The phospholipids can move to the opposite side of a bacterial cell membrane in a few minutes at room temperature. That's is a distance several thousand times the size of the phospholipid. To do so required the hydrophilic head of the phospholipid to pass fully through the highly hydrophobic interior of the membrane, and for the hydrophobic tails to be exposed to the aqueous environment. There is a good diagram of the fluid mosaic model of a biological membrane in The WWW Cell Biology Course . Notice that there are molecules of cholesterol embedded in the membrane.
Cholesterol is a necessary component of biological membranes.
Cholesterol breaks up the Van der Waals interactions and close packing of the phospholipid tails. This disruption makes the membrane more fluid. Therefore, one way for a cell to control the fluidity of its membrane is by regulating its level of cholesterol in the cell membrane. Another way for the cell to control the fluidity of its membrane is to regulate the ratio of saturated to unsaturated hydrocarbon chains of the phospholipids. A group of phospholipids with saturated hydrocarbon chains can pack close together and form numerous Van der Waals bonds that hold the phospholipids to each other.