C4 Plants

<h1>Text Preview</h1> <p>In hot climates where light is intense, the rate of photosynthesis is high. In dry climates, stomata close to prevent the loss of water. The combination of closed stomata and high rates of photosynthesis depletes carbon dioxide levels and raises oxygen levels in the plant cells. Under hot, dry conditions, the rate of photorespiration in C3 plants may approach the rate of photosynthesis. Under such conditions, plants produce less food and grow more slowly. </p> <p>Special adaptations in C4 plants allow them to minimize photorespiration. C4 plants, including corn, sugar cane, and crabgrass, are found primarily in tropical regions where they grow faster and produce more food than C3 plants living in the same environment.</p> <p>Leaf anatomy is adapted to carry out photosynthesis. Leaves contain mesophyll cells with air spaces so carbon dioxide can diffuse in for photosynthesis. They contain veins to supply the water used in photosynthesis. Bundle sheath cells are grouped around the veins.</p> <p>The leaf anatomy of C4 plants differs from that of C3 plants. First, the leaves of C4 plants have more prominent bundle-sheath cells than the leaves of C3 plants. Second, in C4 plants, the Calvin-Benson cycle takes place in the bundle-sheath cells, whereas the Calvin-Benson cycle occurs in the mesophyll cells of C3 plants. </p> <p>Carbon dioxide enters the leaves through the stomata and diffuses into the mesophyll cells. </p> <p>The mesophyll cells of C4 plants lack the enzyme rubisco found in C3 plants. Instead, carbon dioxide reacts with the 3-carbon compound phosphoenolpyruvate, or PEP, to form the four-carbon compound oxaloacetate. Oxaloacetate is converted into malate, which is transported to the bundle-sheath cells, which contain rubisco. </p> <p>Bundle-sheath cells are located deep in the leaf where atmospheric oxygen cannot easily diffuse into them. Inside these cells, malate is broken down into pyruvate and carbon dioxide. The released carbon dioxide now enters the Calvin-Benson cycle. Pyruvate is returned to the mesophyll cells, where it is converted into PEP. The C4 cycle concentrates carbon dioxide in the bundle-sheath cells. This facilitates the binding of carbon dioxide to rubisco and minimizes photorespiration. Because photorespiration is minimized, the C4 cycle can increase the plant’s efficiency even though an additional ATP is required to regenerate the starting reactant PEP.</p> <p>A variation of the C4 cycle is observed in some desert plants, where the C4 cycle is separated from the Calvin-Benson cycle in time rather than in space. In such plants, the stomata open and take in carbon dioxide at night when it’s cooler to minimize water loss. The carbon dioxide enters the C4 cycle and is stored as malate. During the day when sunlight is available, the malate is broken down into carbon dioxide, which now enters the Calvin-Benson cycle. These plants, called CAM plants, are able to carry out photosynthesis with minimal water loss.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>