Alternative Mechanisms of Carbon Fixation

(5.4)

RuBisCO

• RuBisCO is the most abundant protein on Earth

• catalyzes the first reaction of the Calvin cycle

Photorespiration

• in the first reaction of the Calvin cycle, CO2 is “fixed”, forming 2 molecules of 3-PGA

• O2 competes with CO2 for the active site of RuBisCO (ribulose-1,5-bisphosphate carboxylase oxidase)

Photorespiration

• RuBP is combined with O2

• produces only 1 molecule of 3-PGA

• also produces a molecule that is toxic to the cell that must be removed in a series of reactions, which use ATP release 1 CO2

Photorespiration

In hot & dry climates…

• stomata close during the day to reduce water loss

• so less CO2 is present, less is fixed into G3P, and more photorespiration occurs

• How have plants in warmer climates evolved to cope with RuBisCO’s affinity for O2?

C4 Photosynthesis

• CO2 is combined with PEP to form 4-carbon oxaloacetate (enzyme PEP carboxylase) in mesophyll cells

• oxaloacetate is converted to malate, then transported into bundle-sheath cells

• malate is decarboxylated forming pyruvate, and CO2 enters the Calvin cycle

C4 Photosynthesis

C4 Photosynthesis

C4 Plants

CAM Plants

• crassulacean acid metabolism (CAM), first discovered in Crassulacea family of plants

• stomata are closed during the day, so CO2 is only absorbed at night

• C4 pathway at night, C3 pathway (Calvin cycle) during the day

CAM Plants

CAM plants

Animation

Narrated animation of C4 photosynthesis

and CAM plants:

http://www.uic.edu/classes/bios/bios100/lectures/c4.htm