Final answer:
Steroids are considered lipids because they are hydrophobic and insoluble in water, not because they are composed of fatty acids or involved primarily in energy storage. They have a unique structure of four linked carbon rings, and include important substances like cholesterol, which is a component of cell membranes and a precursor for steroid hormones.
Step-by-step explanation:
Steroids are considered lipids primarily because they share one essential characteristic with other lipid fractions: they are hydrophobic and insoluble in water. This characteristic is due to the non-polar nature of steroids, which are composed of hydrocarbon rings. Unlike triglycerides, steroids do not contain fatty acids, nor are they primarily used for energy storage. Instead, steroids have a distinctive structure consisting of four linked carbon rings, making them significantly different in shape from other lipids like fats or oils. Some steroids, such as cholesterol, are crucial components of cell membranes, while many other steroids function as hormones, acting as signaling molecules within the body.
It is this hydrophobic property, coupled with their molecular structure, that classifies steroids as lipids. Cholesterol, a well-known steroid, is not only a key structural component of cell membranes, but also acts as a precursor for the synthesis of steroid hormones like estrogen and testosterone.