A level Biology exam revision resources written by A level Examiners


Xylem vessels are the tissues made from xylem cells. They are responsible for transport of water & mineral salts (and support of the plant). Xylem vessels are:

  • Made from xylem cells, which are elongated and strengthened (with lignin)
  • Long, tubular structures
  • Formed by fusion of several cells, end to end in a row
  • Lignin deposited as rings or spirals in the cell wall providing rigidity
  • The living part of the cell dies, leaving a hollow centre.

Phloem tissue is responsible for transport of solutions of organic solutes (these are the products of photosynthesis). Phloem tissue is made from Phloem sieve tube elements and companion cells:

Figure 5 : Phloem tissue (Transverse Section (T.S.))

  • Living cells which contain obstructions to the flow of solution (i.e. sieve plates and, to a lesser extent, the cytoplasm).
  • No nucleus.
  • Cell walls at ends develop into sieve plates.
  • Closely associated with each sieve element
  • Dense cytoplasm with all the usual cell organelles
  • Metabolically very active deduced from large numbers of mitochondria present.
  • Very close link with sieve tube elements. If companion cell dies, so do some elements. In leaves they function as transfer cells, absorbing sugars and transferring them to sieve tube elements.

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