the appearance of letter ‘Z’ and so known as Z scheme . When there is availability of NADP for reduction and when there is splitting of water molecules both PS I and PS II are activated (Table . ). Non-cyclic electron transport PS I and PS II both are involved co-operatively to transport electrons from water to NADP (Figure .
). . . Bio energetics of light reaction • To release one electron from pigment system it requires two quanta of light.
• One quantum is used for transport of electron from water to PS I. • Second quantum is used for transport of electron from PS I to NADP • Two electrons are required to generate one NADPH H . (Fd), Plastoquinone (PQ), cytochrome b6-f complex, Plastocyanin (PC) and finally back to chlorophyll P700 (PS I). During this movement of electrons Adenosine Di Phosphate (ADP) is phosphorylated, by the addition of inorganic phosphate and generates Adenosine Tri Phosphate (ATP).
Cyclic electron transport produces only ATP and there is no NADPH H formation. At each step of electron transport, electron loses potential energy and is used by the transport chain to pump H ions across the thylakoid membrane. The proton gradient triggers ATP formation in ATP synthase enzyme situated on the thylakoid membrane. Photosystem I need light of longer wave length (> P700 nm).
It operates under low light intensity, less CO and under anaerobic conditions which makes it considered as earlier in evolution (Figure . ). Cyt f Cyt b6 PC P700 PS I LHC I FRS Ferredoxin ADP+ P i ADP+ P i ATP ATP - 2e - 2e - 2e - 2e - 2e - 2e Light Figure . : Cyclic Photophosphorylation Q Pheophytin PQ Cyt b6,f complex PC P680 P700 PS II PS I LHC II LHC I FRS Ferredoxin + 2NADP + 2NADPH+H ADP+ Pi ATP 2H O + + + + C a , , C n l M O Evolving Complex - 4e - 4e - 4e - 4e - 4e - 4e - 4e - 4e