Final answer:
Cholesterol and the tight packing of phospholipids in the lipid bilayer restrict the lateral motion of proteins, limiting their mobility.
Step-by-step explanation:
The lateral motion of proteins in a lipid bilayer can be restricted by several factors. One such factor is the presence of cholesterol molecules, which tend to fill the space between fatty acids in the hydrophobic interior of the membrane. By doing this, cholesterol reduces the fluidity of the membrane, limiting the movement of proteins and other components. Additionally, the tight packing of phospholipids in the bilayer contributes to the restraint of protein mobility. The fatty acid composition of the phospholipids can also affect fluidity: more unsaturated fatty acids with kinks tend to increase fluidity and mobility, while saturated fatty acids make the membrane more rigid.