Topic 9.2: Transport in the Phloem of Plants

Essential Idea:  Structure and function are correlated in the phloem in plants.

At SHS, Topic 9.2 is taught in the following class unit(s):

• Xylem Structure and Function (unit 35)
• Phloem Structure and Function (unit 36)

Statements & Objectives:

9.2.U1  Plants transport organic compounds from sources to sinks.

• ​Define translocation, phloem sap, source and sink.
• List example source and sink tissues.
• State that phloem transport is bidirectional.

9.2.U2  Incompressibility of water allows transport along hydrostatic pressure gradients.​

• ​Outline why pressure in the phloem increases due to the movement of water into the phloem.

9.2.U3  Active transport is used to load organic compounds into phloem sieve tubes at the source.

• State that sucrose is the most prevalent solute in phloem sap.
• Outline why sucrose is used for phloem transport, as opposed to glucose.
• Describe the active transport of sucrose into the phloem via a co-transport protein.

9.2.U4  High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.

• State that the phloem becomes hypertonic to xylem due to the active transport of sucrose into the phloem.
• State that water moves into the phloem by osmosis.

9.2.U5  Raised by hydrostatic pressure causes the contents of the phloem to flow toward sinks.

• State that water moves from area of higher pressure to area of lower pressure and that the movement of water also moves the solutes dissolved in it.

9.2.A1  Structure-function relationships of phloem sieve tubes.

• State that the function of phloem includes loading of carbohydrates at a source, transport of carbohydrates through the plant, and unloading of carbohydrates at a sink.
• Outline the structure and function of sieve tube cells, with specific mention of the rigid cell wall and sieve plates.
• Outline the structure and function of companion cells, with specific mention of mitochondria and cell membrane infolding.

9.2.S1  Identification of xylem and phloem in microscope images of stem and root.

• State two ways xylem cells can be identified in cross sections of stem and root.
• Identify xylem given microscope images of stem and root.
• Identify phloem within the vascular bundle of a stem and root. ​

9.2.S2  Analysis of date from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.

• State that aphids consume phloem sap as the main component of their diet.
• Outline how aphids have been used to measure the rate of flow and composition of phloem sap.

9.2.NOS  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.

• Outline how radioactive carbon isotopes are used to study translocation.

In the News:

• Exploring a Tree One Cell at a Time (2014-09-01)
• Researcher affirms 86-year-old hypothesis (2016-06-06)
• Protein restricts sap uptake by aphids (2017-10-10)