Cohesion-Tension Theory
Biology: IB Diploma Higher Level, topic 7.2.6
Cohesion-Tension Theory
Water and inorganic ions travel up the xylem through cohesion and tension. The steps involved in this process are:
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
- 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
- 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
- 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.
Test yourself on Cohesion-Tension Theory
Steps Involved in Water Transport in the Xylem
Where are the hydrogen bonds formed in cohesion?