Ehleringer, J. R., Monson, R. K. 1993.

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Ehleringer, J. R., Monson, R. K. 1993. Evolutionary and ecological aspects of photosynthetic pathway variation. Annual review of ecology and systematics 24:411-439


C4[1] and CAM[2] photosynthesis are evolutionarily derived from C3[3] photosynthesis. The morphological and biochemical modifications necessary to achieve either C4 or CAM photosynthesis are thought to have independently arisen numerous times within different higher plant taxa. It is thought that C4 photosynthesis evolved in response to the low atmospheric CO2 concentrations that arose sometime after the end of the Cretaceous. Low CO2 concentrations result in significant increases in photorespiration of C3 plants, reducing productivity; both C3-C4 intermediate and C4 plants exhibit reduced photorespiration rates. In contrast, it may be argued that CAM arose either in response to selection of increased water-use efficiency or for increased carbon gain. Globally, all three pathways are widely distributed today, with a tendency toward ecological adaptation of C4 plants into warm, monsoonal climates and CAM plants into water-limited habitats. In an anthropogenically altered CO2 environment, C4 plants may lose their competitive advantage over C3 plants.


  1. ^ . C4 carbon fixation is one of three biochemical processes, along with C3 and CAM photosynthesis, that fixes carbon. It is named for the 4-carbon molecule of the first product of carbon fixation found in the small subset of plants that use the C4 process. This process is in contrast to the 3-carbon molecule products of C3 plants. C4 fixation is an elaboration of the more common C3 carbon fixation and is believed to have evolved more recently. C4 and CAM overcome the tendency of the enzyme RuBisCO to wastefully fix oxygen rather than carbon dioxide in the process of photorespiration. This is achieved in a more efficient environment for RubisCo by shuttling CO2 via malate or aspartate from mesophyll cells to bundle-sheath cells. In these bundle-sheath cells, RuBisCO is isolated from atmospheric oxygen and saturated with the CO2 released by decarboxylation of the malate. C4 plants use PEP carboxylase to capture more CO2 in the mesophyll cells. PEP Carboxylase (3 carbons) binds to CO2 to make oxaloacetic acid (OAA). The OAA then makes malate (4 carbons). Malate enters bundle sheath cells and releases the CO2 where RuBisCO works more efficiently. These additional steps, however, require more energy in the form of ATP. Because of this extra energy requirement, C4 plants are able to more efficiently fix carbon in drought, high temperatures, and limitations of nitrogen or CO2, while the more common C3 pathway is more efficient in the other conditions. C4_carbon_fixation:wikipedia
  2. ^ . Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide (CO2). The CO2 is stored as the four-carbon acid malate in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to CO2, which is then used during photosynthesis. The pre-collected CO2 is concentrated around the enzyme RuBisCO[4] , increasing photosynthetic efficiency. This metabolism was first studied in plants of the Crassulaceae family. These mainly include succulents. The first time it was studied, Crassula was used as a model organism. CAM:wikipedia
  3. ^ . C3 carbon fixation is one of three metabolic pathways for carbon fixation in photosynthesis, along with C4 and CAM. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into 3-phosphoglycerate through the following reaction: CO2 + H2O + RuBP → (2) 3-phosphoglycerate. Plants that survive solely on C3 fixation (C3 plants) tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate, carbon dioxide concentrations are around 200 ppm or higher,[1] and groundwater is plentiful. The C3 plants, originating during Mesozoic and Paleozoic eras, predate the C4 plants and still represent approximately 95% of Earth's plant biomass. C3 plants lose 97% of the water taken up through their roots to transpiration. Examples include rice and barley. C3 plants cannot grow in very hot areas because RuBisCO incorporates more oxygen into RuBP as temperatures increase. C3 carbon fixation: wikipedia
  4. ^ . Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known by the abbreviations RuBisCO or RuBPCase, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants and other photosynthetic organisms to energy-rich molecules such as glucose. In chemical terms, it catalyzes the carboxylation of ribulose-1,5-bisphosphate (also known as RuBP). It is probably the most abundant enzyme on Earth. RuBisCO