Transport213-page17 - The presence of sugar in a sieve tube...

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Transport in Plants - 17 Solutes move through leaf mesophyll or storage parenchyma cells both symplastically and apoplastically to the vascular tissue. Solutes are then actively secreted, or loaded, from the source into a sieve tube. Transfer companion cells do this. Proton pumps, discussed earlier, use cotransport to transfer the sugars, which may accumulate concentrations two to three times the surrounding areas. Once the sugars are in the sieve tubes, movement is facilitated by osmotic potential (turgor increases the hydrostatic pressure in the cells).
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Unformatted text preview: The presence of sugar in a sieve tube attracts water. • Water moves into the sieve tube from adjacent xylem cells increasing the pressure in the sieve tube • Increased pressure forces the solutes into the next cell of the sieve tube. This mechanism is called pressure flow, a type of bulk flow. Active transport (cotransport with proton pumps) is again used at the sink, to move the solutes into the cells where they will be needed or stored....
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