Photosynthesis Lecture Part II


Light Independent, Dark Reactions or the Calvin Cycle


Photosynthesis is the process that plants use to convert CO2 into carbohydrates (sugar)--CO2 fixation. These carbohydrates are stored (as starch) and are used by the plant for food. The basic formula for photosynthesis is:

6CO2 + 6H2O ------> C6H12O6 + 6 O2

The metabolic pathways which convert CO2 into various sugars are called the light independent reactions or the Calvin cycle. All plants eventually use the Calvin cycle but there are 3 ways plants can fix CO2: C3, C4, and CAM pathways.

C3 plants are the most common. These plants attach (fix) CO2 to the 5 carbon sugar RuBP (ribulose bisphosphate) with the assistance of the enzyme RuBP carboxylase (Rubisco). The first organic product is a 3-carbon compound (3PG or PGAL) which can either be converted to sugar or recycled to pick up more CO2. The dark reactions of photosynthesis in C3 plants assimilate CO2 in the Photosynthetic carbon reduction cycle or PCR cycle.

Rubisco can catalyze two reactions. It can either fix carbon (rubisco carboxylase) or in a counterproductive process called photorespiration it can add oxygen to ribulose bisphosphate (rubisco oxygenase).

 

C4 plants, like corn (and other monocots), first convert CO2 into a 4-carbon compound by using a very efficient enzyme called PEP carboxylase found in mesophyll cells. These plants do best in hot tropical environments where CO2 is a limiting factor. C4 plants transport the 4-carbon molecule (malate) to bundle sheath cells where CO2 can be released to begin the Calvin cycle. All this takes place during the day.

 

C4 plants avoid photorespiration. Investigate photorespiration here

 

CAM plants (mostly succulents) first convert CO2 into a variety of organic acids. This happens at night when these plants open their stomata and water loss due to evaporation is greatly reduced. During the day CO2 is released from the organic acids, and converted into sugar using the Calvin cycle. CAM plants are best adapted to the hot dry environment of a desert where water is a limiting factor.


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Page undated Nov. 24, 2002