Cohesion-tension theory

• Cohesion-tension is the main factor that is responsible for the movement of water up the xylem, from the roots to the leaves
• Movement of water up the stem:

  1. Water vapour evaporates out of stomata on leaves

  • This leaves the cells with a negative water potential which causes more water to diffuse in through osmosis
• Movement of water up the stem:

  2. Water molecules form hydrogen bonds between one another and hence tend to stick together - this is known as cohesion

  3. Water forms a continuous, unbroken column across the mesophyll cells and down the xylem
• Movement of water up the stem:

  4. As water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of this cohesion

  5. A column of water is therefore pulled up the xylem as a result of transpiration - called the transpiration pull
• Movement of water up the stem:

  6. Transpiration pull (column of water being pulled up the xylem) creates tension, a negative pressure within the xylem, pulling the xylem in to become narrower
• Transpiration pull - Evaporation of water from the mesophyll cells pulls other water molecules from the xylem tissue into the leaf
• Cohesion - Water molecules form hydrogen bonds between one another and hence tend to stick together

  • So when some water molecules are pulled into the leaf others follow
• As water vapour evaporates it causes an upwards pull on the water column

  • This creates tension and pulls in the xylem walls making their diameter narrower
• Cohesion-tension theory:

  1. Water is lost from leaf because of transpiration
  2. Lowers water potential of mesophyll cells
  3. Water is pulled up the xylem - creating tension
  4. Water molecules cohere (stick together by hydrogen bonds)
  5. This forms a continuous water column
  6. Adhesion of water molecules to walls of xylem