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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?
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Your first question...

What are the factors and laws which decide which chlorophyll a molecule will work as a reaction centre?

... has no answer because it's based on a wrong premise: namely that, regarding the chlorophyll molecules, "any one of them behaves as a Reaction centre in a photosystem" (sic).

On the contrary, the special pair is bound to the reaction center's proteins (called D1 and D2). According to Berg (2002):

The photochemistry of photosystem II begins with excitation of a special pair of chlorophyll molecules that are bound by the D1 and D2 subunits. (emphasis mine).

Regarding your second question, it was already answered here.


Source: Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21154/

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Someone else too had asked your question no. 2. here Photosystem 1 and 2; P680/P700; Chlorophyll a/b

The answer basically quotes Molecular Biology 4 ed. by Lodish as follows.

"The chlorophylls in the two reaction centers differ in their light-absorption maxima because of differences in their protein environment."

[I could've just commented this but I can't locate add comment option on your post. Am new here.]

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  • $\begingroup$ You couldn't find the comment option because you have less than 50 Reputation Points. Once you pass that threshold you'll be able to comment. $\endgroup$ – user24284 Nov 23 '17 at 22:34
  • $\begingroup$ Thanks Gerardo. Could you please answer a question I have put up here biology.stackexchange.com/questions/68190/… $\endgroup$ – Prateek Yadav Nov 24 '17 at 13:44

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