Crassulacean Acid Metabolism  

Crassulacean Acid Metabolism, or CAM, is a very important desert adaptation which prevents water loss in a hot environment for plants. The heart of this elegant adaptation lies in the ability to open the stomates at night while keeping them closed at day. However a number of modifications were necessary for the implementation of this process in the carbon assimilation pathway, or the "dark" reactions of photosynthesis.

Normal carbon assimilation is known as the C3 pathway in which six turns are necessary for the formation of one glucose molecule. A simple equation is: CO2 + RuBP> 2PGA. Essentially, a 5 carbon and 1 carbon are converted into two 3 carbon molecules (PGA) which are later combined to form glucose. However this is an inefficient process for two reasons: (1) there are low levels of carbon in the atmosphere, and (2) rubisco (RuBP) has a low affinity for CO2. In response, plants produce high levels of RuBP. However, when these high concentrations come into contact with O2, a major bi-product of photosynthesis, oxidation occurs and the process is further reduced in efficiency. So as O2 levels increase, carbon assimilation rates are decreased. Plants generally compensate for this by keeping their stomates open during the day (Ricklefs, 1993).

The evolution of an additional step in carbon assimilation pathway is what distinguishes C4 plants from C3 plants. C4 plants have an extra step which allows spatial separation within the leaf. In these plants, CO2 is sequestered into the bundle sheath cells where a new molecule, PEP carboxylase, resides and has a high affinity for CO2. Also, bundle sheath cells are smaller than mesophyll cells, resulting in a higher concentration of CO2 simply by virtue of transport from mesophyll to bundle sheath (see figure).
PEP carboxylase combines PEP with CO2 and results in OAA which then proceeds in the remainder of the dark reactions. Essentially, C4 plants concentrate levels of CO2 and keep RuBP away from O2, both of which make photosynthesis a more efficient process.

In warmer climates, such as deserts, much less water is lost in C4 plants relative to C3 plants, offering a much stronger selective advantage to C4 plants. CAM is a second metabolic adaptation which minimizes water loss along with utilizing a C4 pathway. CAM plants keep their stomates open during the coolest periods of the day, i.e. night, while their stomates are closed during the hottest periods of the day. Rapid growth is reduced in CAM plants relative to C3 plants, but in desert environments where survival is more important than rapid growth, it pays, reproductively, to be a CAM plant (Mauseth 1998).

To recap, CAM plants are C4 plants that keep their stomates open at night and closed during the hottest periods of the day. This enables the plants to maximize water storage. In deserts, most plants are CAM.
 

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