They let some things in and they let some things out but not everything. For example glucose can only pass through the membrane by means of a carrier molecule, such as insulin. Diebetics who either do not produce any insulin, do not produce enough and or have developed an insulin resistance are unable to get the glucose into their cells and tend to get very high levels of glucose on their blood stream. The plasma membrane is made up of a protein phospholipid bilayer. Which typically requires either a lipid carrier or a protein carrier molecule to allow permeability. In the above example glucose is normally carried into the cells by the protein molecule insulin. This is known as carrier protein diffusion.