Explain how the polarity of water molecules enables water to act as a solvent for non-polar molecules in the presence of amphipathic molecules, using phospholipid bilayer formation as an example.

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).

Biological membranes form spontaneously when phospholipids are mixed with water. This self-assembly depends critically on the properties of water as a solvent.

Model answer (4 marks)

Water molecules are polar, with a partial negative charge on oxygen and partial positive charges on hydrogen atoms, allowing them to form hydrogen bonds with other polar or charged species.
Non‑polar molecules cannot hydrogen‑bond and are unfavourably solvated, so water keeps its hydrogen‑bond network intact and excludes non‑polar regions.
Phospholipids are amphipathic: a hydrophilic phosphate head interacts with water, while hydrophobic fatty‑acid tails do not.
The tails therefore cluster together away from water, minimising disruption of hydrogen bonds, and the heads face the aqueous environment, driving spontaneous bilayer formation.

Examiner tips

  • Use the word ‘hydrogen bond’ and ‘hydrophobic’ to show understanding of key terms.
  • Show the two‑step process: water’s polarity → exclusion of non‑polar → amphipathic arrangement drives bilayer.

Common mistakes

  • Confusing ‘non‑polar’ with ‘hydrophilic’; remember non‑polar = hydrophobic.
  • Failing to mention that water’s polarity allows hydrogen bonding with the phospholipid head, not the tail.

Mark scheme (4 marks)

  1. Water molecules are polar/have an uneven distribution of charge, with a partial negative charge on oxygen and partial positive charges on hydrogen atoms, allowing them to form hydrogen bonds with other polar or charged molecules.
  2. Non-polar (hydrophobic) molecules cannot form hydrogen bonds or interact favourably with water, so water molecules maintain hydrogen bonds with each other and exclude non-polar regions.
  3. Phospholipids are amphipathic, having a polar/hydrophilic phosphate head that interacts with water and a non-polar/hydrophobic fatty acid tail that is excluded from water.
  4. The hydrophobic tails cluster together away from water (minimising disruption to hydrogen bonding) while the hydrophilic heads face the aqueous environment, driving spontaneous bilayer formation.

Key terms in this question

polarity · amphipathic · phospholipid bilayer

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