Biology 1202B Lecture Notes - Lecture 6: C4 Carbon Fixation, Pineapple, Vascular Bundle

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30 Apr 2012
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Lecture 6
On hot, dry days plants close the stomata to conserve water, but this causes problems for
photosynthesis
C3 Plants:
o Cotton, rice, spinach, lettuce, carrots, apples, etc.
o Initial fixation of CO2 occurs via rubisco
o When their stomata re closed, CO2 levels drop as CO2 is consumed in the Calvin cycle
o At the same time, O2 levels rise as the light reaction converts light to chemical energy
o While rubisco normally accepts CO2, when the stomata is closed the O2/CO2 ratio
increases, and rubisco can add O2 to RuBP instead of CO2
o When rubisco adds O2 to RuBP, it splits into a 3-carb piece and a two-carbon piece in a
process called photorespiration
The two-carbon fragment is exported from the chloroplast and degraded to CO2
by the mitochondria and peroxisomes
Unlike normal respiration, this process requires no ATP or organic molecules
C4 Plants:
o Corn, sugarcane, crabgrass, pigweed, Russian thistle, etc.
o They fix CO2 first in a four-carbon compound
o Mesophyll cells (spongy cells in leaf) incorporate CO2 into organic molecules
The key enzyme (phosphoenolpyruvate carboxylase) adds CO2 to
phosphoenolpyruvate (PEP) to form oxaloacetate
PEP carboxylase has a very high affinity for CO2 and can fix CO2 efficiently when
rubisco cannot
o Mesophyll cells pump these four-carbon compounds into bundle-sheath cells
Bundle sheath cells strip a carbon (as CO2) from the four-carbon compound and
return the three-carbon remainder to the mesophyll cells
Bundle sheath cells then use rubisco to start the Calvin cycle with an abundant
supply of CO2
Mesophyll cells pump CO2 into the bundle sheath cells, keeping CO2 levels high
enough for rubisco to accept CO2 and not O2
o C4 photosynthesis minimizes photorespiration and enhances sugar production
o C4 plants thrive in hot regions with intense sunlight
CAM Plants:
o Cactus, pineapple, jade plant, agave, etc.
o Open stomata during the night, then close them during the day
Temperature are typically lower at night and humidity is higher
o At night, these plants fix CO2 into a variety of organic acids in mesophyll cells
o During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is
released from the organic acids
Both C4 and CAM plants add CO2 into organic intermediates before it enters the Calvin cycle
o C4 carbon fixation and the Calvin cycle are spatially separated
o CAM carbon fixation and the Calvin cycle are temporarily separated
o Both eventually use the Calvin cycle to incorporate light energy into the production of
sugar
Limiting factors of photosynthesis:
o Light
o CO2
o Temperature
o Water