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

Lipids are vital components in various biological structures and functions within the body. Among the choices provided, protein hormones are primarily composed of amino acids rather than lipids. This distinguishes them from the other options where lipids serve critical structural roles.

Cell membranes, for instance, are largely made up of phospholipids, which create a bilayer that acts as a barrier to protect the cell and manage the passage of substances in and out. In adipose tissue, lipids such as triglycerides are stored as energy reserves and contribute to insulation and protection for the organs. Certain vitamins, specifically fat-soluble vitamins (such as A, D, E, and K), rely on lipids for their absorption and transport within the body, highlighting their structural significance in biological processes.

Thus, while lipids contribute extensively to the structure and function of cell membranes, adipose tissue, and certain vitamins, they do not play a role in the structural composition of protein hormones. Hence, the correct answer reflects the absence of lipids in the structural makeup of protein hormones.