Kranz Anatomy
The leaves of green plants using the C4 or Hatch-Slack pathway for photosynthetic carbon fixation almost invariably have a specialized internal arrangement of cells surrounding the vascular bundles that is called Kranz anatomy. Kranz, the German word for "halo," or "wreath,"refers to a ring of mesophyll cells just to the outside of another ring of large bundle-sheath cells, both of which encircle the vascular bundle. In transverse sections viewed under the microscope, the two cell layers give the appearance of a wreath surrounding each bundle. In addition to the unique "wreaths," other features that typify leaves with Kranz anatomy include small intercellular spaces, and frequent veins.
Kranz anatomy and the C4 photosynthetic pathway are especially characteristic of tropical grasses such as sugar cane, where it was first discovered, and corn, although it has also been found in other plants. It is sometimes possible to distinguish C4 plants by their dark green veins, which are a consequence of the chlorophyll rich "wreaths" surrounding the conductive tissue. The chloroplasts of mesophyll cells typically contain chlorophyll-bearing internal membranous structures called grana, while the large and conspicuous chloroplasts in bundle-sheath cells lack grana or have only a poorly developed type.
During active photosynthesis, bundle-sheath chloroplasts tend to form larger and more numerous starch grains than the mesophyll chloroplasts.
Plants with Kranz anatomy and the C4 photosynthetic pathway tend to be highly efficient in photosynthesis. They generally have higher maximum rates of photosynthesis, and become light saturated at higher light intensities enabling them to capture and store large amounts of light energy even in tropical areas. They are able to photosynthesize more effectively at higher temperatures, and at low carbon dioxide concentrations that severely limit photosynthesis in less efficient C3 plants. The basis for the superior photosynthetic ability of C4 plants with Kranz anatomy is based in part on cooperation between the C3 and C 4 photosynthetic pathways, both of which occur in these plants, and a peculiar system that involves the double fixation of carbon dioxide. Carbon dioxide is first fixed in the mesophyll cells by the C4 pathway with a three-carbon compound, phosphoenolpyruvate, as the acceptor molecule, and the four-carbon molecule, oxaloacetate, as the product. Oxaloacetate is quickly converted to malate using reducing power produced in mesophyll chloroplasts. The malate is transported to bundle-sheath cells where it releases carbon dioxide. The released carbon dioxide is quickly captured by the C 3 pathway, forming a three-carbon compound, 3-phosphoglycerate, and is then incorporated into sugars and starch.
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