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9.2.11 Outline the role of phloem in active translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds, roots).
The movement of organic molecules in plants is called translocation. The organic molecule are dissolved in water and the solution is referred to as phloem sap.
So how does phloem move the phloem sap from the source to the sink?
The movement of phloem sap is explained through the pressure-flow hypothesis
The sieve plate function is not clear. Sometimes the pores are filled (viz callose) and there seems to be an associated protein that can be made to tightly fit in the poor - or change its shape and release the poor. The mechanism is partly understood, but the purpose is currently unknown (see University of Hamburg's Botany hypertext.)
http://www.uic.edu/classes/bios/bios100/lectf03am/sucrosepump.jpg
Structure of Phloem (diagram)
<http://www.clt.astate.edu/mhuss/toppage6.htm>
<http://www.uic.edu/classes/bios/bios100/lectf03am/phloem.jpg>
image from <http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPLANTANAT.html>
Image from <http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPLANTANAT.html>
image from <http://plantphys.info/plant_physiology/basiccytology1.shtml>
<http://www.uic.edu/classes/bios/bios100/lectf03am/phloemwater02.gif>