Which of the following is not classified as a lipid?

Amino acids are the building blocks of proteins and are not classified as lipids. While lipids include substances such as cholesterol, triglycerides, and phospholipids, which are all integral components of biological membranes and energy storage, amino acids serve a different function within the body. They play a crucial role in the synthesis of proteins necessary for countless biological processes, including tissue repair, enzyme function, and immune responses. Thus, identifying amino acids as the answer in this context highlights their distinct role compared to the other options listed, which fall under the lipid classification.