Learning Objectives for this Section
The entry and exit of gasses in plants is through small pores called stomata located on the underside of leaves. Carbon dioxide, the gas required for the Calvin cycle, is not a very abundant gas in nature. Under hot and dry environmental conditions the stomata close to reduce the loss of water vapor, but this also results in a greatly diminished supply of CO2 for the plant. Plants that normally live in dry, hot climates have adapted different ways of initially fixing CO2 prior to its entering the Calvin cycle. These pathways of carbon fixation, know as the C4 and the CAM pathways, take place in the cytoplasm of the cell.
a. The C4 pathway
The C4 pathway is designed to efficiently fix CO2 at low concentrations and plants that use this pathway are known as C4 plants. These plants first fix CO2 into a four carbon compound (C4) called oxaloacetate (see Fig. 9). This occurs in cells called mesophyll cells. First, CO2 is fixed to a three-carbon compound called phosphoenolpyruvate to produce the four-carbon compound oxaloacetate. The enzyme catalyzing this reaction, PEP carboxylase, fixes CO2 very efficiently so the C4 plants don't need to to have their stomata open as much.
The oxaloacetate is then converted to another four-carbon compound called malate in a step requiring the reducing power of NADPH. The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells. Here the four-carbon malate is decarboxylated to produce CO2, a three-carbon compound called pyruvate, and NADPH. The CO2 combines with ribulose bisphosphate and goes through the Calvin cycle while the pyruvate re-enters the mesophyll cells, reacts with ATP, and is converted back to phosphoenolpyruvate, the starting compound of the C4 cycle. The C4 cycle is summarized in Fig. 9.
b. The CAM pathway
CAM plants live in very dry condition and, unlike other plants, open their stomata to fix CO2 only at night. Like C4 plants, the use PEP carboxylase to fix CO2, forming oxaloacetate. The oxaloacetate is converted to malate which is stored in cell vacuoles. During the day when the stomata are closed, CO2 is removed from the stored malate and enters the Calvin cycle.
© Gary E. Kaiser
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Updated: July 12, 2001