Explain how the structures of saturated and unsaturated fatty acids differ, and how these structural differences affect the physical properties of the lipids that contain them.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Saturated fatty acids contain only single C–C bonds; unsaturated fatty acids have one or more C=C double bonds in the hydrocarbon chain.
The C=C double bond(s) in unsaturated fatty acids introduce a kink or bend in the chain.
Because saturated chains are straight, they pack tightly together, giving strong van der Waals (London dispersion) forces and making the lipid solid at room temperature (e.g. animal fat).
The kinks in unsaturated chains prevent tight packing, weakening the intermolecular forces and so the lipid remains liquid at room temperature (e.g. plant oil).
The C=C double bond(s) in unsaturated fatty acids introduce a kink or bend in the chain.
Because saturated chains are straight, they pack tightly together, giving strong van der Waals (London dispersion) forces and making the lipid solid at room temperature (e.g. animal fat).
The kinks in unsaturated chains prevent tight packing, weakening the intermolecular forces and so the lipid remains liquid at room temperature (e.g. plant oil).
Examiner tips
- Use the exact terms ‘single C–C bonds’ and ‘C=C double bonds’.
- Explain the effect on packing and van der Waals forces.
- Give an example for each type of lipid.
Common mistakes
- Confusing ‘kink’ with ‘bend’ or omitting the double bond detail.
- Failing to link the packing to the physical state.
Mark scheme (4 marks)
- Saturated fatty acids have no double bonds between carbon atoms / contain only single C–C bonds, whereas unsaturated fatty acids have one or more C=C double bonds in the hydrocarbon chain.
- The C=C double bond(s) in unsaturated fatty acids introduce a bend / kink in the hydrocarbon chain.
- Saturated fatty acid chains are straight, allowing close packing of molecules and strong van der Waals / London dispersion forces between chains, so lipids containing them are solid at room temperature (e.g. animal fats).
- The kinks in unsaturated fatty acid chains prevent close packing, resulting in weaker intermolecular forces, so lipids containing them are liquid / oils at room temperature (e.g. plant oils).
Key terms in this question
saturated fatty acid · unsaturated fatty acid
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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