The phenomenon of red drop shown by Emerson and Lewis was quite puzzling. This is not due to decrease in light absorption because the quantum yield measures only light that has actually S been absorbed.
This oMy indicates that light of £ f. wavelengths greater than 680 nm is much less efficient than light of shorter wavelengths. In subsequent experiments, Emerson and his colleagues measured photosynthesis using red and far-red light after adjusting their fluence rates to give equal rates of photosynthesis.
They observed that the quantum yield obtained using both red and far-red light simultaneously was much higher than the sum of the yields obtained with red and far-red light separately
This phenomenon is known as Emerson enhancement effect or often as Emerson effect.
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These puzzling phenomena of red drop and enhancement effect led to the conclusion that two different reaction centers or photochemical events are involved in photosynthesis. One event is driven by red light (= 680 nm) and the other driven by far-red light (>680 nm). Optimal photosynthesis occurs when both events are driven simultaneously or in rapid succession. These two photochemical events are now known as Photosystem II and Photosystem I and they operate in series to carry out photosynthesis optimally. Photosystem II absorbs red light of 680 nm well and is driven very poorly by far-red light. On the other hand Photosystem I absorb preferentially far-red light of wavelengths greater than 680 nm.
Electron transport actually begins with the arrival of excitation energy at the PS-II reaction centre chlorophyll, P680. The excited P680 (written as P680*) passes an electron to the pheophytin (Pheo), which is considered as the primary electron acceptor of PS-II. (Pheo is a form of chlorophyll-a in which the magnesium has been replaced by two hydrogen atoms) the result of this photo-oxidation event is the formation of P680 and Pheo” (due to charge separation).
The electron flows from Pheo through plastoquinone (PQ) to another multiprotein complex, cytochrome h6/f complex. From this complex the electrons are picked up by a copper-binding protein, plastocyanin (PC).
P680, formed by initial charge separation, is very strong oxidant and is able to extract electrons from water. The electron that reduces P680 is supplied by Yz which is called as the first electron
donor of PS-II. Y/. is a tyrosine residue in the D. protein of PS-II, which in turn receives the electron from a cluster of four manganese ions bound to a small complex of proteins called the oxygen-evolving complex (OKC). This complex is responsible for the splitting (oxidation) of water by drawing electrons from water with the evolution of molecular oxygen and the formation of II’. Photolysis of water takes place as per the following equation:
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Light-driven charge separation similar to that involving PS-II reaction centre (P68o) also takes place in the reaction centre of PS-I. The PS-I reaction centre chlorophyll, P700, is first excited to P700*, then photo-oxidized to P700T.
The primary electron acceptor in PS-I is special chlorophyll-a molecule (Ao) which then passes the electron to ferredoxin. Ferredoxin subsequently is used to reduce XADP* to XADPII, a reaction mediated by the enzyme ferrrcdoxin- NADP’ -oxidoreductasc. The oxidized P700* is reduced by withdrawing an electron from reduced plastocyanin.
The complete photosynthetic electron transport chain in which there is a continuous flow of electrons starting from water to XADP”, passing through the two different photosystems and cytochrome b6/f complex is shown schematically in. The scheme, in which the components of the photosynthetic electron transport chain are arranged as per their redox potential, is named as the Z-scheme because of its characteristic shape.