Explain how the polarity of water molecules arises from their structure and outline one consequence of this polarity for living organisms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Oxygen is more electronegative than hydrogen, so the shared electrons in each O–H bond are pulled towards the oxygen atom. This creates a partial negative charge (δ−) on oxygen and partial positive charges (δ+) on the hydrogen atoms, giving the molecule a permanent dipole. The bent / V‑shaped geometry of the water molecule means the dipoles do not cancel, so the molecule is polar overall.
One consequence for living organisms is that the polarity allows water to act as a solvent for polar and ionic substances (e.g. glucose, ions, amino acids), enabling transport of nutrients, waste products or metabolites in biological fluids and facilitating metabolic reactions in aqueous solution.
One consequence for living organisms is that the polarity allows water to act as a solvent for polar and ionic substances (e.g. glucose, ions, amino acids), enabling transport of nutrients, waste products or metabolites in biological fluids and facilitating metabolic reactions in aqueous solution.
Examiner tips
- Show the electronegativity difference and dipole formation, then explain the bent shape; link polarity to solvent ability and biological relevance.
- Use terms: electronegative, partial charge, dipole, bent geometry, solvent, transport, metabolic reactions.
Common mistakes
- Claiming water is non‑polar or that the dipoles cancel; forgetting to mention the V‑shape; giving a non‑biological consequence such as boiling point instead of solvent role.
Mark scheme (4 marks)
- Oxygen is more electronegative than hydrogen, so the shared electrons in each O–H bond are pulled towards the oxygen atom.
- This creates partial negative charge (δ−) on oxygen and partial positive charges (δ+) on the hydrogen atoms, giving the molecule a permanent dipole.
- The bent / V-shaped geometry of the water molecule means the dipoles do not cancel, so the molecule is polar overall.
- One consequence for living organisms: polarity allows water to act as a solvent for polar/ionic substances (e.g. glucose, ions, amino acids), enabling transport of nutrients, waste products, or metabolites in biological fluids / facilitating metabolic reactions in aqueous solution.
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 →
More Water questions
- Explain how the hydrogen bonding between water molecules gives water a high spec…
- Explain how the cohesive and adhesive properties of water contribute to the tran…
- Explain why ice is less dense than liquid water and outline one biological signi…
- Explain how hydrogen bonding between water molecules gives water a high latent h…
- Explain how the property of surface tension in water arises from hydrogen bondin…
- Explain how the properties of water make it an effective solvent for ionic compo…