Final answer:
To increase the fluidity of a phospholipid bilayer, one must remove saturated fats and incorporate unsaturated fatty acids that contain double bonds, which prevent tight packing of the molecules and thus increase fluidity.
Step-by-step explanation:
The situation that would increase the fluidity of a phospholipid bilayer is removing unsaturated fatty acids. Unsaturated fatty acids have double bonds in their tails that introduce bends or 'kinks' which prevent the fatty acids from packing closely together. This spacing allows for more movement among the molecules, thus contributing to a more fluid membrane. The presence of unsaturated fatty acids is crucial for maintaining membrane fluidity, especially in cold environments where rigid saturated fatty acid tails can cause the membrane to become too stiff and potentially rupture.
Cholesterol has a different role. It tends to fill in the spaces between phospholipids in the hydrophobic interior of the membrane, reducing the mobility of phospholipids and thus decreasing fluidity. Therefore, adding cholesterol would not increase the fluidity but rather serve to modulate it, making membranes less permeable to certain ions.