Energy acquisition and biochemical
pathways: Photosynthetic pathways
(C3, C4, CAM)
Questions
• Lectures now posted on wiki
• Discussions:
– Mariana: posting paper and questions for this
week
– Dilys: will be picking a paper to post for next week
• Any questions?
Ferns
Gymnosperms
Angiosperms
Carbon dioxide uptake by
plants
• CO2 is
– About 0.038% in atmosphere
– What about inside leaves?
– But remember CO2 uptake is in exchange
for H2O loss. What about differences in
water across the leaf surface?
What is photosynthesis?
RUBISCO
• RUBISCO, drives
– Carbon fixation in photosynthesis and releases
oxygen (but has a low affinity for CO2),
– Carbon release in photorespiration with oxygen
as substrate.
Photosynthesis
RuBP + CO2  2PGA
Photorespiration
RuBP + O2  glycolate
•Photorespiration: No carbon fixed
but glycolate can be used in amino
acid synthesis
Photosynthetic strategies
• Most plants (C3): fix carbon dioxide
initially as phosphoglycerate (PGA), a
three-carbon compound.
– Enzyme is ribulose bi-phosphate carboxylaseoxidase (RUBISCO)
– Biochemical cycle is called Calvin cycle.
But, if it is really hot and dry
water uptake is a problem.
What to do?
Hatch and Slack cycle
• Some plants, mainly tropical grasses, C4:
first combine carbon dioxide with phosphoenol-pyruvate (PEP), into a four carbon
compound – oxaloacetate.
Crassulacean Acid Metabolism
(CAM)
• Third photosynthetic strategy (CAM): first
found in plants in the family Crassulaceae
and was therefore called Crassulacean
Acid Metabolism.
• Plants in other families also use this mode
of carbon fixation (lots of succulents and
epiphytes).
What do you think the main
advantages and disadvantages
of each strategy is?
Evolution of C4 in grasses
Question
• Why did C4 evolve in grasses?
– Dogma: A response to rising temperatures and
lowering CO2
– To test this, for 1230 grass spp, 1.1 million specimens
• GBIF: georeference points
• Climate: CRU (Climate Resource Unit)
• DNA sequences: With PHLAWD, built a phylogeny
– Chloroplast regions: atpB (59 taxa), matK (266 taxa), ndhF (437
taxa), rbcL (251 taxa), rpl16 (176 taxa), and trnL-trnF (810 taxa)
– Nuclear regions: phyB (93 taxa) and the internal transcribed
spacer (ITS; 753 taxa)
Ferns
Gymnosperms
Angiosperms
Poales
Poaceae
Shady
Question
• Why did C4 evolve in grasses?
– Forget dogma:
• Grasses were historically warm adapted
• C4 evolution was in response to a drop in precipitation
• So, the shift was probably from tropical rainforests
understories to open tropical savannahs and
grasslands!!!
• The question then becomes, when, why, and how did
C3 grasses evolve cold tolerance?