Transport213-page16

Transport213-page16 - (The cells collapsed and quit...

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Transport in Plants - 16 Movement of Solutes While some solutes, including many minerals, are moved through xylem, most organic solutes, especially sucrose, are transported, or translocated, in phloem. Phloem is also the vehicle for much of the chemical communication that occurs within the plant. Phloem is responsible for the translocation of many signal molecules, including hormones and various RNA molecules that activate systemic changes such as defense responses and induction of flowering shoots. Molecules produced in one area are moved through phloem to their target destination. Electrical signals also move through phloem, including electrical signals that activate some rapid turgor responses. Phloem Movement The discovery of phloem movement is credited to Malpighi who recorded that when one rings a tree, the tree dies by lack of “nourishment” below the ring. Although this was noted a long time ago, learning how phloem transports solutes was inhibited because access to the phloem tissue was difficult.
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Unformatted text preview: (The cells collapsed and quit functioning when manipulated.) Ultimately, the common aphid was used as a research tool. Aphids normally penetrate into phloem to feed, and their actions do not stop the phloem activity in the plant. Aphids merely divert the flow into (and sometimes through) the aphid body. Sucrose has a high osmotic potential (hydrostatic pressure). While sucrose, like any substance, can move by diffusion, the normal rate of phloem movement is much faster than simple diffusion. Movement in phloem is always in a direction of more concentrated solute to less concentrated, or from where you have solutes (mostly sugars from photosynthesis) to where you need the solutes. Phloem can also be used to move solutes from storage areas to where solutes are needed. This pressure-flow gradient between the source of solutes and the sink, the location to which the solutes are being moved, explains how solutes are moved through phloem....
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This note was uploaded on 01/08/2012 for the course BIO 213 taught by Professor Makina during the Fall '09 term at SUNY Stony Brook.

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