Final answer:
A fatty acid has a carboxyl group (-COOH) at the end of its hydrocarbon chain, making it amphiphilic due to the hydrophilic nature of the carboxyl group and the hydrophobic long chain.
Step-by-step explanation:
A fatty acid typically consists of a long hydrocarbon chain with a terminal carboxyl group (-COOH). This carboxyl group is what makes the fatty acid an acid. The molecule as a whole is amphiphilic, meaning it has both hydrophilic and hydrophobic parts. The carboxyl group is hydrophilic because it can release an H+ ion, which makes it water-soluble, while the long hydrocarbon chain is hydrophobic and does not mix well with water.
Fatty acids are important components of triglycerides and phospholipids, which are essential in the structure of cell membranes. Phospholipids are composed of a glycerol backbone, two fatty acid tails which are hydrophobic, and a phosphate group which is hydrophilic. This structure allows phospholipids to form bilayers that make up the cell membrane, giving the cell a flexible yet selectively permeable barrier.
Therefore, the correct answer is A. Carboxyl.