Membranes are of crucial importance to life, because a cell must separate itself from the outside environment for two major reasons.
However, while the cell must always abide by these two principles, it must also communicate with the environment to continuously monitor the external conditions and adapt to them. For example, if an E. coli bacterium detects a high concentration of lactose in the surrounding environment it begins synthesizing proteins to take in and metabolize the lactose. But, if the E. coli also detects a high concentration of glucose in the environment it instead begins synthesizing proteins to take in and metabolize the glucose. It also needs to pump in nutrients and release toxic products of its metabolism. How does the E. coli know? It depends on its membrane proteins to gather information about the environment in various ways.
Cells within the complex tissue of a multi-cellular organism also need to exchange metabolites and waste products with the outside. But they require information that is far more complex than that demanded by a protozoan cell. Each of the cells in our tissues communicates with dozens if not hundreds of other types of cells about a variety of important issues -- when it should grow or differentiate or die, when it should release certain protein products or metabolites needed by other cells at distant sites in the body, and what other cells it should associate with in order to build complex tissue architectures. Ironically, the intrinsic structure of cells creates a formidable obstacle to these important processes of intercellular communication. The cell shields itself behind its membrane, which is relatively impermeable to most types of molecules. This barrier enables the cell to create its own intracellular environment that is distinct and very different from the world around it. But this arrangement also creates a problem, in that it prevents the free interchange of materials between inside and outside. This barrier must be breached somehow in order for a cell to inform itself on what is happening in the world outside.
Therefore, cells have two major types of membrane proteins :
Some proteins serve both of these functions simultaneously, but most proteins are specialized to do one or the other.
The cell has therefore evolved an array of proteins to cope with these problems. Some are embedded in the lipid bilayer and even extend through it. They are called intrinsic membrane proteins; yet others are adsorbed to the surface of the lipid bilayer and are termed extrinsic membrane proteins in that they can usually be readily separated from the lipid bilayer without destroying the bilayer itself.