Editing Photosystem (section)

==Structure of PSI and PSII==
A reaction center comprises several (about 25-30)<ref>{{Cite journal|last1=Caffarri|first1=Stefano|last2=Tibiletti|first2=Tania|last3=Jennings|first3=Robert C.|last4=Santabarbara|first4=Stefano|date=June 2014|title=A Comparison Between Plant Photosystem I and Photosystem II Architecture and Functioning|journal=Current Protein & Peptide Science|volume=15|issue=4|pages=296–331|doi=10.2174/1389203715666140327102218|issn=1389-2037|pmc=4030627|pmid=24678674}}</ref> protein subunits, which provide a scaffold for a series of cofactors. The cofactors can be pigments (like [[chlorophyll]], [[pheophytin]], [[carotenoids]]), quinones, or [[Iron–sulfur cluster|iron-sulfur clusters]].<ref>{{cite book|last1=Jagannathan|first1=B|last2=Golbeck|first2=JH|title=Photosynthesis:Microbial|journal=Encyclopedia of Microbiology, 3rd ed|date=2009|pages=325–341|doi=10.1016/B978-012373944-5.00352-7|isbn=9780123739445}}</ref>

Each photosystem has two main subunits: an [[Light-harvesting complexes of green plants|antenna complex]] (a light harvesting complex or LHC) and a reaction center. The antenna complex is where light is captured, while the reaction center is where this light energy is transformed into chemical energy. At the reaction center, there are many polypeptides that are surrounded by pigment proteins. At the center of the reaction center is a special pair of chlorophyll molecules.

Each PSII has about 8 LHCII. These contain about 14 [[Chlorophyll a|chlorophyll ''a'']] and chlorophyll ''b'' molecules, as well as about four [[carotenoid]]s. In the reaction center of PSII of plants and cyanobacteria, the light energy is used to split water into oxygen, protons, and electrons. The protons will be used in proton pumping to fuel the ATP synthase at the end of an [[electron transport chain]]. A majority of the reactions occur at the D1 and D2 subunits of PSII.