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Carotenoids have three absorption peaks in the blue-violet range of the spectrum. In photosynthetic organisms, carotenoids play a vital role in the photosynthetic reaction centre. They either participate in the energy-transfer process, or protect the reaction center from auto-oxidation. In non-photosynthesizing organisms, carotenoids have been linked to oxidation-preventing mechanisms.

1. Each pigment has its own absorption spectrum, chlorophyll absorbs light energy in the blue and red region.
2. The optimum temperature of photosynthesis is 10°–25°C for C3 plants and 30°–45°C for C4 plants.
3. Protochlorophyll differs from chlorophyll in lacking 2 hydrogen atoms in one of its pyrrole rings.
4. Chlorophyll a has a methyl group attached to the third carbon, which is replaced by an aldehyde group in chlorophyll b.

Correct answer: Chlorophyll a has a methyl group attached to the third carbon, which is replaced by an aldehyde group in chlorophyll b.

Solution

Chlorophyll a and b differ by a single functional group on one ring position: chlorophyll a has a methyl group, while chlorophyll b has an aldehyde group there. That structural change shifts their absorption properties, making this the correct statement.

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