Editing Stoma (section)

===Opening and closing===
{{further|Guard cell}}
[[File:Opening and Closing of Stoma.svg|upright=1.9|thumb|Opening and closing of stoma]]

However, most plants do not have CAM and must therefore open and close their stomata during the daytime, in response to changing conditions, such as light intensity, humidity, and carbon dioxide concentration. When conditions are conducive to stomatal opening (e.g., high light intensity and high humidity), a [[proton pump]] drives [[proton]]s (H<sup>+</sup>) from the guard cells. This means that the cells' [[electrical potential]] becomes increasingly negative. The negative potential opens potassium voltage-gated channels and so an uptake of [[potassium]] ions (K<sup>+</sup>) occurs. To maintain this internal negative voltage so that entry of potassium ions does not stop, negative ions balance the influx of potassium. In some cases, chloride ions enter, while in other plants the organic ion [[malate]] is produced in guard cells. This increase in solute concentration lowers the [[water potential]] inside the cell, which results in the diffusion of water into the cell through [[osmosis]]. This increases the cell's volume and [[osmotic pressure|turgor pressure]]. Then, because of rings of cellulose [[microfibrils]] that prevent the width of the guard cells from swelling, and thus only allow the extra turgor pressure to elongate the guard cells, whose ends are held firmly in place by surrounding [[epidermis (botany)|epidermal]] cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which gas can diffuse.<ref>{{cite journal
|journal=Annals of Botany
|volume=89
|issue=1
|date=January 2002
|pages=23–29
|title=Structure and Development of Stomata on the Primary Root of ''Ceratonia siliqua'' L.
|author=N. S. CHRISTODOULAKIS |author2=J. MENTI |author3=B. GALATIS
|pmid=12096815 | doi = 10.1093/aob/mcf002
|pmc=4233769}}</ref>

When the roots begin to sense a water shortage in the soil, [[abscisic acid]] (ABA) is released.<ref>{{cite journal
|journal=Plant Physiology
|volume=102
|issue=2
|year=1993
|pages=497–502
|title=Sensitivity of Stomata to Abscisic Acid (An Effect of the Mesophyll)
|author=C. L. Trejo
|author2=W. J. Davies |author3=LdMP. Ruiz
|pmid=12231838
|pmc=158804 
|doi=10.1104/pp.102.2.497}}</ref> ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of the [[cytosol]] of the cells and cause the concentration of free Ca<sup>2+</sup> to increase in the cytosol due to influx from outside the cell and release of Ca<sup>2+</sup> from internal stores such as the endoplasmic reticulum and vacuoles.<ref>{{cite journal
|journal=Journal of Experimental Botany
|volume=52
|issue=363
|pages=1959–1967
|date=October 2001
|title=The role of ion channels in light-dependent stomatal opening
|author=Petra Dietrich
|author2=Dale Sanders |author3=Rainer Hedrich
 |pmid=11559731
|doi=10.1093/jexbot/52.363.1959 |doi-access=free
}}</ref> This causes the chloride (Cl<sup>−</sup>) and organic ions to exit the cells. Second, this stops the uptake of any further K<sup>+</sup> into the cells and, subsequently, the loss of K<sup>+</sup>. The loss of these solutes causes an increase in [[water potential]], which results in the diffusion of water back out of the cell by [[osmosis]]. This makes the cell [[plasmolysed]], which results in the closing of the stomatal pores.

Guard cells have more chloroplasts than the other epidermal cells from which guard cells are derived. Their function is controversial.<ref>{{cite web |url=http://6e.plantphys.net/essay10.01.html |title=Guard Cell Photosynthesis |access-date=2015-10-04 }}</ref><ref>{{cite journal
|title=The Guard Cell Chloroplast: A Perspective for the Twenty-First Century
|author=Eduardo Zeiger
|author2=Lawrence D. Talbott |author3=Silvia Frechilla |author4=Alaka Srivastava |author5= Jianxin Zhu
|journal=New Phytologist
|volume=153
|issue=3 Special Issue: Stomata
|date=March 2002
|pages=415–424
|doi=10.1046/j.0028-646X.2001.NPH328.doc.x
|pmid=33863211
|bibcode=2002NewPh.153..415Z
}}</ref>