Final answer:
Cell membranes regulate bilayer fluidity through factors such as the number of C-C double bonds in hydrocarbon tails, temperature, cholesterol, and proteins.
Step-by-step explanation:
The fluidity of the bilayer in cell membranes is regulated by various factors, including the number of C-C double bonds in the hydrocarbon tails of lipids and the temperature surrounding the cell. The presence of double bonds in the hydrocarbon tails introduces kinks in the lipid bilayer, making it more fluid. This fluidity allows for the movement of particles across the membrane. Cholesterol, another component of cell membranes, also plays a role in regulating fluidity by adjusting rigidity. Additionally, proteins in the membrane control the movement of particles in and out of the cell.