Xylem & Phloem (Edexcel GCSE Biology: Combined Science)

A xylem vessel

Xylem cells lose their top and bottom walls to form a continuous tube through which water moves through from the roots to the leaves

• Function: transport tissue for water and dissolved mineral ions
• Adaptations:
  • No top and bottom walls between cells to form continuous hollow tubes through which water is drawn upwards towards the leaves by transpiration
  • Cells are essentially dead, without organelles or cytoplasm, to allow free passage of water
  • Outer walls are thickened with a substance called lignin, strengthening the tubes, which helps support the plant

Phloem cells

Phloem cells form tubes similar to xylem vessels, except the cells still retain some subcellular structures and are therefore living

• Function: transport of dissolved sugars (e.g. sucrose) and amino acids
• Adaptations:
  • Made of living cells (as opposed to xylem vessels, which are made of dead cells) that are supported by companion cells
  • Cells are joined end-to-end and contain holes in the end cell walls (sieve plates) forming tubes which allow sugars and amino acids to flow easily through (by translocation)
  • Cells also have very few subcellular structures to aid the flow of materials