Cyclic and Non-Cyclic Photophosphorylation

The synthesis of ATP that is coupled to the light-driven electron transport chain described in the previous sections is given a specific name: photophosphorylation, meaning, quite literally, light-driven phosphorylation of ADP to ATP. Depending on the precise path electrons take through the two photosystems, this process can occur in either of two distinct modes, called non-cyclic and cyclic photophosphorylation.

Non-cyclic photophosphorylation describes the default, linear route of electron flow already traced through the previous two sections: electrons are ejected from Photosystem II, replaced there by electrons from the splitting of water, and travel via plastoquinone, the cytochrome b6f complex and plastocyanin to Photosystem I; there, a second photoactivation event ejects a fresh, re-energised electron from PS I, which is passed via the small iron-sulfur protein ferredoxin (Fd) to the enzyme NADP+ reductase, which uses it (together with a proton) to reduce NADP+ to NADPH. Because the redox potential profile of this whole sequence, plotted from PS II through to NADP+ reduction, traces a shape resembling the letter Z lying on its side, this entire linear pathway is often called the Z-scheme. Non-cyclic photophosphorylation therefore simultaneously produces three products from a single pass of electron flow: ATP (via the proton gradient it generates), NADPH (at PS I), and O2 (from the splitting of water at PS II) -- and it is the pathway responsible for essentially all of the NADPH a chloroplast produces.

Cyclic photophosphorylation, by contrast, involves only Photosystem I operating on its own, entirely independently of Photosystem II. In this mode, the electron ejected from photoactivated P700 is not passed on to NADP+ reductase at all; instead, it is redirected back through the cytochrome b6f complex and plastocyanin, and returned to Photosystem I itself, forming a closed loop (hence "cyclic") rather than a one-way, linear path. Because the electron returns to its own point of origin rather than ending up on NADP+, cyclic photophosphorylation produces no NADPH; and because Photosystem II and the water-splitting complex are not involved at all, no water is split and no O2 is evolved either. What cyclic flow does still accomplish, because the electron still passes through the proton-pumping cytochrome b6f complex on its circular path, is to contribute further to the trans-thylakoid proton gradient -- so its sole net product is additional ATP. …