Non-Cyclic Photophosphorylation

Non-cyclic photophosphorylation involves both photosystems working cooperatively in series, moving electrons one-way from water all the way to NADP+, rather than in a closed loop. When photons activate the P680 reaction centre of PS II, an electron is raised to a high energy level and then passes downhill through a defined chain of carriers - pheophytin, then plastoquinone, then the cytochrome b6-f complex, then plastocyanin - before finally being accepted by the P700 reaction centre of PS I; ATP is generated during this segment of electron transport, as the electron moves from PS II to PS I. Separately, light also directly excites P700 itself, raising a second electron to a high energy level, where it is captured by the acceptor molecule ferredoxin-reducing substance (FRS); as this electron then travels downhill through ferredoxin, it is used to reduce NADP+ to NADPH+H+ (with the required H+ supplied by water photolysis). Because electrons lost by PS II are never returned to it - unlike in the cyclic pathway - and instead end up permanently reducing NADP+, this pathway is called non-cyclic, and because the shape traced by the electron's changing energy level across this whole sequence resembles the letter Z, it is also known as the Z-scheme. The full non-cyclic pathway can be broken into three sequential stages: first, electron transport from water to P680, in which the electrons PS II loses on excitation are replaced by the electrons, protons and oxygen released from water splitting; second, electron transport from P680 to P700, via the pheophytin-plastoquinone-cytochrome b6-f-plastocyanin chain described above; and third, electron transport from P700 to NADP+, in which the freshly excited P700 electron travels downhill throug …