Explain how the cohesive and adhesive properties of water contribute to the transport of water through the xylem of a plant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Cohesion is caused by hydrogen bonds between water molecules, so they stick together.
Because of cohesion, water molecules form a continuous, unbroken column in the xylem vessel. The tension generated by transpiration at the leaf pulls the entire column upward.
Adhesion occurs when hydrogen bonds form between water and the hydrophilic cellulose walls of the xylem, so water sticks to the walls.
Adhesion helps counteract gravity: the water clinging to the walls reduces the tendency of the column to fall and supports capillary action, aiding upward transport.
Because of cohesion, water molecules form a continuous, unbroken column in the xylem vessel. The tension generated by transpiration at the leaf pulls the entire column upward.
Adhesion occurs when hydrogen bonds form between water and the hydrophilic cellulose walls of the xylem, so water sticks to the walls.
Adhesion helps counteract gravity: the water clinging to the walls reduces the tendency of the column to fall and supports capillary action, aiding upward transport.
Examiner tips
- Use the terms cohesion and adhesion explicitly; explain the role of hydrogen bonds.
- Show how cohesion creates a continuous column and how tension pulls it up.
- Explain how adhesion to xylem walls counteracts gravity and aids capillary action.
- Keep each point concise and directly linked to the transport mechanism.
Common mistakes
- Confusing cohesion with adhesion or vice versa.
- Omitting the role of tension from transpiration.
- Failing to mention hydrogen bonds as the cause of cohesion/adhesion.
Mark scheme (4 marks)
- Cohesion arises because hydrogen bonds form between adjacent water molecules, causing them to stick together.
- Cohesion allows water molecules to form a continuous, unbroken column within the xylem vessel, so tension created by transpiration at the leaf surface pulls the entire column upward.
- Adhesion occurs because hydrogen bonds form between water molecules and the hydrophilic cellulose walls of the xylem, causing water to stick to the walls.
- Adhesion helps counteract gravity by allowing water to cling to the xylem walls, reducing the tendency of the water column to fall and supporting capillary action.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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