Editing Photosynthesis (section)

==Light-dependent reactions==
{{Main|Light-dependent reactions}}
[[File:Thylakoid membrane 3.svg|thumb|upright=1.4|right|Light-dependent reactions of photosynthesis at the thylakoid membrane]]
In the [[light-dependent reactions]], one [[molecule]] of the pigment [[chlorophyll]] absorbs one [[photon]] and loses one [[electron]]. This electron is taken up by a modified form of chlorophyll called [[pheophytin]], which passes the electron to a [[quinone]] molecule, starting the flow of electrons down an [[electron transport chain]] that leads to the ultimate [[Redox|reduction]] of [[Nicotinamide adenine dinucleotide phosphate|NADP]] to [[Nicotinamide adenine dinucleotide phosphate|NADPH]]. In addition, this creates a [[Electrochemical gradient|proton gradient]] (energy gradient) across the [[chloroplast membrane]], which is used by [[ATP synthase]] in the synthesis of [[Adenosine triphosphate|ATP]]. The chlorophyll molecule ultimately regains the electron it lost when a [[water]] molecule is split in a process called [[Photodissociation|photolysis]], which releases [[oxygen]].

The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:<ref name="Raven-2005">{{cite book |vauthors= Raven PH, Evert RF, Eichhorn SE |year= 2005 |title= Biology of Plants |edition= 7th |location= New York |publisher= [[W. H. Freeman and Company]] |pages= [https://archive.org/details/biologyofplants00rave_0/page/124 124–127] |isbn= 978-0-7167-1007-3 |url= https://archive.org/details/biologyofplants00rave_0 |url-access= registration }}</ref>

{{block indent|2 H<sub>2</sub>O + 2 NADP<sup>+</sup> + 3 ADP + 3 P<sub>i</sub> + light → 2 NADPH + 2 H<sup>+</sup> + 3 ATP + O<sub>2</sub>}}

Not all [[wavelength]]s of [[light]] can support photosynthesis. The photosynthetic [[action spectrum]] depends on the type of [[accessory pigment]]s present. For example, in [[green plants]], the action spectrum resembles the [[absorption spectrum]] for [[chlorophyll]]s and [[carotenoid]]s with absorption peaks in violet-blue and red light. In [[red algae]], the action spectrum is blue-green light, which allows these [[algae]] to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths (red light) used by above-ground green plants. The non-absorbed part of the [[Electromagnetic spectrum|light spectrum]] is what gives [[photosynthetic organism]]s their [[color]] (e.g., green plants, red algae, [[purple bacteria]]) and is the least effective for photosynthesis in the respective [[organism]]s.

===Z scheme===
[[File:Z-scheme.png|thumb|upright=3|center|The "Z scheme"]]
In [[plant]]s, [[light-dependent reaction]]s occur in the [[thylakoid membrane]]s of the [[chloroplast]]s where they drive the synthesis of [[Adenosine triphosphate|ATP]] and [[NADPH]]. The light-dependent reactions are of two forms: [[Photophosphorylation|cyclic and non-cyclic]].

