Crassulacean Acid Metabolism

As mentioned in my previous post (Narrow Leaf Stonecrop 02-10-22), narrow leaf stonecrop (Sedum stenopetalum) is a plant that employs crassulacean acid metabolism (CAM).

CAM occurs in cacti and other plants that live in hot and water-deprived environments. Oversimplifying, plants photosynthesize during the day converting carbon dioxide and sunlight into glucose (energy). This is accomplished through the Calvin-Benson cycle (a C-3 cycle). Carbon dioxide enters plant through stomata in the leaves. All green plants contain the enzymes necessary for the Calvin-Benson cycle.

On one hand, desert plants need to restrict water loss by keeping their stomata closed during the day when it is hot and evaporation is at a maximum. But when the stomata are closed carbon dioxide cannot enter the leaf from the outside so photosynthesis is retarded. Yet during the day narrow leaf stonecrop can continue to photosynthesize. This is possible because of CAM.

At night when it is cooler and evaporation is less, the stomata open and carbon dioxide enters the leaf. It is chemically turned into malic acid, a four carbon compound. Then during the day when the stomata are closed to prevent water loss the malic acid is turned back into carbon dioxide and is incorporated into the Calvin-Benson cycle.

There are biochemical, physiological and anatomical differences between plants that utilize CAM (C-4) and only C-3 photosynthesis. CAM plants have cell layers of the leaf differentiated into a mesophyll and bundle sheath.

There is another type of C-4 system that I can mention in a later post.

As in my previous post, the pictured narrow leaf stonecrop was growing along Modoc National Forest Road 43N96A in Modoc County CA.

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