Nature of science:

Developments in scientific research follow improvements in apparatus—experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available. (1.8)

• Plants transport organic compounds from sources to sinks.
• Incompressibility of water allows transport along hydrostatic pressure gradients.
• 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.
• Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.

Applications and skills:

• Application: Structure–function relationships of phloem sieve tubes.
• Skill: Identification of xylem and phloem in microscope images of stem and root.
• Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.

• Syllabus and cross-curricular links:
• Biology
• Topic 1.4 Membrane transport