Mechanism of photophosphorylation
Synthesis of ATP from ADP and inorganic phosphate requires energy. The energy ultimately comes from light. There is conclusive evidence to show that electron transport and phosphorylation are coupled, where released energy is trapped in the synthesis of ATP.
Three mechanisms have been proposed to explain the process of ATP synthesis, of which chemoosmotic coupling hypothesis proposed by mitchell seems to be convincing.
According to this hypothesis a multi-component enzymatic complex known as ATPase complex sporadically spanned thalakoid membrane at many place. The ATPase complex is also known as coupling factor comprises of protein GF1 binds to a hydrophobic compound CF0. The CF0 protein sub unit serves as a proton channel. The proton motive force generated during this process is utilised by ATPase complex to catalyse sythesis of ATP from ADP and inorganic phosphate. The light driven electron transport is accompanied by release of protons(H+) in the internal space of thallakoid sac. The protons are released into the lumen from oxidation of water, plastoquinone pool and during cyclic electron transport(PS-I). There are 8H+ ion released by non cyclic electron transfer, 4H+ by cyclic electron transport. The release of total 12 protons into the inter space of thalakoid creates huge difference in pH between the outside and inside of the thalakoid membrane. The pH of outside medium becomes 8 when compare to pH of 5 in thalakoid lumen. As a result concentration of protons inside the lumen becomes more than one thousand times than the stroma.
Since, thalakoid membranes are impermeable to protons they flow towards outside of the thalakoid membranes through ATPase complex. The flow of proton down the proton electrochemical gradient generate proton motive force. The movement of proton in electrochemical gradient reveals the coupling factors CF1 results in the synthesis of ATP. A single molecule of ATP is synthesized for every 3H+ passing through the coupling factor(ATPase complex). Thus about 4ATP molecules will be formed for total 12H+ released during two types of electron transport systems. The synthesis of energy rich ATP and NADPH are then utilised to reduce CO2 into carbohydrates.
