Which of the following is NOT considered a type of lipid?

Glycosaminoglycans (GAGs) are polysaccharides composed of repeating disaccharide units and are part of a group known as mucopolysaccharides. They play a crucial role in providing structural support in tissues and facilitating various biological processes, but they do not fit the classification of lipids.

Lipids are a broad category of molecules that are primarily hydrophobic or amphipathic in nature and include fats, oils, steroids, and phospholipids, among others. Cholesterol is a sterol, which is a type of lipid crucial for membrane structure and steroid hormone synthesis. Isoprenoids, also known as terpenoids, are derived from isoprene units and include a variety of natural compounds that function in cellular signaling and as building blocks for other biomolecules. Sphingomyelins are a class of sphingolipids, which are important components of cell membranes, particularly in the myelin sheath surrounding nerve cells.

Thus, while cholesterol, isoprenoids, and sphingomyelins are all recognized as lipid types due to their hydrophobic characteristics or their roles in membrane structure, GAGs do not qualify under this classification.