The idea
Hydrogen bonding gives water strong cohesion and contributes to surface tension. Adhesion allows water to interact with suitable surfaces, including components of xylem walls. During transpiration, evaporation from leaves creates tension in a connected water column. Cohesion helps transmit this tension through the xylem. Capillary action alone is not a complete explanation of water rising through a tall tree.
Worked example
A drop of water clings to a clean glass surface. Identify the roles of cohesion and adhesion.
- Cohesion helps water molecules remain together in a drop.
- Adhesion attracts water to the glass surface.
- The balance between these interactions affects whether water beads or spreads.
A mistake to avoid
Cohesion and adhesion are different. Always identify whether the attraction is between water molecules or between water and another surface.
Check your understanding
What helps maintain a continuous column of water under tension in xylem?
Show the explanation
Cohesion between water molecules helps maintain the column; adhesion to xylem walls also contributes to its behaviour.
Explore the lesson
This guide connects to Cohesion, adhesion and the properties of water in the IB Biology course. The first two published lessons per subject are free previews; further interactive lessons require an account.