here that the water splitting complex is associated with the PS II, which itself is physically located on the inner side of the membrane of the thylakoid. Then, where are the protons and O formed likely to be released – in the lumen? or on the outer side of the membrane? .
. Cyclic and Non-cyclic Photo-phosphorylation Living organisms have the capability of extracting energy from oxidisable substances and store this in the form of bond energy. Special substances like ATP, carry this energy in their chemical bonds. The process through which Electron transport system - - e acceptor e acceptor Light Photosystem II Photosystem I NADPH NADP + LHC LHC H O 2e + 2H + [O] - ADP+ iP ATP Figure .
Z scheme of light reaction ATP is synthesised by cells (in mitochondria and chloroplasts) is named phosphorylation. Photo- phosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light. When the two photosystems work in a series, first PS II and then the PS I, a process called non-cyclic photo-phosphorylation occurs. The two photosystems are connected through an electron transport chain, as seen earlier – in the Z scheme.
Both ATP and NADPH + H + are synthesised by this kind of electron flow (Figure . ). When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons (Figure . ).
A possible location where this could be happening is in the stroma lamellae. While the membrane or lamellae of the grana have both PS I and PS II the stroma lamellae membranes lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to NADP + but is cycled back to the PS I complex through the electron transport chain (Figure . ).
The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH + H + . Cyclic photophosphorylation also occurs when only light of wavelengths beyond nm are available for excitation. . .
Chemiosmotic Hypothesis Let us now try