Chloroplasts contained the naturally occurring electron acceptor NADP+ and that it was reduced to NADPH2 in the light by addition of electrons and hydrogen ions. The hydrogen ions for the reduction are provide by water molecule which split into hydrogen ions and electrons releasing the oxygen (photolysis of water). The chloroplasts supplied with ADP and inorganic phosphate, manufacture ATP in the light (photophosphorylation). The chloroplasts in the dark can convert carbon dioxide to carbohydrate provided they were supplied with ATP and NADPH2. Thus, photochemical phase of photosynthesis (light reaction involved photochemical splitting of water and production of ATP). Robert Hill studied the chemical reactions leading to formation of NADPH2 and ATP in the presence of light, therefore, these reactions are called light reaction or Hill’s reaction.
Thylakoid protein Complexes
Almost all the chemical processes in the light reaction of photosynthesis are carried out in foul-protein complexes located in thylakoid membranes. These are called thylakoid protein complexes and include photosystem reaction Centre II complex:
Photosystems II Reaction Centre Complex: it comprises of two membranes proteins to which P680 (the trap), pheophytin (primary electron acceptor) and plastoquinone (proton acceptor) are bound. In addition, the cytochrome is associated with it. Oxidation of water occurs in this Centre.
2H2O + 2PQ —————————————- O2 + 2PQH2
PS II membranes
Cytochrome b6-f Complex: It contains four major proteins: cvt b6, cyt f, tile Rieske iron-sulphur protein (FS R) and subunit IV. It is responsible for transfer of electrons from PS II to PS I during electron flow from water to NADP+. The electron transfer sequence within the complex is:
PQH2 ———- FESR ———- cyt f ———- PC ———- PSI
The complex also trans-locates protons (H+) from stromal to lumen side of the membrane which result in generation of electrochemical potential across the membrane of the two side of the membrane due to I-1+ protons concentration difference.
Photosystem I reaction Centre Complex: This reaction Centre comprises of two proteins to which P700 and about 200 chlorophyll molecules are bound. The primary electron acceptor is an Fe-S protein. Additionally, the electron acceptors are in the form of series of three Fe-S proteins called Fe-S Centers X, A and B are also bound to the membranes. The complex catalyzes light-driven transfer of electrons from reduced plastocyanin (PC) which is located on the lumen side of the thylakoid membrane, to ferredoxin located on the stromal side of the membrane which reduces NADP to NADPH2.
Two mobile electron carriers, a CU-containing protein plastocyanin (PC) and a group of quinones carry two electrons and two H + from PS II to PS I.
ATP Synthase Complex: It is a large complex, also called coupling factor where synthesis of ATP occurs during light reaction. The complex consists of two parts:
A hydrophobic membrane bound portion called CF1 in the form of a stalk piece,
And a portion in the form of a spherical head that sticks out into the stroma called CF1.
Z-Scheme – Electron & Proton Transport
The electron and proton transport and the production of NADPH2 and ATP in chloroplasts is referred to as Z-Scheme, so named because of its shape (Z for zigzag).
Electron & Proton Transport from H2O to NADP+
The electron and proton transport from water to NADP+ involves following steps:
Photolysis of Water
When a proton is absorbed by a pigment in the light-harvesting complex associated with photosystems II (LHC II) its energy is transferred to P680. This excites an electron in P680 so much that it can be removed by pheophytin (Pheo). Loss of electron causes P680 to become positively charged and it then attracts an electron from adjacent Mn-proteins. As the Mn-protein becomes oxidized (loses an electron), it in turn strongly attracts an electron from water. Each water can give up only two electrons.
Each electron moves from Q to plastoquinone (PQ) and reduces it to PQH2. This reduction requires two electrons and two H+. The H+ are absorbed from stroma side of the membrane. When POH2 is re-oxidized to PQ, the H+ are transported into the thylakoid channel and join those released from water oxidation. These protons cause decrease of pH in the channel that help in synthesis of ATP.
Synthesis of NADPH2
From PQH2, electrons move one at a time, to cytochrome b6 or to the Fe-S protein and then to cytochrome F in the complex between PS II and PS I. From there they go to plastocyanin (PC) that moves along the edge of the membrane to PS I. The reaction Centre of PSI (P700) cannot accepts electron unless it loses an electron. Such loss can only occur by light excitation. Light energy is transferred from LHCI to P700 and excited P700 gives its electrons to iron in one of the Fe-S proteins associated with it, and the electron is then passed to ferredoxin (Fd) molecule, reducing its iron. Ferredoxin then reduces NADP+.