Membrane Proteins
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
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 hydrophobic,
made up of non-polar amino acids. Like the lipid bilayer, the exposed ends of
the integral protein are hydrophilic.
When a protein crosses the
lipid bilayer it adopts an alpha-helical configuration. Transmembrane proteins
can either cross the lipid bilayer one or multiple times. The former are
referred to as single-pass proteins and the later as multi-pass proteins. As a
result of their structure, transmembrane proteins are the only class of
proteins that can perform functions both inside and outside of the cell.
Peripheral Proteins
Peripheral proteins are
attached to the exterior of the lipid bilayer. They are easily separable from
the lipid bilayer, able to be removed without harming the bilayer in any way.
Peripheral proteins are less mobile within the lipid bilayer.
Lipid-Bound Proteins
Lipid-bound proteins are
located entirely within the boundaries of the lipid bilayer.
The Cell Surface
The protein and lipid
cell membrane is covered with a layer of carbohydrate chains on its outer
surface. This layer is called a cell coat or glycocalyx. The exact composition
and distribution of these chains is very diverse. The chains are thought to provide
the cell with protection against damage. Glycocalyx are only found on the
surface of the cells of higher organism's.
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