Explain how the cohesive and adhesive properties of water, both arising from hydrogen bonding, allow water to move upward through the xylem of tall plants against gravity.

IB DP Biology Higher Level (2023 syllabus) — A1.1 Water · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Cohesion, caused by hydrogen bonds between water molecules, keeps the water molecules joined into a continuous column. Adhesion, also due to hydrogen bonds, occurs between water and the hydrophilic cellulose walls of the xylem vessels, so the water sticks to the walls. When water evaporates from the leaf surface (transpiration) it creates a negative pressure or tension that pulls the water column upward. Cohesion transmits this tension through the entire column from leaves to roots, while adhesion prevents the column from breaking away from the vessel walls under tension. Together, cohesion and adhesion allow the column to remain intact and to move water upward against gravity (capillary action also contributes).

Examiner tips

  • Use the terms cohesion, adhesion, hydrogen bonding, tension, and capillary action explicitly.
  • Show the sequence: cohesion → continuous column, adhesion → cling to walls, transpiration → tension, tension transmitted by cohesion, adhesion keeps column intact.

Common mistakes

  • Confusing cohesion with adhesion or vice‑versa.
  • Omitting the role of transpiration or tension in pulling the column up.
  • Using vague phrases like "water moves up" without linking to hydrogen bonding or pressure.

Mark scheme (4 marks)

  1. Hydrogen bonds form between adjacent water molecules, producing cohesion that keeps water molecules joined in a continuous column.
  2. Adhesion occurs because water molecules form hydrogen bonds with the cellulose/hydrophilic walls of xylem vessels, allowing water to cling to the vessel walls.
  3. Transpiration at the leaf surface creates tension / a negative pressure that pulls the water column upward, with cohesion transmitting this tension throughout the entire column from leaves to roots.
  4. Adhesion to xylem walls prevents the water column from pulling away from the sides under tension, so the column remains intact and continuous movement against gravity is sustained (capillary action contributes).

Key terms in this question

hydrogen bonding · xylem

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