In the non-cyclic reaction, the photons are captured in the light-harvesting [[antenna complex]]es of [[Photosystem|photosystem II]] by [[chlorophyll]] and other [[accessory pigments]] (see diagram "Z-scheme"). The absorption of a photon by the antenna complex loosens an electron by a process called [[photoinduced charge separation]]. The antenna system is at the core of the [[chlorophyll]] molecule of the photosystem II reaction center. That loosened electron is taken up by the primary [[Electron acceptor|electron-acceptor]] molecule, [[pheophytin]]. As the electrons are shuttled through an [[Electron transfer chain|electron transport chain]] (the so-called ''Z-scheme'' shown in the diagram), a [[chemiosmotic potential]] is generated by pumping [[Hydron (chemistry)|proton cations]] ([[Hydrogen|H]]<sup>+</sup>) across the [[Cell membrane|membrane]] and into the [[thylakoid space]]. An ATP synthase [[enzyme]] uses that [[chemiosmotic potential]] to make ATP during [[photophosphorylation]], whereas [[NADPH]] is a product of the terminal [[redox]] reaction in the ''Z-scheme''. The electron enters a chlorophyll [[molecule]] in [[Photosystem I]]. There it is further excited by the [[light]] absorbed by that [[photosystem]]. The electron is then passed along a chain of [[electron acceptor]]s to which it transfers some of its [[energy]]. The energy delivered to the electron acceptors is used to move [[hydrogen ion]]s across the thylakoid membrane into the [[Lumen (anatomy)|lumen]]. The electron is eventually used to [[Redox|reduce]] the coenzyme [[Nicotinamide adenine dinucleotide phosphate|NADP]] with an [[Hydron (chemistry)|H<sup>+</sup>]] to NADPH (which has functions in the light-independent reaction); at that point, the path of that electron ends.

The cyclic reaction is similar to that of the non-cyclic but differs in that it generates only ATP, and no reduced NADP (NADPH) is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns to photosystem I, from where it was emitted, hence the name ''cyclic reaction''.

===Water photolysis===
{{Main|Photodissociation|Oxygen evolution}}
[[#Z scheme|Linear electron transport]] through a photosystem will leave the [[Photosynthetic reaction centre|reaction center]] of that photosystem [[oxidized]]. Elevating another electron will first require re-reduction of the reaction center. The excited electrons lost from the reaction center ([[P700]]) of [[photosystem I]] are replaced by transfer from [[plastocyanin]], whose electrons come from electron transport through [[photosystem II]]. Photosystem II, as the first step of the ''Z-scheme'', requires an external source of electrons to reduce its oxidized [[Chlorophyll a|chlorophyll ''a'']] reaction center. The source of electrons for photosynthesis in green plants and [[cyanobacteria]] is water. Two water molecules are oxidized by the energy of four successive charge-separation reactions of photosystem II to yield a molecule of [[Diatomic molecule|diatomic]] oxygen and four [[hydrogen]] ions. The electrons yielded are transferred to a redox-active [[tyrosine]] residue that is oxidized by the energy of [[P680|P680{{sup|+}}]]. This resets the ability of P680 to absorb another photon and release another [[Photodissociation|photo-dissociated]] electron. The oxidation of water is [[catalysis|catalyzed]] in photosystem II by a redox-active structure that contains four [[manganese]] ions and a [[calcium ion]]; this [[oxygen-evolving complex]] binds two [[water molecules]] and contains the four oxidizing equivalents that are used to drive the water-oxidizing reaction (Kok's S-state diagrams). The hydrogen ions are released in the [[Thylakoid#Lumen|thylakoid lumen]] and therefore contribute to the transmembrane chemiosmotic potential that leads to [[ATP synthesis]]. Oxygen is a [[By-product|waste product]] of light-dependent reactions, but the majority of organisms on [[Earth]] use oxygen and its energy for [[cellular respiration]], including [[photosynthetic organism]]s.<ref>{{Cite web |url= https://www2.lbl.gov/vkyachan/ |title= Yachandra / Yano Group |publisher= Lawrence Berkeley National Laboratory |access-date= 2019-07-22 |archive-url= https://web.archive.org/web/20190722054431/https://www2.lbl.gov/vkyachan/ |archive-date= 2019-07-22 |url-status= dead }}</ref><ref>{{cite journal |vauthors= Pushkar Y, Yano J, Sauer K, Boussac A, Yachandra VK |date= February 2008 |title= Structural changes in the Mn4Ca cluster and the mechanism of photosynthetic water splitting |journal= [[Proceedings of the National Academy of Sciences of the United States of America]] |volume= 105 |issue= 6 |pages= 1879–1884 |bibcode= 2008PNAS..105.1879P |doi= 10.1073/pnas.0707092105 |doi-access= free |pmc= 2542863 |pmid= 18250316 }}</ref>