Among the items listed, the one that is not a lipid is cellulose. Cellulose is a complex carbohydrate, also known as a polysaccharide, which is made up of glucose units linked together. It is a major component of plant cell walls and provides structural support to plants.
On the other hand, cholesterol, triglycerides, phospholipids, and steroids are all types of lipids. Cholesterol is a sterol that plays essential roles in cell membrane structure and fluidity. Triglycerides are the primary form of stored energy in animals, consisting of glycerol and fatty acids. Phospholipids are crucial components of cellular membranes, with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. Lastly, steroids are a class of lipids characterized by a structure of four fused carbon rings, with various functional groups attached, influencing a variety of biological functions.