What effect do fatty acids have on the phospholipid bilayer?

• A. Rigid and brittle
• B. Flexible and fluid, allowing rapid lateral movement of molecules
• C. Completely impermeable to water
• D. Increases thickness of the bilayer

Answer: (B)

Fatty acids in the phospholipid tails control how tightly the molecules pack in the bilayer, which sets the membrane’s fluidity. When tails are unsaturated, they have kinks that prevent tight packing, making the membrane more fluid and allowing lipids and proteins to move laterally quickly within the layer. This flexible, dynamic nature is a hallmark of the phospholipid bilayer, described by the fluid mosaic model. In contrast, more saturated, longer tails pack more tightly and make the membrane more rigid, and the bilayer isn’t simply impermeable to water or necessarily thicker due to fatty acids alone.