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

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On hot, dry days plants close the stomata to conserve water, but this causes problems for photosynthesis. 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: corn, sugarcane, crabgrass, pigweed, russian thistle, etc, they fix co2 first in a four-carbon compound, 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: 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.

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