Download Photosynthesis

Photosynthesis
Alternative Mechanisms of Carbon
Fixation
RuBisCO
Image 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.
Photorespiration
Image 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.
Photorespiration
Image 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 Photosynthesis
Image 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 Photosynthesis
Image from: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/calvin.html
C4 Photosynthesis
Image 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 Plants
Image 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/le
ctures/c4.htm