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Biology Question 102 – NEET-UG 2026

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Which pigment has absorption peak at 700 nm in the photosynthetic reaction centre PS I (P700) ?

Photosynthesis involves two photosystems, PS I and PS II, each with a specific reaction center pigment.

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Ninja Strategy•Reaction Center Pigment Recall

Recognize that P700 refers to the specific absorption maximum of the reaction center chlorophyll, which is always chlorophyll a, while other options are accessory pigments.

Step 1: Understand Photosystem I (P700)✦ Active

Photosystem I (PS I) is a complex of proteins and pigments involved in the light-dependent reactions of photosynthesis. The 'P700' designation indicates that its reaction center absorbs light most efficiently at a wavelength of 700 nm.

Step 2: Identify the Reaction Center Pigment○ Expand

The reaction center of both Photosystem I (P700) and Photosystem II (P680) is composed of a special pair of chlorophyll molecules. Specifically, it is a form of chlorophyll a that acts as the primary electron donor.

Step 3: Evaluate Options○ Expand

Chlorophyll b, Xanthophylls, and Carotenoids are accessory pigments that capture light energy and transfer it to chlorophyll a, but they do not form the reaction center itself. Therefore, Chlorophyll a is the pigment with an absorption peak at 700 nm in the PS I reaction centre.

💡 Teacher's Secret Hint

Remember that accessory pigments broaden the absorption spectrum but chlorophyll a is the core reaction center pigment.

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