Photosynthesis

Alternative Mechanisms of Carbon Fixation

RuBisCOImage from: http://www.ncbi.nlm.nih.gov/books/NBK22344/

• RuBisCO is the most abundant protein on Earth.

• It catalyzes the first reaction of the Calvin cycle.

PhotorespirationImage from: http://access.mmhs.ca/docs/Science/MMHS%20Web%20folder/Kamla/20102008.htm

• 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.

PhotorespirationImage from: http://access.mmhs.ca/docs/Science/MMHS%20Web%20folder/Kamla/20102008.htm

• When RuBP is oxidized, it produces only 1 molecule of 3-PGA.

• This process is called photorespiration.

Photorespiration

• In warmer temperatures, RuBisCO’s affinity for O2 increases, resulting in less CO2 being fixed into G3P.

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

C4 PhotosynthesisImage from: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/calvin.html

• 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 PhotosynthesisImage from: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/calvin.html

C4 PhotosynthesisImage from: http://www.botany.hawaii.edu/faculty/webb/bot311/bot311-00/psyn/PsynDark2.htm

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 during the day

CAM PlantsImage from: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/calvin.html

CAM plants

Animation

Narrated animation of C4 photosynthesis

and CAM plants:

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