Cells, whether it is animal, plant or bacteria, have many different parts or organelles. Each organelle has a different and important function within the cell. If any of the parts fail then the cell ceases to function and it begins to die.
One of the most important parts of a cell is called the plasma membrane (or cell membrane). The cell membrane has many important functions including, cell to cell recognition, regulation of what goes into and out of the cell, and cell to cell communication. The unique chemistry of the plasma membrane allows it to perform these functions.
The plasma membrane is composed of a phospholipid bilayer that has proteins and carbohydrates embedded in it. A typical lipid or triglyceride is composed of three fatty acids chemically bonded to a glycerol molecule. Regular lipids are insoluble in water. An example of this insolubility can be seen in salad dressing such as Italian salad dressing in which the layers separate when allowed to settle. One layer is the water layer and the other the oil or lipid layer. The lipids found in the lipid bilayer of the plasma membrane have exchanged one of the fatty acid molecules for a phosphate molecule. This changes the chemistry of the lipid molecule giving it a polar or charged "head" and a "nonpolar" or uncharged "tail".
The lipid bilayer is composed of two layers of these phospholipids with the heads pointing "out." This means that one polar head is in contact with the exterior of the cell and one polar head is in contact with the interior of the cell. And both the exterior and interior environments are water based environments. The tails or the unpolar portions of the phospholipids are sequestered together away from the water environment. If you place phosopholipids into a water based environment, they will spontaneously form the bilayer described above.
Embedded in the plasma membrane are proteins which can have various functions including: being marker molecules, attachment proteins, transport proteins, receptor proteins or enzymes. This protein embedded lipid model is called the fluid mosaic model because the proteins are not stationary within the lipid membrane.
Marker molecule proteins help the cells help the cells recognize each other. For example, marker molecules allow the immune system to tell the difference between self-cells and invading cells or bodies. Attachment proteins can attach the cell to other cells or to other extracellular molecules. Transport proteins allow molecules that are too big or that have a charge to enter into the membrane since only small and uncharged molecules can get through the membrane without the aid of transport proteins. This is because of the hydrophilic and hydrophobic nature of the membrane. Receptor proteins function in cell to cell communication. They accept or recognize the incoming chemical signal from other cells. And the enzymes in the membrane function as any other enzymes in that they lower the activation energy for chemical reactions.
There can also be carbohydrate molecules attached to the protein or lipid molecules of the plasma membrane. These carbohydrates help in the cell recognition and cell to cell signaling functions. There are also cholesterol molecules in the membrane which determine the degree of fluidity of the membrane.
It is the above mentioned proteins, lipids and carbohydrates that give the membrane its chemical properties and therefore determine the function of the plasma membrane which is the regulation of what goes into and out of the cell.