text: Since carbon dioxide and oxygen compete at the active site of RuBisCO, carbon fixation by RuBisCO can be enhanced by increasing the carbon dioxide level in the compartment containing RuBisCO (chloroplast stroma). Several times during the evolution of plants, mechanisms have evolved for increasing the level of carbon dioxide in the stroma (see  carbon fixation). The use of oxygen as a substrate appears to be a puzzling process, since it seems to throw away captured energy. However, it may be a mechanism for preventing carbohydrate overload during periods of high light flux. This weakness in the enzyme is the cause of photorespiration, such that healthy leaves in bright light may have zero net carbon fixation when the ratio of O to  available to RuBisCO shifts too far towards oxygen. This phenomenon is primarily temperature-dependent: high temperatures can decrease the concentration of  dissolved in the moisture of leaf tissues. This phenomenon is also related to water stress: since plant leaves are evaporatively cooled, limited water causes high leaf temperatures.  plants use the enzyme PEP carboxylase initially, which has a higher affinity for . The process first makes a 4-carbon intermediate compound, hence the name  plants, which is shuttled into a site of  photosynthesis then decarboxylated, releasing  to boost the concentration of .
  Crassulacean acid metabolism (CAM) plants keep their stomata closed during the day, which conserves water but prevents the light-independent reactions (a.k.a. the Calvin Cycle) from taking place, since these reactions require  to pass by gas exchange through these openings. Evaporation through the upper side of a leaf is prevented by a layer of wax.