XADPII is one of the main products of the light reactions of photosynthesis. The other important product is ATP.
The formation of ATP molecules coupled to the light-driven transfer of electrons through the thylakoidal electron transfer chain in the chloroplast is known as photophosphor-ylation. When electron transport takes place continuously from water to XADPI I, as shown in, it is known as non-cyclic or linear electron transport. The formation of ATP in association with non-cyclic electron transport is known as non-cyclic photophosphorylation.
The PS-I may transport electrons independent of PS-II, a process known as the cyclic electron transport. In this case ferredoxin transfers the electrons back to plastoquinon. The electron then returns to P700+ passing through cytochrome b6/f complex and plastocyanin This cyclic electron transport supports ATP synthesis, a process known as CQ cyclic photophosphorylation.
This is thought to be an N^ additional source of ATP to support chloroplast activities, j Cyclic photophophorylation does not generate XADPII.
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The molecular mechanism of ATP synthesis during photosynthesis in the thylakoid membrane of chloroplasts can be explained through the chemiosmotic hypothesis. The detail of this hypothesis is described in the chapter on respiration.
The light-driven elect transport creates a proton motive force across the thylakoid membrane. This proton motive for drives the synthesis of ATP when the protons force their way into the stroma through an integ£ thylakoid protein complex known as ATP synthase.
This complex is also described in many I books under the name CFo-CFi particle (C for chloroplast) or coupling factor. It is estimated th; for 4 protons transported through ATP synthase, one ATP is synthesized. Because 12 protons a transported for two pairs of electrons (derived from the splitting of two molecules of H20) transfers trough the photosynthetic electron transport chain, a total of 3 ATP molecules are synthesized fo( the photooxidation of two molecules of H20. For the splitting of one molecule of HoO, one pair electrons are transported and 1.5 ATP molecules are synthesized.