Proteins function as structural components in all of the following except __________.

The correct choice highlights that proteins do not function as structural components in glycolipids. Glycolipids are primarily composed of carbohydrates and lipids, and their role is mainly in cell membrane structure and signaling, rather than providing structural support through proteins.

In contrast, proteins are key structural components in cell membranes, muscle tissue, and biological structures like hair and nails. In cell membranes, proteins help create a framework that supports the membrane's integrity and functionality, often assisting in communication and transport. In muscle tissue, proteins such as actin and myosin are crucial for contraction and movement. Additionally, keratin, a structural protein, is a main component of hair and nails, providing strength and resilience. Each of these examples illustrates the central role that proteins play in structural biology.