Which unsaturated fatty acid packs tightly?

• A. Saturated fatty acids
• B. Trans unsaturated fatty acids
• C. Cis unsaturated fatty acids
• D. Polyunsaturated fatty acids

Answer: (B)

Understanding how fatty acid shape affects packing is key. The tightness of packing depends on whether the hydrocarbon chain is straight or kinked. Trans unsaturated fatty acids have double bonds with hydrogens on opposite sides, which keeps the chain nearly straight and allows close alignment of molecules. In contrast, cis unsaturated fats have hydrogens on the same side, causing a bend that disrupts packing. Polyunsaturated fats have multiple cis bonds, adding more kinks and reducing packing density. So, among unsaturated fats, trans unsaturated fatty acids pack the most tightly.