The cohesion-tension theory explains how water moves up the xylem from the roots to the leaves, even in tall trees.Water molecules are polar: the oxygen atom carries a slight negative charge and the hydrogen atoms a slight positive charge. Neighbouring water molecules are therefore attracted to each other by hydrogen bonds. This attraction between water molecules is called cohesion.The theory works as follows:1. Water evaporates from the mesophyll cells in the leaf and is lost by transpiration.
2. This lowers the water potential of the mesophyll cells, so water moves into them from the xylem in the leaf.
3. Because of cohesion, water molecules leaving the top of the xylem pull the molecules below them upwards.
4. The whole column of water is pulled up the xylem as a continuous column, like a rope being pulled from the top.
5. The pull puts the water in the xylem under tension (negative pressure). Lignin stops the vessels collapsing.
6. Water enters the xylem in the root to replace the water that has moved up.
The process is passive: the plant uses no ATP to move water up the xylem. The energy comes from the sun, which heats the leaf and evaporates the water.If an air bubble enters a vessel, the column breaks and water can no longer be pulled up that vessel.
2. This lowers the water potential of the mesophyll cells, so water moves into them from the xylem in the leaf.
3. Because of cohesion, water molecules leaving the top of the xylem pull the molecules below them upwards.
4. The whole column of water is pulled up the xylem as a continuous column, like a rope being pulled from the top.
5. The pull puts the water in the xylem under tension (negative pressure). Lignin stops the vessels collapsing.
6. Water enters the xylem in the root to replace the water that has moved up.
The process is passive: the plant uses no ATP to move water up the xylem. The energy comes from the sun, which heats the leaf and evaporates the water.If an air bubble enters a vessel, the column breaks and water can no longer be pulled up that vessel.