Xylem & Phloem (OCR Gateway GCSE Biology: Combined Science)

• Plants possess two specialist transport vessels called the xylem and phloem
• The xylem and phloem are arranged throughout the root, stem and leaves in groups called vascular bundles

Vascular tissue in a dicotyledonous plant

Structure and function of the xylem

• Xylem vessels (pronounced: zi-lem) transport water and minerals from the roots to the stem and leaves in the transpiration stream
• The xylem has some key structural features that makes it adapted for the transport of water 
  • It is composed of dead cells joined end to end with no end walls between them
  • This creates a hole (lumen) down the middle which forms a hollow tube
  • Xylem cells are strengthened by a woody material called lignin which makes the cell walls strong, waterproof and stiff

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

• Phloem tubes transport nutrients like dissolved sugars (e.g. sucrose) and amino acids both up and down the stem
  • Nutrients are transported to new growing and storage tissue in a process known as translocation
• Phloem tubes are made of columns of living cells called sieve-tube elements which are joined end to end
• Sieve tube elements contain end-plates with holes (sieve plates) which allow sugars and amino acids to flow easily through
• Sieve tube elements are supported by companion cells that carry out the living functions for the sieve tube elements
  • This is because sieve tube elements contain no nucleus so can't survive on their own

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

Comparison between Xylem and Phloem Tissue Table