast, the reduction of carbon dioxide to carbohydrate takes place in the

  • (a)thylakoid membrane
  • (b)outer membrane
  • (c)stroma
  • (d)granum

  Answer(c) stroma — the light reactions occur on the thylakoid membranes stacked as grana, while the carbon reactions run in the stroma around them.