Explain how the cohesive and adhesive properties of water are essential for its transport in the xylem of plants.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water molecules exhibit cohesion, meaning they are attracted to each other.
This cohesive force is due to the formation of hydrogen bonds between adjacent water molecules.
Water molecules also exhibit adhesion, forming attractions with the hydrophilic surfaces of the xylem vessel walls.
Together, cohesion and adhesion allow for the formation of a continuous column of water that can be pulled upwards by transpiration, resisting gravity.
This cohesive force is due to the formation of hydrogen bonds between adjacent water molecules.
Water molecules also exhibit adhesion, forming attractions with the hydrophilic surfaces of the xylem vessel walls.
Together, cohesion and adhesion allow for the formation of a continuous column of water that can be pulled upwards by transpiration, resisting gravity.
Examiner tips
- Use the terms cohesion and adhesion explicitly; link hydrogen bonds to cohesion.
- Mention the continuous column and its resistance to gravity; show how transpiration pulls the column up.
Common mistakes
- Confusing cohesion with adhesion or vice versa.
- Omitting the role of hydrogen bonds or the continuous column in the explanation.
Mark scheme (4 marks)
- Water molecules exhibit cohesion, meaning they are attracted to each other.
- This cohesive force is due to the formation of hydrogen bonds between adjacent water molecules.
- Water molecules also exhibit adhesion, forming attractions with the hydrophilic surfaces of the xylem vessel walls.
- Together, cohesion and adhesion allow for the formation of a continuous column of water that can be pulled upwards by transpiration, resisting gravity.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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