Which statement best explains the amphipathicity of fatty acids?

• A. Ionic and hydrophilic carboxyl group
• B. Non-polar and hydrophobic hydrocarbon tail
• C. Either A or B could be correct
• D. Both A and B

Answer: (D)

Amphipathicity means a molecule has both a hydrophilic (water-loving) part and a hydrophobic (water-fearing) part. For fatty acids, the hydrophilic part is the carboxyl head, which is polar and, at physiological pH, can be ionized to a charged carboxylate, making it strongly water-attracting. The hydrophobic part is the long hydrocarbon tail, which is nonpolar and does not interact well with water. Having both features in one molecule explains why fatty acids can orient themselves at interfaces and form structures like micelles or lipid bilayers, with the heads in contact with water and the tails tucked away from it. Therefore, both aspects are necessary to describe amphipathicity, making the combined statement the best explanation.