CIE A Level Biology (9700) 2019-2021

Revision Notes

7.1.4 Phloem Sieve Tube Elements

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

Phloem sieve tubes elements and companion cells 1, downloadable AS & A Level Biology revision notes

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)

Phloem sieve tubes elements and companion cells 2, downloadable AS & A Level Biology revision notes

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

Phloem sieve tube elements structure & function table, downloadable AS & A Level Biology revision notes

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

Phloem companion cells structure & function table, downloadable AS & A Level Biology revision notes

Comparison of xylem & phloem tissue table

, downloadable AS & A Level Biology revision notes by Save My Exams teachers for AQA, CIE, Edexcel & OCR exam boards

Exam Tip

Understand the difference between sieve tube elements and companion cells, and how they are different to xylem tissue. Remember that mature xylem tissue is dead, so there is no evidence of organelles, and they have lignified cell walls, whereas sieve tube elements have no lignin, do have sieve plates, and their companion cells contain nuclei and dense cytoplasm.

Author: Catherine

Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.
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