Plant Ecology
Photolysis
B. Adaptations to the Light Environment
3
1. Photosynthesis
b. Calvin Cycle (Dark Reactions)
ATP and NADPH used as energy source to: Produce glucose from atmospheric CO2 Enzyme rubisco catalyzes CO2 + 5C (Ribulose Biphosphate RuBP)
*rubisco has a strong affinity for O2
Splits into 3-C compound used to make glucose
B. Plant Adaptations to the Light Environment
3
1. Photosynthesis
c. Uptake of CO2 and Transpiration:
The Tale of the Terribly Tragic Trade-off
C source for glucose is atmospheric CO2 CO2 diffuses into leaf via stomata Rubisco fixes atmospheric CO2
Lowers [CO2] inside leaf and maintains gradient
CO2
H2O
B. Plant Adaptations to the Light Environment
3
1. Photosynthesis
c. Uptake of CO2 and Transpiration:
The Tale of the Terribly Tragic Trade-off
Inner leaf space saturated with H2O from root uptake H2O diffuses out of cell as long as stomata are open
Transpiration-cohesion-tension model 900 g water lost/gC assimilated
When plant becomes water stressed, stomata close conserving water, but shutting off CO2 supply.
B. Plant Adaptations to the Light Environment
3
1. Photosynthesis
c. Photorespiration
As stomata close, CO2 levels drop relative to O2 Rubisco catalyzes RuBP and O2 reaction
Favored under high temps and when CO2 levels are low
Produces an amino acid and CO2 Uses up RuBP, Releases CO2 May reduce net photosynthesis by 25 to 50%
Bad bad bad
Why does it exist??
B. Plant Adaptations to the Light Environment
3
2. Photosynthetic Adaptations
Balance constraints of water loss and photorespiration
a. C4 Plants
Different leaf anatomy: 2 types of photosynthetic cells
Mesophyll cells Bundle sheath cells
CO2 fixed by phosphoenolpyruvate (PEP) carboxylase in the cells
mesophyll PEP carboxylase: has much higher affinity for CO2 than Rubisco AND lacks an oxygenase function
Makes a 4-C compound (malic acid)
B. Plant Adaptations to the Light Environment
3
2. Photosynthetic Adaptations
a. C4 Plants
4-C compound transported to bundle sheath cells Decarboxylation occurs and CO2 is released [CO2] may be 8 to 10 times greater than atmosphere and mesophyll Normal C3 pathway runs in bundle sheath
B. Plant Adaptations to the Light Environment
3
2. Photosynthetic Adaptations
a. C4 Plants
Advantages **Avoids photorespiration
High [CO2] in bundle sheath cells
**maintains steep CO2 gradient btwn mesophyll and bundle sheath cells even when stomata are almost closed Much higher rate of net photosynthesis Decreased water loss
250-350 g water/gC assimilated
B. Plant Adaptations to the Light Environment
3
2. Photosynthetic Adaptations
a. C4 Plants
Most are tropical grasses
Corn Sugar Cane
Few semi-arid shrubs
and saline habitat dicots
Evolved more than once
B. Plant Adaptations to the Light E...