The Phloem (OCR AS Biology): Revision Note

Phloem Sieve Tube Elements & Companion Cells: Structure & Function

• The function of phloem tissue in a plant is to:

  • Transport organic compounds (assimilates), particularly sucrose, from the source (eg. leaf) to the sink (eg. roots). The transport of these compounds can occur up and down the plant

• The organic compounds are dissolved in water to form sap

• Phloem is a complex tissue made up of various cell types; its bulk is made up of sieve tube elements which are the main conducting cells and companion cells

• Other cell types of phloem tissue also include parenchyma for storage and strengthening fibres

• Mature phloem tissue contains living cells, unlike xylem tissue

Structure of phloem tissue. (a) Microscope slide image and drawing of a sieve tube element and companion cell in transverse section (TS), (b) photomicrograph image and drawing of a sieve tube element and companion cell in longitudinal section (LS).

Structure of phloem tissue. (c) Transmission electron micrograph image and drawing of a sieve tube element and companion cell in transverse section (TS)

Sieve tube elements

• Sieve tube elements line up end to end to form a continuous tube

Phloem sieve tube elements structure & function table

Companion cells

• Each sieve tube element has a companion cell associated with it as companion cells control the metabolism of their associated sieve tube member

• They also play a role in loading and unloading of sugars into the phloem

Phloem companion cells structure & function table

Comparison of xylem & phloem tissue table