Editing Cactus (section)

===Metabolism===
[[Photosynthesis]] requires plants to take in [[carbon dioxide]] gas ({{CO2}}). As they do so, they lose water through [[transpiration]]. Like other types of [[Succulent plant|succulents]], cacti reduce this water loss by the way in which they carry out photosynthesis. "Normal" leafy plants use the [[C3 carbon fixation|C<sub>3</sub> mechanism]]: during daylight hours, {{CO2}} is continually drawn out of the air present in spaces inside leaves and converted first into a compound containing three carbon atoms ([[3-phosphoglycerate]]) and then into products such as [[carbohydrate]]s. The access of air to internal spaces within a plant is controlled by [[stoma]]ta, which are able to open and close. The need for a continuous supply of {{CO2}} during photosynthesis means the stomata must be open, so water vapor is continuously being lost. Plants using the C<sub>3</sub> mechanism lose as much as 97% of the water taken up through their roots in this way.<ref name=RaveEdwa01/> A further problem is that as temperatures rise, the [[enzyme]] that captures {{CO2}} starts to capture more and more oxygen instead, reducing the efficiency of photosynthesis by up to 25%.<ref name=Shar88/>

{{multiple image
|direction = vertical
|width = 260
|header = Schematic illustration of CAM
|image1 = CAM_schema_night.svg
|alt1 =
|caption1 = '''Night''': stomata open; {{CO2}} enters and is stored as malic acid; water vapor is able to escape.
|image2 = CAM_schema_day_cropped.svg
|alt2 =
|caption2 = '''Day''': stomata close; malic acid is converted back to {{CO2}} and used to make carbohydrate; water vapor is confined.
}}
[[Crassulacean acid metabolism]] (CAM) is a mechanism adopted by cacti and other succulents to avoid the problems of the  C<sub>3</sub> mechanism. In full CAM, the stomata open only at night, when temperatures and water loss are lowest. {{CO2}} enters the plant and is captured in the form of organic acids stored inside cells (in [[vacuole]]s). The stomata remain closed throughout the day, and photosynthesis uses only this stored {{CO2}}. CAM uses water much more efficiently at the price of limiting the amount of carbon fixed from the atmosphere and thus available for growth.<ref name=KeelRund03/> CAM-cycling is a less water-efficient system whereby stomata open in the day, just as in plants using the C<sub>3</sub> mechanism. At night, or when the plant is short of water, the stomata close and the CAM mechanism is used to store {{CO2}} produced by [[Cellular respiration|respiration]] for use later in photosynthesis. CAM-cycling is present in ''Pereskia'' species.<ref name=EdwaDono06/>

By studying the ratio of <sup>14</sup>C to <sup>13</sup>C incorporated into a plant—its [[isotopic signature]]—it is possible to deduce how much {{CO2}} is taken up at night and how much in the daytime. Using this approach, most of the ''Pereskia'' species investigated exhibit some degree of CAM-cycling, suggesting this ability was present in the ancestor of all cacti.<ref name=EdwaDono06/> ''Pereskia'' leaves are claimed to only have the  C<sub>3</sub> mechanism with CAM restricted to stems.{{sfnp|ps=none|Anderson|2001|p=37}} More recent studies show that "it is highly unlikely that significant carbon assimilation occurs in the stem"; ''Pereskia'' species are described as having "C<sub>3</sub> with inducible CAM."<ref name=EdwaDono06/> Leafless cacti carry out all their photosynthesis in the stem, using full CAM. {{As of|2012|February}}, it is not clear whether stem-based CAM evolved once only in the core cacti, or separately in the opuntias and cactoids;<ref name=EdwaDono06/> CAM is known to have [[convergent evolution|evolved convergently]] many times.<ref name=KeelRund03/>

To carry out photosynthesis, cactus stems have undergone many adaptations. Early in their evolutionary history, the ancestors of modern cacti (other than ''Leuenbergeria'' species) developed [[stoma]]ta on their stems and began to delay developing bark. However, this alone was not sufficient; cacti with only these adaptations appear to do very little photosynthesis in their stems. Stems needed to develop structures similar to those normally found only in leaves. Immediately below the outer epidermis, a hypodermal layer developed made up of cells with thickened walls, offering mechanical support. Air spaces were needed between the cells to allow carbon dioxide to diffuse inwards. The center of the stem, the cortex, developed "[[Ground tissue|chlorenchyma]]" – a plant tissue made up of relatively unspecialized cells containing [[chloroplast]]s, arranged into a "spongy layer" and a "[[Palisade cell|palisade layer]]" where most of the photosynthesis occurs.{{sfnp|ps=none|Edwards|Nyffeler|Donoghue|2005|p=1184}}
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