In addition to the lipid bilayer, the cell membrane also contains a number of proteins. We have
already mentioned the presence of certain proteins in the cell membrane. In this section we will
discuss the different classes of proteins found there. While the lipid bilayer provides the structure for
the cell membrane, membrane proteins allow for many of the interactions that occur between cells.
As we discussed in the previous section, membrane proteins are free to move within the lipid bilayer
as a result of its fluidity. Although this is true for most proteins, they can also be confined to certain
areas of the bilayer with enzymes. Membrane proteins perform various functions, and this diversity is
reflected in the significantly different types of proteins associated with the lipid bilayer.
Classifications of Membrane Proteins
Proteins are generally broken down into the smaller classifications of integral proteins, peripheral
proteins, and lipid-bound proteins.
Integral proteins are embedded within the lipid bilayer. They cannot easily be removed from the cell
membrane without the use of harsh detergents that destroy the lipid bilayer. Integral proteins float
rather freely within the bilayer, much like oceans in the sea. In addition, integral proteins are usually
transmembrane proteins, extending through the lipid bilayer so that one end contacts the interior of
the cell and the other touches the exterior. The stretch of the integral protein within the hydrophobic
interior of the bilayer is also