Types of Photosynthesis: III. CAM Photosynthesis

Crassulacean Acid Metabolism or CAM Photosynthesis

This type of photosynthesis was named after the plant family Crassulaceae. According to Hopkins (1999), it may have evolved ahead of the C4 cycle based on its presence in primitive ferns and Welwitschia.

The Crassulacean Acid Metabolism is similar to C4 photosynthesis based on their having both C3 and C4 cycles of CO2 fixation and reduction. However, they differ in that in C4 photosynthesis the C3 and C4 cycles occur spatially in two different cells, that is, CO2 is initially fixed via the C4 cycle in the mesophylls cell followed by C3 cycle in the bundle sheath cells.

But in CAM photosynthesis, C3 and C4 cycles occur both spatially in different parts of the same cell (Moore et al. 2003) and temporally, that is, C4 cycle at night and C3 cycle during daytime. Plants exhibiting this mechanism of photosynthesis, referred to as CAM plants, open their stomata at night to allow the passage of CO2 from the atmosphere but close them during the day to conserve water.

The Process of CAM Photosynthesis

CO2 from the atmosphere first enters the leaf at night via the stoma when it is open. It diffuses into the cytoplasm of the mesophyll cell, undergoes hydration with carbonic anhydrase (CA) as catalyst, and converts it into bicarbonate (HCO3-). HCO3- is subsequently fixed by the enzyme phosphoenulpyruvate carboxylase (PEPcase or PEPC or PEPCO), adding it to the substrate phosphoenolpyruvate (PEP, C3H5O6P), a three-carbon acid. The product is the four-carbon oxaloacetate (OAA, C4H4O5). The OAA is quickly reduced to organic acid (e.g. malic acid) and temporarily stored in the vacuole of the cell.

During the day when the stomata are closed, the acid is retrieved from the vacuole and decarboxylated. The resulting CO2 enters the chloroplast and is fixed in the C3 cycle. C3 cycle proceeds as usual. Consequently, CAM plants are more acidic at night and progressively become more basic during the day.

As in C3 and C4 photosynthesis, the product of the C3 cycle in CAM photosynthesis is the glyceraldehyde-3-phosphate (G3P, C3H7O6P), also called triose phosphate and phosphoglyceraldehyde (PGAL). This three-carbon sugar leaves the cycle and go through further reactions to form glucose and other organic compounds including starch. Subsequently, the CO2 acceptor phosphoenolpyruvate (PEP) is regenerated as a breakdown product of starch for use in CO2 fixation via the C4 cycle during the night.

REFERENCES

HATCH MD, BURNELL JN. 1990. Carbonic anhydrase activity in leaves and its role in the first step of C4 photosynthesis. Plant Physiol. 93:825-828. Retrieved Aug. 10, 2013 from http://www.plantphysiol.org/content/93/2/825.full.pdf.

HOPKINS WG. 1999. Introduction to Plant Physiology. 2nd ed. New York, NY: John Wiley & Sons, Inc. p. 189-214.

MATHEWS CK, VAN HOLDE KE. 1990. Biochemistry. Redwood City, CA: The Benjamin/Cummings Publishing Co., Inc. p. 643-669.

MOORE R, CLARK WD, VODOPICH DS. 2003. Botany. 2nd ed. Boston, Massachusetts: McGraw-Hill. p. 130-162.

SIMPSON MG. 2010. Plant Systematics. 2nd ed. San Diego, CA, USA: Elsevier Inc. P. 535-539.

(Ben G. Bareja Aug. 2013)

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