C4 photosynthesis |how C4 plants overcome photorespiration|
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C4 Plants The C4 plants
C4 Plants
The C4 plants are so named because they preface the Calvin cycle with an alternate mode of carbon fixation that forms a four-carbon compound as its first product.
The anatomy of a C4 leaf is correlated with the mechanism of C4 photosynthesis. In C4 plants, there are two distinct types of photosynthetic cells: bundle-sheath cells and mesophyll cells. Bundle-sheath cells are arranged into tightly packed sheaths around the veins of the leaf. Between the bundle sheath and the leaf surface are the more loosely arranged mesophyll cells, which, in C4 leaves, are closely associated and never more than two to three cells away from the bundle-sheath cells. The Calvin cycle is confined to the chloroplasts of the bundle-sheath cells. However, the Calvin cycle is preceded by incorporation of CO2 into organic compounds in the mesophyll cells.
The first step is carried out by an enzyme present only in mesophyll cells called PEP carboxylase. This enzyme adds CO2 to phosphoenolpyruvate (PEP), forming the four-carbon product oxaloacetate. PEP carboxylase has
a much higher affinity for CO2 than does rubisco and no affinity for O2. Therefore, PEP carboxylase can fix carbon efficiently when rubisco cannot—that is, when it is hot and dry and stomata are partially closed, causing CO2 concentration in the leaf to be lower and O2 concentration to be relatively higher.
After the C4 plant fixes carbon from CO2, the mesophyll cells export their four-carbon products. Within the bundle-sheath cells, the four-carbon compounds release CO2, which is reassimilated into organic material by rubisco and the Calvin cycle. The same reaction regenerates pyruvate, which is transported to mesophyll cells. There, ATP is used to convert pyruvate
to PEP, allowing the reaction cycle to continue.
#C4Cycle
#C4pathwayOfCarbonFixation
The C4 plants are so named because they preface the Calvin cycle with an alternate mode of carbon fixation that forms a four-carbon compound as its first product.
The anatomy of a C4 leaf is correlated with the mechanism of C4 photosynthesis. In C4 plants, there are two distinct types of photosynthetic cells: bundle-sheath cells and mesophyll cells. Bundle-sheath cells are arranged into tightly packed sheaths around the veins of the leaf. Between the bundle sheath and the leaf surface are the more loosely arranged mesophyll cells, which, in C4 leaves, are closely associated and never more than two to three cells away from the bundle-sheath cells. The Calvin cycle is confined to the chloroplasts of the bundle-sheath cells. However, the Calvin cycle is preceded by incorporation of CO2 into organic compounds in the mesophyll cells.
The first step is carried out by an enzyme present only in mesophyll cells called PEP carboxylase. This enzyme adds CO2 to phosphoenolpyruvate (PEP), forming the four-carbon product oxaloacetate. PEP carboxylase has
a much higher affinity for CO2 than does rubisco and no affinity for O2. Therefore, PEP carboxylase can fix carbon efficiently when rubisco cannot—that is, when it is hot and dry and stomata are partially closed, causing CO2 concentration in the leaf to be lower and O2 concentration to be relatively higher.
After the C4 plant fixes carbon from CO2, the mesophyll cells export their four-carbon products. Within the bundle-sheath cells, the four-carbon compounds release CO2, which is reassimilated into organic material by rubisco and the Calvin cycle. The same reaction regenerates pyruvate, which is transported to mesophyll cells. There, ATP is used to convert pyruvate
to PEP, allowing the reaction cycle to continue.
#C4Cycle
#C4pathwayOfCarbonFixation
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