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Alternative Mechanisms of
Carbon Fixation
C3 plants
C4 plants
CAM plants
The story so far….
Rubisco
• Catalyzes 2 reactions:
▫ Addition of CO2 to RuBP
(carboxylation)
 Photosynthesis
▫ Addition of O2 to RuBP
(oxidation)
 Photorespiration
BOTH happening inside plants!
What Weather Has To Do With It…
• Stomata  gas exchange & temperature control
• When it is hot, stomata openings shrink to
conserve water
▫ CO2 concentrations decline inside the leaf
• Cells continue to undergo photosynthesis
▫ oxygen levels increase – can’t exit via stomata
• Oxygen competes with CO2 for Rubisco’s active
site
▫ Oxygen has a higher affinity ∴ less CO2 binds
Photorespiration
• “Photo” – needs products
from light reactions
• Removes PGA from
Calvin Cycle…slowing
carbohydrate production.
• C3 plants lose up to ¼ of
the carbon they fix to
photorespiration.
Don’t’ Copy…
Just Look
Photosynthesis
Rubisco Substrate:
Products:
CO2
Glucose
O2
What Happens?
Rubisco Substrate:
Products:
O2
PGA
CO2
What Happens?
Calvin cycle
proceeds “normally”
Optimal Temperature:
Photorespiration
15-25°C
- No ATP produced
- No G3P produced
- Loses C
Optimal Temperature:
30-47°C
Photorespiration
• As C3 photosynthesis declines,
photorespiration increases …
could kill the plant due to
lack of food.
• Then why???
▫ Rubisco a remnant of earlier
mechanisms better suited to
high atmospheric CO2
C4 Plants
(4-Carbon molecule)
Enzyme:
Phosphoenol pyruvate
Carboxylase (PEP carboxylase)
Intermediary Molecule:
Oxaloacetate (OAA) & Malate
Drought resistant
Ex. corn, sugarcane
Types of Plants:
Types of Cells:
Bundle sheath: surround veins
Mesophyll – around bundle sheath
Purpose:
Keep [CO2] high to compete
with O2 for Rubisco
Energy Comparison to C3 Plants:
Requires more energy
ATP required to convert pyruvate into PEP
C4 Plant Photosynthetic Cells
• Bundle Sheath: site of Calvin cycle
• Mesophyll
▫ No mesophyll cells are more than 3 cells away
from any bundle sheath cell at anytime.
How Do C4 Plants Reduce
Photorespiration?
• Continually move CO2 into
bundle sheath cells, via malate
• Keeps CO2 concentrations high
so that it may outcompete
oxygen.
• 10-120x higher than normal
• Using more ATP is still
advantageous in a HOT
climate!
CAM Plants
Crassulacean Acid Metabolism
Stomata Action:
NIGHT:
Enzyme:
Night – open
Day – closed
PEP carboxylase
Storage Location:
Vacuole
DAY:
What happens?
- Malic acid broken down
- CO2 released
- Calvin cycle occurs
Why?
Conserve water
Intermediary:
Malic acid
In Summary…
Classwork/Homework