Which factor does NOT influence membrane fluidity?

The factor that does not influence membrane fluidity is the concentration of proteins. Membrane fluidity primarily depends on the composition and structure of the lipid components in the membrane. Cholesterol is known to modulate fluidity by inserting itself between phospholipids, which can stabilize membranes at varying temperatures. Temperature directly affects the kinetic energy of the lipid molecules, thereby influencing fluidity; higher temperatures generally increase fluidity while lower temperatures reduce it. The types of fatty acids present in the membrane also play a significant role, as unsaturated fatty acids introduce kinks in the fatty acid tails, preventing tight packing and thereby increasing fluidity.

While proteins can be integral or peripheral components of membranes, their concentration does not directly affect the fluidity of the lipid bilayer itself. Instead, they can influence other properties of the membrane, such as function and organization, but they do not change the fluid nature of the lipid bilayer. Thus, it is the lipid components that are primarily responsible for determining the fluidity of a membrane.