In eukaryotic cells, membranes also serve as partitions dividing cells into compartments. In varying ratio. The phospholipids establish the physical integrity of the membrane and serve as a barrier to passage of hydrophilic molecules. Cholesterol causes more rigidity at membrane face, less rigidity in membrane core. Generally tends to increase fluidity.
Biological membranes are as fluid as salad oil.
See Figure 8.6 for an example of a transmembrane protein. The inherent asymmetry of proteins contributes to asymmetry in the 2 faces of the membrane. A lipid lake in which proteins float . For this reason, diffusion over short distances is rather fast, but over long distances, is very long. Neither lipids nor proteins readily flip-flop in the membrane, so that the hydrophilic head groups or protein regions pass through the hydrophobic membrane lipid core.