In photosystems of higher plants, there are about 250-400 pigments (number wise) in a particular photosystem. Out of which,approx 170-180 pigments are of *chlorophyll a* molecules. And **any one** of them behaves as a *Reaction centre* in a photosystem.

My question is:

1. What are the factors and laws which decide **which** *chlorophyll a* molecule will work as a reaction centre?
2. What's the difference in **chlorophyll a molecules** present in reaction centres of PS I and PS II since they have different threshold energy. Is there any structural difference?