Editing Chloroplast (section)

=== pH ===

Because of the [[Hydron (chemistry)|H<sup>+</sup>]] gradient across the thylakoid membrane, the interior of the thylakoid is [[acidic]], with a [[pH]] around 4,<ref>{{cite book|title=Biochemistry.|year=2002|publisher=W H Freeman|pages=Section 19.4|url=https://www.ncbi.nlm.nih.gov/books/NBK22519/| vauthors=Berg JM, Tymoczko JL, Stryer L |edition=5th |access-date=30 October 2012}}</ref> while the stroma is slightly basic, with a pH of around 8.<ref name="Hauser-1995">{{cite journal | vauthors=Hauser M, Eichelmann H, Oja V, Heber U, Laisk A | title=Stimulation by Light of Rapid pH Regulation in the Chloroplast Stroma in Vivo as Indicated by CO2 Solubilization in Leaves | journal=Plant Physiology | volume=108 | issue=3 | pages=1059–1066 | date=July 1995 | pmid=12228527 | pmc=157457 | doi=10.1104/pp.108.3.1059 }}</ref>
The optimal stroma pH for the Calvin cycle is 8.1, with the reaction nearly stopping when the pH falls below 7.3.<ref name="Werdan-1975" />

[[CO2|CO<sub>2</sub>]] in water can form [[carbonic acid]], which can disturb the pH of isolated chloroplasts, interfering with photosynthesis, even though CO<sub>2</sub> is [[Enzyme substrate (biology)|used]] in photosynthesis. However, chloroplasts in living [[plant cells]] are not affected by this as much.<ref name="Hauser-1995" />

Chloroplasts can pump [[Potassium|K<sup>+</sup>]] and H<sup>+</sup> ions in and out of themselves using a poorly understood light-driven transport system.<ref name="Hauser-1995" />

In the presence of light, the pH of the thylakoid lumen can drop up to 1.5 pH units, while the pH of the stroma can rise by nearly one pH unit.<ref name="Werdan-1975">{{cite journal | vauthors=Werdan K, Heldt HW, Milovancev M | title=The role of pH in the regulation of carbon fixation in the chloroplast stroma. Studies on CO2 fixation in the light and dark | journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume=396 | issue=2 | pages=276–92 | date=August 1975 | pmid=239746 | doi=10.1016/0005-2728(75)90041-9 }}</ref>