Of the two carbohydrate products of photosynthesis, starch remains in the mesophyll cells of C3 plants and the bundle sheaf cells in C4 plants. In these cells, starch is subjected to glycolysis, mainly in the dark, forming ATP, NADH, and small molecules that are used as building blocks for the synthesis of amino acids, lipids, and other cellular constituents. Sucrose, in contrast, is exported from the photosynthetic cells and transported throughout the plant. The vascular system used by higher plants to transport water, ions, sucrose, and other water-soluble substances has two components: the xylem and the phloem, which generally are grouped together in the vascular bundle (see Figure 8-44).
As illustrated in Figure 8-45a, the xylem conducts salts and water from the roots through the stems to the leaves. Water transported upward through the xylem is lost from the plant by evaporation, primarily from the leaves. In young plants the xylem is built of cells interconnected by plasmo-desmata, but in mature tissues the cell body degenerates, leaving only the cell walls. The phloem, in contrast, transports dissolved sucrose and organic molecules such as amino acids from their sites of origin in leaves to tissues throughout the plant; water also is transported downward in the phloem.
A phloem vessel consists of long, narrow cells, called sieve-tube cells, interconnected by sieve plates, a type of cell wall that contains many plasmodesmata and is highly perforated (Figure 8-45b). Numerous plasmodesmata also connect the sieve-tube cells to companion cells, which line the phloem vessels, and mesophyll cells to companion cells. Sieve-tube cells have lost their nuclei and most other organelles but retain a water-permeable plasma membrane and cytoplasm, through which sucrose and water move. In effect, the sieve-tube cells form one continuous tube of cytosol that extends throughout the plant. Differences in osmotic strength cause the movement of sucrose from the photosynthetic mesophyll cells in the leaves, through the phloem, to the roots and other nonphotosynthetic tissues that catabolize sucrose.
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