Electron Transport in Photosynthesis

Photosynthesis of prokaryotic cyanobacteria as well as that of eukaryotic algae and higher plants produces oxygen and the basic process is similar in them all.

A form of the Hill and Bendall ‘Z’ scheme of the sequence of processes and electron transport leading from water-splitting through NADP+ reduction is explained below.

Photon capture by the photosystem antennae and excitation transfer to PSII & I provide the energy for oxidation of water and electron movement to acceptors, which donate e to biochemical processes, and for passage of protons into thylakoid lumen, for synthesis of ATP. The electron transport system may be considered in five parts:

a)A water-splitting complex
b)A photosystem complex[userpro_private]
c)An electron carrier chain
d)A PS I complex
e)A group of e carriers which reduce electron acceptors (NADP+, O2)

Electron transport starts with the capture of photons by chlorophylls and accessory pigments. Transfer of the energy to reaction centers of PS I & II excites the dimer chlorophylls and causes ejection of electrons to acceptors, starting e transport along the chain of redox components. Excitation of P-680 of PS II results in an oxidized reaction centre P-680* PSII is defined as that part of oxygenic photosynthesis catalyzing photo induced transfer of e from water to plastoqiunone (PQ) Passage of electrons along the electron transport chain moves the protons from the stroma into the thylakoid space and thus creates a proton gradient. This is used to drive the synthesis of ATP. Plastoquinone, plastocyanin and ferredoxin are mobile and can transport electrons between the complexes.

 

With the transfer of H+ from the stromal to lumen side of the thylakoid membrane. This oxidized PS II is reduced by e from a water-splitting complex via intermediate states M & Z which are components of the water splitting complex and electron carrier system between it and the reaction centre. The energized e passes, from more to less negative potential, to the primary acceptor pheophytin and then sequence to the quinine acceptors Qa, Qb, and PQ. Quinones are important carriers of e and H+ in many biological processes. From PQ the electron passes to cytochrome f and plastocyanin before reducing an oxidized PS I reaction centre. Here it is energized again by excitation energy derived from photon energy trapped in the chlorophyll matrix, and passed via intermediate states AD, A and B to oxidized ferredoxin (Fd) and NADPH+, which are reduced and are able to enter into biochemical reactions in chloroplast stroma.

Electron transport chains bridge the thylakoid membranes, allowing electron removed from water held in the water-splitting complex of proteins, manganese ions and other components inside thylakoid lumen to pass across the membrane to ferredoxin on the stromal side. Plastoqiunone in the membrane is reduced by the electrons; the H+ from the stroma attaches to the reduced plastoqiunone and is carried to the lumen, where it is released oxidized. Thus the electron transport is coupled to plasto quinine cycle which carries which carries (‘pumps’) H+ from stroma to the thylakoid lumen in reverse direction to electron transport, increasing H+ concentration in the thylakoid lumen and forming the protein concentration gradient, the energy of which derives ATP synthesis.[/userpro_private]

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