Different phases of CAM pathway

·         Crussulacean acid metabolism (CAM) enables many plant to grow in extremely dry environments such as deserts. This pathway is commonly found in Cactaceae, Euphorbiacae and Aizoaceae.

·         CAM photosynthesis involves a temporal separation of the functioning of two carboxylating enzymes. PEP carboxylase and Rubisco. CAM open their stomata at night when temperature are usually below the day time, so there is reduced risk of dehydration.

·         There are four phases in CAM pathway. The sequence are as follows:

·         PHASE I : (night) Co2 entering the leaf at night is first assimilated (as bicarbonate) by PEP carboxylase, forming OAA from HCO3- and PEP as it does in C4 plant.

·         The regulation of PEP carboxylase is different as it active now in the dark and inactive in light.

·         The OAA formed in this reaction is reduced to malate, which is moved into the vacuole by a translocator mechanism and stored their overnight. This accumulation of malate in the form of malic acid in the vacuole, can reach high conc. so that the leaf as a whole become distinctly acidic.

·         PHASE II: While the end of phase I, is the marked closure of stomata, they may reopen briefly at the very beginning of the light period providing the plant is not experience drought condition. This defined as Phase II, where external CO2 can be assimilated by both PEP carboxylase and Rubisco.

·         The signal for the stomata to open under these conditions results from combined effect of light together with low internal conc. of CO2.

·         Malate decarboxylation releases CO2 and as this build ups internally the stomata close and plant enter Phase III.

·         PHASE III: (light) In the morning, stomata close, the calvin cycle enzymes are active and Rubisco is available for CO2 assimilation.

·         Malate moves out of vacuole and into cytosol where it is decarboxylated by malic enzyme. This reaction releases CO2 and because stomata closed it cannot escape.

·         Rubisco is operating in CO2- enriched environment and photorespiration is kept to a minimum. Rubisco produces 3-PGA and the Calvin cycle function exactly same as C3 and C4 plants.

·         Phase IV: As the pool of malate become depleted in vacuole and CO2 conc. begin to fall again, the stomata may reopens once more in the light period. This marks onset of phase IV where again, both PEP carboxylase and Rubisco able to fix CO2 from the surrounding air.