Question

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

• A. Carotenoids
• B. Chlorophyll b
• C. Chlorophyll a
• D. Xanthophylls

Hint:

Remember the golden rule of photosynthesis: Chlorophyll a is the "King" pigment. It is the *only* pigment that can form the operational reaction centres (P680 in PS II and P700 in PS I). All other pigments (Chlorophyll b, Carotenoids, Xanthophylls) are merely "soldiers" or accessory pigments forming the antenna complex to harvest light and protect the King!

Answer 1

The Correct Option is C

Concept: Photosynthesis relies on specialized light-harvesting complexes called photosystems embedded within the thylakoid membranes of chloroplasts. A photosystem consists of a reaction centre surrounded by accessory light-harvesting complexes (antennae). The core molecule at the reaction centre of both Photosystem I (PS I) and Photosystem II (PS II) is always a specific molecule of

Chlorophyll a, which acts as the primary electron donor upon photoexcitation.

Step 1: Analysis of Photosystem I (PS I) and its Reaction Centre.
In Photosystem I, the reaction centre contains a specialized pair of chlorophyll a molecules designated as P700. The letter 'P' stands for pigment, and the number '700' indicates the specific wavelength of light (in nanometers, nm) where this pigment shows its maximum absorption peak in the red region of the electromagnetic spectrum. When light energy is transferred from the accessory pigments to this reaction centre, the Chlorophyll a P700 molecule absorbs this specific energy and gets photo-excited, thereby ejecting an energized electron to a primary electron acceptor.

Step 2: Evaluating the role of other photosynthetic pigments.
To gain absolute clarity, let us assess the functions of the alternative pigments listed:

• (1) Carotenoids & (4) Xanthophylls: These are lipid-soluble accessory pigments. Carotenoids (pure hydrocarbons like \(\beta\)-carotene) and Xanthophylls (oxygenated derivatives) absorb light energy in the blue-violet and blue-green regions of the spectrum and transfer it to chlorophyll a. Additionally, they perform a crucial protective role by quenching excess light energy, thus shielding the delicate chlorophyll molecules from photo-oxidation. They never form the core reaction centre.

• (2) Chlorophyll b: This is an accessory green pigment that possesses an aldehyde group (\(-\text{CHO}\)) in place of the methyl group (\(-\text{CH}_3\)) found in chlorophyll a. It absorbs light at slightly different wavelengths (mostly around 450 nm and 640 nm) to broaden the spectrum of light usable for photosynthesis. It delivers its collected excitation energy down to the reaction centre chlorophyll a.
Therefore, the core pigment molecule located at the reaction centre P700 is explicitly Chlorophyll a.