Figure 6.

Thylakoid pathways for enhancing ATP production. (a) Water–water cycles involving the respiratory terminal oxidase in prokaryotes and the Mehler reaction. (b) Decoupled electron flow where electrons from PSII are transferred back to water by a midstream terminal oxidase and electron flow around PSI is cyclic. Proton pumping for ATP generation is accomplished in this scheme through water splitting at PSII and cyclic electron flow around PSI by PQH₂ oxidation and Q-cycle pumping at cytochrome b₆f.