Which one of the following statements about the process of photosynthesis is correct?
- (a)Chemical energy is converted into light energy.
- (b)Carbon dioxide is oxidised to form carbohydrate.
- (c)Water molecule splits into hydrogen and oxygen.
- (d)Light energy is directly used to split water.
Correct — C, Water molecule splits into hydrogen and oxygen. In the light reactions of photosynthesis, water undergoes photolysis: it is broken apart to release oxygen gas (which the plant gives off) along with hydrogen ions and electrons that go on to make the energy carriers. Describing this as water splitting into hydrogen and oxygen is the accurate statement among the four.
- (a)Chemical energy is converted into light energy. — Photosynthesis works the other way round: light energy is captured and stored as chemical energy in carbohydrates. Converting chemical energy into light is bioluminescence, not photosynthesis.
- (b)Carbon dioxide is oxidised to form carbohydrate. — Carbon dioxide is reduced, not oxidised, when it is built into carbohydrate; it gains hydrogen and electrons, so calling it oxidation is wrong.
- (d)Light energy is directly used to split water. — The splitting of water is driven by chlorophyll in the photosystem after it absorbs light, not by light acting directly on the water molecule, so 'directly' makes the statement inaccurate.
Photosynthesis captures light energy and stores it as chemical energy in glucose. It has two stages: the light reactions, which occur in the presence of light and split water (photolysis) to release oxygen, and the light-independent reactions, which fix carbon dioxide into carbohydrate. Overall, carbon dioxide is reduced while water is split.
The four statements test common misconceptions. The candidate must recall that light is absorbed by chlorophyll, water is split to give off oxygen, and carbon dioxide is reduced. Only the statement about water splitting into hydrogen and oxygen survives all these checks.
- In the light reaction, water is split by photolysis, releasing oxygen.
- Light energy is absorbed by chlorophyll and stored as chemical energy in carbohydrates.
- Carbon dioxide is reduced (not oxidised) when it is fixed into carbohydrate.
- The oxygen released by green plants comes from water, not from carbon dioxide.
- Thinking the oxygen released comes from carbon dioxide rather than from water.
- Confusing reduction of carbon dioxide with oxidation, or reversing the light-to-chemical energy conversion.
Asked as which statement about photosynthesis is correct; test each against the facts that water is split, oxygen comes from water, and carbon dioxide is reduced.
Which one of the following is the process involved in photosynthesis?
- (a) Potential energy is released to form free energy
- (b) Free energy is converted into potential energy and stored
- (c) Food is oxidized to release carbon dioxide and water
- (d) Oxygen is taken in, and carbon dioxide and water vapour are given out
Answer(b) Free energy is converted into potential energy and stored
The gaseous product of a process in plants is a requirement for another vital process that releases energy. Given below are four combinations of the process and product. Identify the correct answer.
- (a) Respiration and Nitric oxide
- (b) Transpiration and Water vapour
- (c) Photosynthesis and Oxygen
- (d) Germination and Carbon dioxide
Answer(c) Photosynthesis and Oxygen
Same process, output side — photosynthesis gives off oxygen, and that oxygen comes from the water split in the light reaction that this NDA (II) 2021 item asks about.
- practice — not a real PYQ
The oxygen released during photosynthesis comes from
- (a)Carbon dioxide
- (b)Water
- (c)Glucose
- (d)Chlorophyll
Answer(b) Water — photolysis of water in the light reaction releases the oxygen.
- practice — not a real PYQ
During photosynthesis, carbon dioxide is
- (a)oxidised to carbohydrate
- (b)reduced to carbohydrate
- (c)converted to oxygen
- (d)released unchanged
Answer(b) reduced to carbohydrate — it gains hydrogen and electrons as it is fixed.