Which two factors influence membrane fluidity?

• A. Temperature and lipid composition
• B. Carbohydrate size and protein density
• C. pH level only
• D. External pressure only

Answer: (A)

Membrane fluidity is governed mainly by two things: how warm the membrane is and what the lipids are made of. Temperature boosts the kinetic energy of the lipid molecules, so at higher temperatures they move more and the bilayer becomes more fluid; at lower temperatures, movement slows and the membrane becomes more rigid.

Lipid composition also plays a big role. Unsaturated fatty acids have kinked tails that prevent tight packing, which increases fluidity. Saturated fatty acids, with straight tails, pack closely and reduce fluidity. Cholesterol helps fine-tune this: it stiffens the membrane at higher temperatures to prevent too much flow, and at lower temperatures it prevents the membrane from solidifying by keeping some space between lipids. Together, temperature and lipid composition determine how easily the membrane can move and rearrange.

Other factors mentioned, like pH or external pressure, can influence certain interactions in the membrane but are not the primary determinants of overall membrane fluidity. Carbohydrate size and protein density affect surface properties more than the bulk mobility of the lipid bilayer.