7.2.6

Cohesion-Tension Theory

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Cohesion-Tension Theory

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

1) Transpiration

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.
2) Tension

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.
3) Cohesion

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.
4) Diffusion in the roots

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.
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Practice questions on Cohesion-Tension Theory

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  1. 1
    Steps Involved in Water Transport in the XylemPut in order
  2. 2
    Where are the hydrogen bonds formed in cohesion?Multiple choice
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Xylem Structure

Investigating Transpiration Rate