What are the functions of cell membranes?

You can find two types of membranes in the cell: the plasma membrane (or cell surface membrane) that is surrounding the whole cell and intracellular membranes that are found within the cell surrounding the organelles. The composition of the membranes is described as fluid mosaic model where phospholipid bilayer makes up the fluid portion and embedded proteins make up the mosaic portion. The functions of these membranes include: a) Creating different biological environments within the cell so different processes can take place at the same time. This function can also be called compartmentalisation. For instance, lysosomes are membrane bound organelles that contain hydrolytic (digestive) enzymes preventing them from damaging the cell and containing an optimum pH for these enzymes; b) Controlling what gets in and out of the cell/organelle as this depends on what channel or carrier proteins are present in the membrane; c) Cell surface membrane contains antigens that allow the immune system to recognize them; d) Membrane might contain receptors that are sites of attachment for hormones and drugs (the process of binding of receptor to a hormone or drug can result in reactions starting at the cell); e) Intracellular membranes can be sites of reaction (e.g. inner membrane of mitochondria)