Which function is NOT associated with proteins in the cell membrane?

Membrane fluidity is primarily a characteristic of the lipid bilayer that composes the cell membrane rather than a direct function of proteins. While proteins play essential roles in various functions, such as facilitating the transport of molecules across the membrane, enabling cell recognition, and participating in signaling pathways, they do not directly contribute to the fluid movement properties of the membrane itself.

Membrane fluidity is determined largely by the composition of the lipids, including the saturated and unsaturated fatty acid chains in the phospholipids, which influence how closely the lipid molecules can pack together. The presence of cholesterol also plays a significant role in modulating membrane fluidity, helping to maintain the balance between rigidity and flexibility. Thus, while proteins are essential for many functions in the membrane, they do not govern fluidity, making it the correct answer in this context.