The process of photosynthesis begins with photosystem II, which is named as such because photosystem I was discovered first. Photosystem II contains a chlorophyll molecule known as P680, which effectively absorbs light at a wavelength of 680 nanometers. When photosystem II loses electrons to electron acceptors, it compensates for this loss by splitting water molecules. This reaction not only generates electrons but also produces oxygen and protons. The splitting of water involves manganese, which plays a crucial role in this process by utilizing all five redox states of manganese.
After the electrons are released from photosystem II, they travel to the cytochrome complex, specifically cytochrome B6F. This complex, which includes hemes and iron-sulfur proteins, functions as a proton pump. It receives electrons from plastoquinone (PQ) and facilitates the movement of protons across the membrane, contributing to a proton gradient. The electrons then proceed through the cytochrome complex, moving from cytochrome B6 to cytochrome F, and finally to plastocyanin, which delivers them to photosystem I.
Photosystem I contains chlorophyll P700, which absorbs light best at 700 nanometers. The energy of the electrons decreases slightly as they transition from photosystem II to photosystem I. Photosystem I also requires electrons to replace those lost to electron acceptors. The electrons from photosystem II serve this purpose, moving from the reaction center to ferredoxin. Ferredoxin can either transfer these electrons to NADP+ reductase, resulting in the formation of NADPH, or return them to the cytochrome complex, where they can be reused.
This leads to two distinct pathways: cyclic and non-cyclic electron flow. In cyclic electron flow, electrons cycle from photosystem I to ferredoxin, then to the cytochrome complex, and back to photosystem I. This pathway generates a proton motive force but does not produce NADPH; it only results in ATP production. Conversely, non-cyclic electron flow produces both ATP and NADPH, as it involves the transfer of electrons from photosystem II to photosystem I and ultimately to NADP+.
The proton gradient established during these processes is essential for ATP synthesis, similar to oxidative phosphorylation. This process, known as photophosphorylation, harnesses sunlight energy to drive ATP synthase, contrasting with oxidative phosphorylation, which relies on the oxidation of nutrients. Thus, photophosphorylation represents a key mechanism for ATP generation in photosynthesis, complementing the energy production achieved through oxidative phosphorylation.