Fluid mosaic theory of cell membrane structure:

The cell membrane can be visualised as a double skin of phospholipid molecules trapped between two bodies of water, the cytoplasm and the extracellular fluid. The molecules in this layer are not fixed in position but can float and move around. Trapped in this sea of phospholipids are protein molecules. There are two main kinds of trapped proteins, the integral (or transmembrane) proteins and the surface proteins. The proteins of the cell membrane are vital to the survival of the cell. The integral proteins act as channels enabling water molecules and water soluble molecules to pass into or out of the cytoplasm. It would only be possible for molecules that were both fat and water soluble to pass through the phospholipid bilayer itself. By altering the activity of these ion channels it is possible to maintain control over the entry and exit of different substances.

The surface proteins often act as receptors for chemicals that regulate cell activity, such as hormones. The shape of the surface protein is designed to lock onto the particular transmitter molecule, which is often a simple protein itself. Having locked onto the receptor the transmitter will cause the protein to activate a mechanism either within the cell or by opening an integral protein pore.