The C4 and CAM Pathways

THE C4 AND CAM PATHWAYS

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.

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