In the photosynthetic reaction center, the biomolecule in which solar energy con

Question

In the photosynthetic reaction center, the biomolecule in which solar energy conversion takes place, an unusual closely packed arrangement of two chlorophyll molecules, called the ‘special pair’, absorbs light at 870 nm (not coincidentally the solar emission maximum). Chlorophylls look like hemes but are designed to better do photochemistry:

This photon absorption causes an electron to be transferred from the special pair to another, single molecule of chlorophyll. If the transformation from light energy to chemical energy is 100% efficient (and it actually is very nearly so), what is the minimum reduction potential that the accepter cholorphyll can have with respect to the donor?

Explanation / Answer

The photosystem consists of a donor, a photosynthetic unit and an acceptor to four light harvesting antenna pigments. The transfer of energy from the antennas to the reaction center is described by the Forster mechanism in which the energy transfer is between two light-sensitive molecules or chromophores. Here the photosystem II refers as the special chlorophyll dimers and functions as a single entity, gets excited as a single molecule. during the excitation, an electron is excited to the higher level which is subsequently captured by the primary electron acceptor which is the pheophytin molecule in the photosystem II. This oxidized P680+is further reduced by electron originating from the water. The estimated redox potential of the oxidized P680+ is 1.3V. The photosystem I is at a lower reduction potential than P680+ and is reduced by the final electron carrier of PSII, plastocyanin. The higher reduction potential of PSII is essential for the establishment of the chain of electron carriers for the further reduction of PS I.

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