How is Sugar Transported in Vascular Plants?
  • Sugars, usually sucrose, are formed by plants during the process of photosynthesis. (S) They may also be produced by the breaking down of carbohydrates in storage organs, such as tubers. (TB)
  • Phloem is the vascular tissue responsible for transporting nutrients around the plant; it carries the dissolve sugars from the leaves.
  • Osmotic concentration is the concentration of solutes or sugars.
  • Sugars travel from ‘sources’ which are areas of high osmotic concentration and high water pressures, to regions called ‘sinks’ which are areas of low osmotic concentration and low water pressure.



Direction of Transport
Sugars in plants can move up or down in plants, depending on the location of source cells and sink cells. Generally sugars move from source to sink. Depending on the season, the location of source and sink cells can be reversed.


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Source to Phloem
After sugar molecules are produced they must be transported from source cell to phloem cells, where the concentration is generally higher.

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Translocation
The transport of substances through the phloem is called translocation. Phloem tubes are not hollow so the substances have to move between living cells. Sugar cells can move quickly enough to supply energy to cells throughout the plant.

Transport of Sugars from Phloem to Sink
When sugar reaches a sink cell it leaves the phloem. The sink cells have a lower concentration of sugars than the phloem cells. Therefore the transport is called passive transport.

Interesting Facts!
The concentration of sucrose inside the phloem cells is up to 100 times greater than that outside. "This attracts water into the cells and increases the hydrostatic pressure of the leaf phloem," -ARS plant physiologist Daniel R. Bush



GLOSSARY OF TERMS

Osmotic Concentration
Refers to the concentration of solutes, or sugars in this case; where the concentration of solutes is highest, so is the osmotic concentration.
Source
a plant cell with a high
concentration of sugars and other solutes,
such as a leaf cell.
Sink
a plant cell with a low concentration
of sugars; sugars may be converted to
starch for storage or used rapidly for
energy or as building blocks of other
carbohydrates.
Translocation
Transport of substances through the phloem.

References:
Nelson Biology 11. LeDrewB. FraserD. DulsonJ. Nelson Education Ltd. Pages 568-570 Transport of Sugars.
SparkNotes Editors. “SparkNote on Plants: Essential Processes.” SparkNotes LLC. n.d.. http://www.sparknotes.com/biology/plants/essentialprocesses/. Accessed June 14, 2012.