Explain how the properties of water make it an effective solvent for ionic compounds and polar molecules within living cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water is polar: the O atom is δ− and the H atoms are δ+. This uneven charge distribution allows water molecules to orient around ions – the δ− oxygen faces cations and the δ+ hydrogen faces anions – forming hydration shells that keep the ions separated in solution.
For polar molecules such as glucose or amino acids, water forms hydrogen bonds and dipole–dipole interactions with the solute’s polar groups, dispersing the molecules throughout the solvent.
These interactions make water an effective solvent, enabling ions and metabolites to move freely and interact, which is essential for biochemical reactions in the cytoplasm and organelles.
For polar molecules such as glucose or amino acids, water forms hydrogen bonds and dipole–dipole interactions with the solute’s polar groups, dispersing the molecules throughout the solvent.
These interactions make water an effective solvent, enabling ions and metabolites to move freely and interact, which is essential for biochemical reactions in the cytoplasm and organelles.
Examiner tips
- Use the word ‘polar’ and describe the δ charges; mention hydration shells for ions; explain hydrogen bonding for polar solutes; link to cellular metabolism and free movement of reactants.
- Structure answer in clear points to match the four marks; include both ionic and polar solute explanations.
Common mistakes
- Confusing water as non‑polar; forgetting to mention the orientation of water around ions; not linking solvent properties to cellular metabolism; using vague terms like ‘good solvent’ without explanation.
Mark scheme (4 marks)
- Water molecules are polar / have an uneven distribution of charge due to the electronegativity of oxygen, creating partial negative (δ−) and partial positive (δ+) charges.
- Water molecules orient around ions / polar solutes — the δ− oxygen faces cations and the δ+ hydrogen faces anions — forming hydration shells that keep solute particles separated in solution.
- For polar molecules (e.g. glucose, amino acids), hydrogen bonds and dipole–dipole interactions form between the polar groups of the solute and water, dispersing the molecules throughout the solvent.
- Being an effective solvent is essential for cell metabolism because it allows reactants and products (ions, metabolites) to move freely and interact, enabling biochemical reactions to occur in the cytoplasm / organelles.
Key terms in this question
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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