Learning focus
Develop accurate biological explanations, interpret plant diagrams and data, and connect practical evidence with theory.
Formation of tension
When water evaporates from mesophyll, it is replaced from xylem. This lowers pressure in leaf xylem and creates a pull or tension.
Cohesion
Water molecules attract one another. Cohesion allows pulling at the top of the column to be transmitted downward through xylem.
Continuous column
The absence of end walls in xylem and the narrow vessel lumen help maintain a continuous water column. If the column breaks, transport efficiency falls.
Root replacement
Water entering roots by osmosis replaces water drawn upward. Root pressure may contribute under some conditions, but the syllabus explanation centres on transpiration pull and cohesion.
Lignin under tension
The water column is under tension rather than being pushed like water through a hose. Lignified walls prevent vessels from collapsing inward.
Practical or data skill
Explain the ascent of water using a chain: evaporation → lower water potential/tension in leaf → pull on xylem → cohesion → upward movement.
Examination tip
Use “cohesion between water molecules”; adhesion is not required by this syllabus point.
Review questions and suggested answers
Question 1
What creates transpiration pull?
Suggested answer
Evaporation and water loss from leaves create tension in xylem.
Question 2
What keeps the water column together?
Suggested answer
Cohesive forces between water molecules.
Question 3
Why do xylem walls need lignin?
Suggested answer
To prevent collapse under tension and provide support.
