How is NADPH produced in (a) Photosynthetic AND (b) Non-photosynthetic cells? Give examples of how NADPH may be used in each case.
NADPH is produced in photosynthetic cells by the light-dependent reactions of photosynthesis in order to be used for reduction of carbon atoms in the light-independent stage, or the Calvin Cycle.
NADPH is produced in the light reactions which convert light energy into chemical energy. Photons absorbed by photosystem II in the thylakoid membrane, and the energy is used to break down water and release two electrons which are excited and begin to move along an electron transport chain in the membrane. As they move along, they release energy and pass through cytochrome bf, which is used to generate a proton-motive force and generate ATP until the electrons reach photosystem I. Photosystem I absorbs photons with wavelength 700nm and uses the energy to excite the electrons again, so that they can move further along the transport chain and eventually combine with NADP+ and H+ to give NADPH.
NADPH is used in the Calvin cycle, which takes place in the stroma of chloroplasts and synthesises hexoses from carbon dioxide and water. In order to oxidise NADPH by this process, ribulose 5-phosphate is first phosphorylated by R5P kinase to generate ribulose-1,5-bisphosphate. This is carboxylated and broken down by CO2 to give two molecules of 3-phosphoglycerate in a reaction catalysed by the enzyme Rubisco, and which features two intermediates – one highly unstable and the other an enediolate. The enediolate forms first as the ketone group on R-1,5-BP is reduced, before carboxylate adds to form the unstable intermediate which immediately breaks down via hydrolysis to give two 3-phosphoglycerates. Each of these is then phosphorylated by ATP with 3-PG kinase to give 1,3-bisphosphoglycerate. It is at this stage that NADPH is involved in the reaction, as it donates a proton to a 1,3-BPG so that it will reduce and lose Pi to give glyceraldehyde 3-phosphate. Two G3P molecules can then combine to regenerate the R5P at the start of the cycle so that it can restart.