Editing Photosystem (section)

==In oxygenic photosynthesis==

Both [[photosystem I]] and [[Photosystem II|II]] are required for oxygenic photosynthesis. Oxygenic photosynthesis can be performed by plants and cyanobacteria; cyanobacteria are believed to be the progenitors of the photosystem-containing chloroplasts of [[eukaryotes]]. Photosynthetic bacteria that cannot produce oxygen have only one photosystem, which is [[Light-dependent reactions#In bacteria|similar to either PSI or PSII]].

At the core of photosystem II is P680, a special chlorophyll to which incoming excitation energy from the antenna complex is funneled. One of the electrons of excited P680* will be transferred to a non-[[Fluorescence|fluorescent]] molecule, which ionizes the chlorophyll and boosts its energy further, enough that it can split water in the oxygen evolving complex (OEC) of PSII and recover its electron.{{cn|date=June 2022}} At the heart of the OEC are 4 Mn atoms, each of which can trap one electron. The electrons harvested from the splitting of two waters fill the OEC complex in its highest-energy state, which holds 4 excess electrons.<ref name=":0" />

Electrons travel through the [[cytochrome b6f complex|cytochrome ''b6f'' complex]] to photosystem I via an electron transport chain within the [[thylakoid membrane]]. Energy from PSI drives this process{{cn|date=June 2022}} and is harnessed (the whole process is termed [[chemiosmosis]]) to pump protons across the membrane, into the thylakoid lumen space from the chloroplast stroma. This will provide a potential energy difference between lumen and stroma, which amounts to a proton-motive force that can be utilized by the proton-driven [[ATP synthase]] to generate ATP. If electrons only pass through once, the process is termed noncyclic photophosphorylation, but if they pass through PSI and the proton pump multiple times it is called cyclic photophosphorylation.

When the electron reaches photosystem I, it fills the electron deficit of light-excited reaction-center chlorophyll P700{{sup|+}} of PSI. The electron may either continue to go through cyclic electron transport around PSI or pass, via ferredoxin, to the enzyme NADP{{sup|+}} reductase. Electrons and protons are added to NADP{{sup|+}} to form NADPH. 
This reducing (hydrogenation) agent is transported to the Calvin cycle to react with [[glycerate 3-phosphate]], along with ATP to form [[glyceraldehyde 3-phosphate]], the basic building block from which plants can make a variety of substances.