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In lipid biochemistry, why do unsaturated fatty acids tend to be liquid at room temperature?
Question

In lipid biochemistry, why do unsaturated fatty acids tend to be liquid at room temperature?

A.

The cis-double bonds introduce kinks, reducing van der Waals interactions.

B.

They have lower molecular weights than saturated fatty acids.

C.

They form ionic bonds that prevent solidification.

D.

Their double bonds increase molecular symmetry leading to less crystal packing.

Correct option is A

The correct answer is (a) The cis-double bonds introduce kinks, reducing van der Waals interactions.

  • Cis-double bonds in unsaturated fatty acids create kinks in the hydrocarbon chains.

  • These kinks prevent tight packing of the molecules, thereby reducing van der Waals interactions between them.

  • As a result, less thermal energy is needed to keep them in a liquid state, unlike saturated fatty acids, which pack tightly and are usually solid at room temperature.

Information Booster:

  • Saturated fatty acids: No double bonds; straight chains that pack tightly, leading to higher melting points.

  • Unsaturated fatty acids: One or more double bonds (usually cis), causing bends and reducing intermolecular attractions.

  • Example: Olive oil (rich in unsaturated fats) is liquid at room temperature; butter (rich in saturated fats) is solid.

  • Van der Waals forces: Weak attractions between molecules; stronger in well-packed (straight) chains.

Additional Information (Why Other Options Are Incorrect):

  • (B) Lower molecular weights: Not always true; unsaturated fatty acids can have similar or higher molecular weights than saturated ones.

  • (C) Ionic bonds: Fatty acids do not form ionic bonds in biological systems; they are primarily held together by hydrophobic interactions.

  • (D) Increased molecular symmetry: Cis-double bonds reduce symmetry, making packing more difficult, not easier.

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