Which of the following substances comprises the majority of the human cell membrane?

Proteins are indeed an integral component of the cell membrane, but they do not make up the majority of its composition. There are two types of membrane proteins: peripheral and integral. Peripheral proteins are located on the surface, while integral proteins span through the membrane. They play crucial roles in many cellular processes such as signal transduction, transport of molecules, and cell adhesion. However, they are interspersed within the phospholipid bilayer rather than forming the primary structure of the membrane.

Ergosterol is a type of sterol found in the cell membranes of fungi and protozoa, not in human cells. It functions similarly to cholesterol in these organisms, providing fluidity and stability to their cell membranes. Therefore, ergosterol would not be found in the majority of the human cell membrane.

Cholesterol is present in the cell membrane, where it contributes to membrane fluidity and stability. It is interspersed among the phospholipids in the bilayer and prevents fatty acid chains from sticking together, thus maintaining membrane flexibility. However, while important, cholesterol does not make up the majority of the cell membrane. Its role is more of a regulatory one, adjusting the fluidity of the membrane rather than forming its primary structure.