Explain how the structural differences between saturated and unsaturated fatty acids affect the physical properties of the lipids that contain them.

IB DP Biology Higher Level (2023 syllabus) — B1.1 Carbohydrates and lipids · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Saturated fatty acids have no C=C bonds, so the hydrocarbon chain is straight and can pack tightly, giving strong van der Waals forces and a high melting point – they are solid at room temperature.
Unsaturated fatty acids contain one or more C=C bonds, which create kinks in the chain. These kinks prevent tight packing, weaken van der Waals forces and lower the melting point – they are liquid (oils) at room temperature.

Examiner tips

Use the key terms ‘straight chain’, ‘kink’, ‘close packing’, ‘van der Waals forces’, ‘melting point’
Show the causal link: double bond → kink → less packing → lower melting point
Keep the answer concise – 4 marks, one sentence per point

Common mistakes

Confusing saturated with unsaturated in the description of kinks
Forgetting to mention the effect on melting point or physical state
Using vague terms like ‘fat’ instead of ‘fatty acid’

Mark scheme (4 marks)

Saturated fatty acids have no carbon–carbon double bonds, so the hydrocarbon chain is straight/unbranched.
Unsaturated fatty acids have one or more C=C double bonds, which introduce a kink/bend in the hydrocarbon chain.
Straight chains in saturated fatty acids allow close packing / stronger van der Waals forces between adjacent molecules, so saturated fats are solid at room temperature.
Kinks in unsaturated fatty acids prevent close packing / reduce van der Waals forces, so unsaturated fats (oils) are liquid at room temperature / have a lower melting point.

Key terms in this question

saturated fatty acid · unsaturated fatty acid