Membrane fluidity depends on which factors?

• A. pH and cholesterol content
• B. Outer surface charge
• C. Temperature and lipid composition
• D. Protein content and carbohydrate attachments

Answer: (C)

Membrane fluidity is about how freely lipid molecules and proteins move within the bilayer, and the main controls are temperature and the lipids themselves. Temperature changes how much kinetic energy the molecules have: higher temperature makes the membrane more fluid, lower temperature makes it firmer. The lipid composition sets how those molecules pack together—unsaturated fatty acids add kinks that prevent tight packing, cholesterol can either dampen or maintain fluidity depending on temperature, and overall chain length and saturation shape movement. Because both temperature and lipid composition directly alter how easily lipids can slide past one another, this combination best explains membrane fluidity.

Other factors don’t drive the core mobility of the bilayer as much. Outer surface charge affects interactions with the environment rather than the intrinsic ability of lipid molecules to move, and while pH can influence the charge state of surface molecules, it doesn’t set the membrane’s fluid state. Protein content and carbohydrate attachments influence structure and signaling on the membrane surface but don’t determine the lipid bilayer’s fundamental fluidity as directly as temperature and lipid composition.