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Biological Molecules: Question 8

Syllabus 2.2

Multiple choice AS 1 mark

Triglyceride X is made from three saturated fatty acids. Triglyceride Y is made from three unsaturated fatty acids, each containing several cis carbon-carbon double bonds.

Which statement correctly compares the melting points of X and Y, and correctly explains the reason for the difference?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Saturated versus unsaturated fatty acids

A saturated fatty acid has no carbon-carbon double bonds in its hydrocarbon chain; the chain is fully “saturated” with hydrogen atoms and is straight, allowing neighbouring molecules to pack closely together. An unsaturated fatty acid contains one or more carbon-carbon double bonds, which (in the common cis configuration) introduce a kink or bend in the chain.

Effect on melting point

Because straight, saturated fatty acid tails can pack closely together, the weak intermolecular (van der Waals) forces between neighbouring triglyceride molecules add up over a large area of close contact, giving triglyceride X a relatively high melting point, such triglycerides tend to be solid at room temperature. In triglyceride Y, the kinks introduced by the cis double bonds prevent the unsaturated tails from packing as closely together, reducing the area of contact between neighbouring molecules and so weakening the total intermolecular forces between them. As a result, Y has a lower melting point than X and is more likely to be liquid at room temperature.

Why the other options are wrong

  • B reverses the relationship: kinked, unsaturated tails pack less closely, not more closely, so they give a lower, not higher, melting point.
  • C reverses the definitions of saturated and unsaturated: it is unsaturated fatty acids that contain carbon-carbon double bonds, not saturated ones.
  • D wrongly claims melting point depends only on the (identical) glycerol backbone; in fact, the fatty acid tails, which differ between X and Y, are what determine the melting point.

Final answer

A. Y has a lower melting point than X, because the cis double bonds in Y’s fatty acid tails create kinks that prevent close packing, weakening the intermolecular forces between neighbouring molecules.