9.2  Transport in the Phloem of Plants

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Essential Idea:

Structure and function are  

correlated in the phloem of plants


  • Plants transport organic compounds from sources to sinks
  • Active transport is used to load organic compounds into phloem sieve tubes at the source
  • High concentrations of solutes in the phloem at the source lead to water uptake by osmosis
  • Incompressibility of water allows transport along hydrostatic pressure gradients
  • Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks


  • Structure-function relationships of phloem sieve tubes


  • Identification of xylem and phloem in microscope images of stem and root
  • Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide