Explain cohesion, adhesion and transpiration pull
Hydrogen bonding supports cohesion between water molecules and adhesion to cellulose, helping a transpiration pull move water through xylem.
Section 7.2 · Transport mechanisms
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; identifying tissues from real slides, drawing biological sections, supervised practical competence and human subject-teacher review remain necessary.
Before you begin
- Distinguish plant cell walls from cell membranes and recognise that vascular tissues form pathways through roots, stems and leaves.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does cohesion describe in a xylem water column?
What you will learn
- Explain cohesion between water molecules through hydrogen bonding
- Explain adhesion of water to cellulose cell walls and its role in xylem movement
Cohesion keeps water molecules associated
Water molecules form hydrogen bonds with neighbouring water molecules. These intermolecular attractions create cohesion, helping a continuous water column remain connected in xylem as water is pulled upwards.
When water evaporates from leaf surfaces, replacement at the evaporating surfaces creates tension transmitted through the cohesive water column. This is the cohesion–tension explanation of transpiration pull.
Adhesion supports contact with vessel walls
Water molecules also form attractions with cellulose in xylem cell walls. This adhesion helps water remain in contact with the vessel wall as it moves. Lignified walls strengthen vessels and resist collapse; adhesion to cellulose and wall support are related but distinct ideas.
Do not say hydrogen bonds are covalent bonds within a water molecule. The relevant hydrogen bonds are intermolecular attractions between water molecules or between water and cellulose groups.
Pause and explain
What does adhesion describe in the xylem explanation?
Worked example
Explain how leaf evaporation can contribute to upward water movement in a xylem vessel.
Show the worked solution
- Evaporation creates tension at the leaf end of the water column.
- Cohesion transmits this pull; adhesion helps water remain associated with cellulose walls.
Answer Evaporation from leaf surfaces creates a transpiration pull and tension in the xylem. Hydrogen-bond cohesion between water molecules helps transmit this pull through a continuous column; adhesion to cellulose in the walls helps maintain contact with the vessel wall.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Distinguish cohesion from adhesion in xylem water transport.
Give me a hint
Name the two partners in each attraction.
Compare my reasoning
- Cohesion is attraction between water molecules.
- Adhesion is attraction between water and cellulose in xylem walls.
Cohesion is hydrogen-bond attraction between water molecules, helping keep the water column continuous. Adhesion is attraction between water and cellulose in xylem cell walls, helping water remain in contact with the walls.
Look for these in your work
- Cohesion partners stated
- Adhesion partners stated
- Roles distinguished
Explain why water evaporation from a leaf can create an upward pull in xylem.
Give me a hint
Link evaporation, tension and the continuous water column.
Compare my reasoning
- Evaporation creates tension at the leaf end.
- Cohesion transmits tension through the water column; adhesion helps maintain wall contact.
Evaporation from internal leaf surfaces creates tension as water is replaced. Cohesion between water molecules allows the pull to be transmitted along a continuous xylem water column, while adhesion to cellulose supports contact with vessel walls.
Look for these in your work
- Evaporation/tension link
- Cohesion link
- Adhesion distinction
A student says lignin pulls water upwards by hydrogen bonding. Correct the statement.
Give me a hint
Separate wall reinforcement from the water interactions.
Compare my reasoning
- Hydrogen-bond cohesion is between water molecules.
- Lignin strengthens xylem walls; adhesion is between water and cellulose.
Hydrogen-bond cohesion between water molecules helps transmit the transpiration pull, and adhesion occurs between water and cellulose in xylem walls. Lignin strengthens the vessel wall and helps it resist collapse; it is not the substance that pulls water upwards.
Look for these in your work
- Cohesion corrected
- Adhesion corrected
- Lignin role corrected
Common mistakes
- Using cohesion and adhesion as synonyms
- Saying the entire water column is pumped upward by each molecule
- Confusing cellulose adhesion with lignin wall strength
What can you explain now?
What does cohesion describe in a xylem water column?
Compare with the explanation
Attraction between water molecules
Hydrogen bonding supports cohesion between water molecules.
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Make it stick
How do cohesion between water molecules and adhesion to cellulose each contribute to water movement in xylem?
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