The Concept of two photosystems originated in the work of Emerson & Lewis in 1943. Working on the action spectrum  for the pigments of Chlorella (a green alga), they found that at 440-520 nm, there was very little evolution of oxygen. This is quite understandable because those wavelengths of light are absorbed by carotenoids and they are known to be least efficient of photosynthetic pigments, chlorophyll-a and chlorophyll-b, absorb light maximally at these wavelengths: indeed these wavelengths correspond to the red region of the spectrum. But when light of wavelengths beyond 60 nm, a region of the spectrum referred to as ‘far-red’ was supplied, there was a drop in the evolution of oxygen, indicative of lessened photosynthetic efficiency. This observation was called red drop.  In 1957, Emerson found that the ‘red-drop’ could be corrected if along with the longer wavelengths of the far-red region, shorter wavelengths of the red region also are supplied. Then the rate of evolution of oxygen picked up. This has been called Emerson’s enhancement effect. These two observations i.e., the ‘red-drop effect’ and ‘enhancement effect’ led to the first suspicion that the light reaction has two sites of action, one in the red region of the spectrum and the other in the far-red.

The explanation offered for the red trap and enhancement effect is that the light reaction  actually consists of two photoacts, photoact I and II, mediated by two photosystems, photosystem I and II. Photosystem I is driven by the far red light and when it operates alone, it produces the red drop effect. But when it operates  along with photosystem II, which functions in the red region, enhancement effect is produced. The latter effect is caused obviously, because both the photosystems are operating in a concerted manner.  Physical separation of the two photosystems had been successfully carried out and the physico-chemical nature of each has been established. Employment of detergents like digitonin and triton X, and techniques like ultracentrifugation, have been useful in these efforts. It was found that photosystem II sediments first, being made up of heavier particles.

The reaction center of photosystem I is a protein-bound chlorophyll-a molecule, P700. Likewise , the reaction center of photosystem II is P690. These reaction centers are the actual sites where light energy is converted to chemical energy.

The rest of the chlorophylls and the other pigments merely serve to transfer the light absorbed by them  to active centres. The electron carriers for the two systems differ: Photosystem I has X, plastocyanins, cytochrome-f and ferredoxin and photosystem II has plastoquinone  & cytochrome -b-559.

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