Cohesion-tension Theory

This lesson covers: 

  1. How water enters the root
  2. How water moves through the xylem
  3. The mechanisms driving water upwards

Water movement through a plant

Water enters plants through their roots.


Water then moves through a plant as follows:

  1. Water enters a plant's root hair cells via osmosis.
  2. It moves through the cell cytoplasm or cell walls towards the xylem.
  3. The xylem transports water from the roots up to the leaves.

The cohesion-tension theory

The cohesion-tension theory explains how water moves upwards through the xylem against gravity.

Diagram showing cohesion between water molecules, adhesion to cellulose in xylem vessel wall, and movement of water in xylem.

This occurs due to various factors:

  • Cohesion - Hydrogen bonding causes water molecules to stick together and move as one continuous column.
  • Adhesion - Hydrogen bonding between polar water molecules and non-polar cellulose in xylem vessel walls pulls water upwards through the xylem.
  • Transpiration pull - Evaporation of water at leaves creates the transpiration pull, and this tension is transmitted down the whole water column due to cohesion.


This causes water to be pulled up through the xylem vessels.