Proteins are the functional part of the membrane allowing for the transport of materials through the membrane and sending and receiving signals to and from other cells. Carbohydrates serve to modify the lipid and protein molecules, with these modified molecules being called glycoproteins and glycolipids. Some cells have a glycocalyx, which is a carbohydrate-rich surface on the outside the membrane. Different types of membranes have different compositions. The lipids in cell membranes act as a barrier between the aqueous solutions inside and outide of the cell.
Membranes can also separate the contents of an organelle from the cell cytoplasm.
For example, the nuclear membrane keeps the DNA inside of the nucleus. The lipid molecules comprising a cell membrane are called phospholipids. Phospholipids are made up of fatty acids, glycerol, phosphate, and a hydrophilic organic derivative. The lipid bilayer Phospholipids in cell membranes are in the form of lipid bilayers. These bilayers form due primarily to hydrophobic forces.