I believe that the protein in a plasma membrane can determine many functions that a membrane can perform.
According to http://en.wikipedia.org/wiki/Cell_Membrane, "Specific proteins embedded in the cell membrane can act as molecular signals that allow cells to communicate with each other." Apparently protein receptors receive signals from other cells and from the environment in which they're contained. These proteins help identify the cell to other cells. This is very important in maintaining good "communication" between cells forming our immune systems. Otherwise, a good cell might be destroyed along with disease-causing cells.
Another important function of the cell membrane proteins is to provide a skeletal framework for other cells to attach to, thus forming tissue. They also give form and structure to the cell itself.
An additional function of the cell membrane proteins is to form finger-like projections called villi or organelles that increase the cell surface making its ability for absorption that much greater.
Other functions of the membrane proteins can also include regulating enzymatic activity within the cell itself, and transporting substances into and out of the cell across the membrane. This is accomplished mainly by ionic charges in the lipid layers directly underneath the cell membrane. Only substances with the right electrical charge are allowed to pass through the membrane into the cell. Once inside, substances have a harder time getting back out of the cell.
The work done on a microscopic level by these little "machines" is nothing short of a miracle!
each protein in a membrane has a specific function that it performs. the carrier proteins undergo facilitated diffusion and active transport that gives the membrane. also, the channel proteins allow the polar molecules (that are unable to pass through the hydrophobic tails of the membrane bilayer) to pass in and out of the cell. hence both of these types of protein give the membrane its selective permeability. also, some membrane proteins act as enzymes for speeding up the chemical reactions. similarly the glycoproteins enables cell recognition and act as hormonal receptors of the cell.