Essential idea: Structure and function are correlated in the phloem of plants.
The lower power scanning electron micrograph images above show the sieve end plates found on phloem sieve tubes. These perforated walls (in combination with the reduced cytoplasm in sieve cells) gives sieve cells a low resistance to the flow of sap enabling efficient translocation of substances, e.g. sucrose throughout the plant.
Understandings, applications and skills
| 9.2.U1 | Plants transport organic compounds from sources to sinks. |
| 9.2.U2 | Incompressibility of water allows transport along hydrostatic pressure gradients. |
| 9.2.U3 | Active transport is used to load organic compounds into phloem sieve tubes at the source. |
| 9.2.U4 | High concentrations of solutes in the phloem at the source lead to water uptake by osmosis. |
| 9.2.U5 | Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks. |
| 9.2.A1 | Structure–function relationships of phloem sieve tubes. |
| 9.2.S1 | Identification of xylem and phloem in microscope images of stem and root. |
| 9.2.S2 | Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide. |
[Text in square brackets indicates guidance notes]
Presentation and Notes
The presentation is designed to help your understanding. The notes outline is intended to be used as a framework for the development of student notes to aid revision.
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Use the Cornell style template to collate your own notes for Topic 9 - Plant biology
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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)
