each photosystem has all the pigments except one molecule of chlorophyll A forming a light harvesting system called antenna. Please explain this statement.

Dear Student,
The photosystems comprise of all types of pigments, majorly chlorophyll b, xanthophylls and carotenoids. They also consist of chlorophyll a in small quantities. These combinations of pigments enables the photosystem to absorb light in various wavelengths of the absorption spectra and thus increase the overall rate of photosynthesis.

The statement says "Except one molecule of chlorophyll a" as it means that the photosystems contain all the pigments which are same in both of them but only one particular molecule of chlorophyll is different in both the photosystems P-I and P-II. This one molecule acts as a reaction centre in both the photosystems. The 'chlorophyll a' which acts as the reaction centre in Photosystem (P-I) absorbs light at wavelength 700 nm (and is thus called P700) and the one which acts as the reaction centre in Photosystem (P-II) absorbs light at wavelength 680 nm (and is thus called P680).

The statement thus sums up the information that the pigments together form the Light harvesting complex, which is also called Antennae in each photosystem. These Light harvesting complexes absorb the light and pass the photons to the Reaction centre (The single molecule of chlorophyll a, P700 or P680) which further gets excited and releases electrons to the electron transport system carrying forward the process of light reaction.
Hope this information will clear your doubts about topic.

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