7.1.2

Xylem

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Structure

The xylem are plant vessels that are responsible for transporting water and mineral ions. The structure of the xylem is specialised for this role.

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Vessel elements

  • The xylem vessels are long, tubes of cells that run up the stem of plants.
  • The cells of the xylem are called vessel elements.
  • The vessel elements in the xylem are dead.
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End walls

  • The vessel elements are stacked on top one another.
  • There are no cell walls at the ends of each vessel element.
  • This creates a continuous tube for water to flow through.
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Lignin

  • The walls of the xylem are lined with a waterproof polymer called lignin.
  • Lignin reinforces the walls of the vessel elements to provide structural support.

Cohesion-Tension Theory

Water and inorganic ions travel up the xylem through cohesion and tension. The steps involved in this process are:

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1) Transpiration

  • Some of the water in the leaves is used in photosynthesis.
  • Most of the water in the leaves evaporates in a process called transpiration.
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2) Tension

  • The loss of water from the leaves creates tension in the xylem.
  • Tension is the formation of hydrogen bonds between water molecules and the sides of the xylem vessel elements.
  • Water in the xylem is pulled upwards by this tension towards the leaves.
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3) Cohesion

  • Individual water molecules also form hydrogen bonds with each other. This process is called cohesion.
  • When water molecules are pulled up the xylem, other molecules of water are also pulled upwards due to cohesion.
  • The combination of cohesion and tension together continuously pull water upwards to replace water that has been lost in the leaves by transpiration.
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4) Diffusion in the roots

  • When water is pulled up the stem, the water potential at the bottom of the plant decreases.
  • Water diffuses into the roots via osmosis down its water potential gradient.
  • Water in the roots travels via the apoplast or symplast pathway into the xylem.
    • In the apoplastic pathway, water moves through the cell walls of adjacent root cells.
    • In the symplastic pathway, water moves through the cytoplasm & vacuoles of adjacent root cells via the plasmodesmata.
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The Casparian strip

  • Near the root xylem vessels, there is an impermeable strip in the cell walls of root cells.
    • This is called the Capsarian strip.
  • The Casparian strip blocks the apoplast pathway as water can't pass through the strip.
  • This means water must pass through the cytoplasm to continue into the xylem.

Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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