Editing Chloroplast (section)

==== Light reactions ====
{{main|Light reactions}}

{{Plain image with caption|File:Thylakoid membrane 3.svg|The [[light reactions]] of photosynthesis take place across the [[thylakoid]] membranes.<!-- [[Photosystem&nbsp;II]] is usually found in the appressed thylakoid membranes between stacked granal thylakoids, while [[photosystem&nbsp;I]] and [[ATP synthase]] is found in portions of the thylakoid membrane in contact with the stroma.-->|450px|right|top|triangle|#ccc}}

The light reactions take place on the thylakoid membranes. They take [[light energy]] and store it in [[NADPH]], a form of NADP<sup>+</sup>, and [[Adenosine triphosphate|ATP]] to fuel the [[dark reactions]].

===== Energy carriers =====

{{Main|Adenosine triphosphate|NADPH}}

ATP is the phosphorylated version of [[adenosine diphosphate]] (ADP), which stores energy in a cell and powers most cellular activities. ATP is the energized form, while ADP is the (partially) depleted form. NADP<sup>+</sup> is an electron carrier which ferries high energy electrons. In the light reactions, it gets [[redox reaction|reduced]], meaning it picks up electrons, becoming [[NADPH]].

===== Photophosphorylation =====

{{main|Photophosphorylation}}

Like mitochondria, chloroplasts use the [[potential energy]] stored in an [[Hydron (chemistry)|H<sup>+</sup>]], or hydrogen ion, gradient to generate ATP energy. The two [[photosystems]] capture light energy to energize [[electrons]] taken from [[water]], and release them down an [[electron transport chain]]. The [[Plastoquinone|molecules]] between the photosystems harness the electrons' energy to pump hydrogen ions into the thylakoid space, creating a [[concentration gradient]], with more hydrogen ions (up to a thousand times as many)<ref name="Campbell-2009b" /> inside the thylakoid system than in the stroma. The hydrogen ions in the thylakoid space then [[diffuse]] back down their concentration gradient, flowing back out into the stroma through [[ATP synthase]]. ATP synthase uses the energy from the flowing hydrogen ions to [[phosphorylate]] [[adenosine diphosphate]] into [[adenosine triphosphate]], or ATP.<ref name="Campbell-2009b" /><ref>{{cite journal | vauthors=Jagendorf AT, Uribe E | title=ATP formation caused by acid-base transition of spinach chloroplasts | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=55 | issue=1 | pages=170–7 | date=January 1966 | pmid=5220864 | pmc=285771 | doi=10.1073/pnas.55.1.170 | bibcode=1966PNAS...55..170J | doi-access=free }}</ref> Because chloroplast ATP synthase projects out into the stroma, the ATP is synthesized there, in position to be used in the dark reactions.<ref name="Berg-2002b">{{cite book| first1=Jeremy M | last1=Berg | first2=John L | last2=Tymoczko | first3=Lubert | last3=Stryer | name-list-style=vanc |title=Biochemistry|year=2002|publisher=W. H. Freeman|location=New York, NY [u.a.]|isbn=0-7167-3051-0|pages=Section 19.4|edition=5. ed., 4. print.|url=https://archive.org/details/biochemistrychap00jere| url-access=registration }}</ref>

===== NADP<sup>+</sup> reduction =====

{{See also|Redox reaction}}

[[Electrons]] are often removed from the [[electron transport chains]] to charge [[NADP+|NADP<sup>+</sup>]] with electrons, [[reduction reaction|reducing]] it to [[NADPH]]. Like ATP synthase, [[ferredoxin—NADP+ reductase|ferredoxin-NADP<sup>+</sup> reductase]], the enzyme that reduces NADP<sup>+</sup>, releases the NADPH it makes into the stroma, right where it is needed for the dark reactions.<ref name="Berg-2002b" />

Because NADP<sup>+</sup> reduction removes electrons from the electron transport chains, they must be replaced—the job of [[photosystem II]], which splits [[water]] molecules (H<sub>2</sub>O) to obtain the electrons from its [[hydrogen atoms]].<ref name="Campbell-2009b" /><ref name="Pearson-2009">{{cite book|title=Biology—Concepts and Connections|year=2009|publisher=Pearson|pages=108–118}}</ref>

===== Cyclic photophosphorylation =====

{{Main|Cyclic photophosphorylation}}

While [[photosystem II]] [[photolyzes]] water to obtain and energize new electrons, [[photosystem I]] simply reenergizes depleted electrons at the end of an electron transport chain. Normally, the reenergized electrons are taken by NADP<sup>+</sup>, though sometimes they can flow back down more H<sup>+</sup>-pumping electron transport chains to transport more hydrogen ions into the thylakoid space to generate more ATP. This is termed [[cyclic photophosphorylation]] because the electrons are recycled. Cyclic photophosphorylation is common in [[C4 plants|{{C4}} plants]], which need more [[Adenosine triphosphate|ATP]] than [[NADPH]].<ref name="Campbell-2009d" />