Final answer:
Waxes are esters of long-chain fatty acids and alcohols, and they serve as protective coatings, contrasting with the ring structure of steroids and the functional components of phospholipids in cell membranes.
Step-by-step explanation:
The compounds that are carboxylic acid esters with both R groups being long straight hydrocarbon chains and perform external protective functions are waxes. Unlike steroids and phospholipids, waxes consist of esters formed from long-chain fatty acids and long-chain alcohols. They are not soluble in water and typically have low melting points. Waxes are found as protective coatings on various plant and animal surfaces, such as the waxy covering on leaves. An example of a wax is bee wax, which is an ester of oleic acid with 18 carbon atoms and oleyl alcohol, also with 18 carbons.
Steroids, on the other hand, differ significantly in structure, featuring four fused carbon rings and occasionally a short tail, like cholesterol. While they share water insolubility with waxes, their ring structure sets them apart.
Phospholipids, which are also lipid molecules important for the structure and function of cell membranes, are composed of glycerol, two fatty acid chains, and a phosphate group, significantly different in structure to waxes and steroids.