The lipid bilayer is not miscible with either the extracellular
fluid or the intracellular fluid. Therefore, it constitutes
a barrier against movement of water molecules
and water-soluble substances between the extracellular
and intracellular fluid compartments. However, as demonstrated
in Figure 4-2 by the leftmost arrow, a few substances
can penetrate this lipid bilayer, diffusing directly
through the lipid substance itself; this is true mainly of
lipid-soluble substances, as described later.
The protein molecules in the membrane have entirely
different properties for transporting substances. Their
molecular structures interrupt the continuity of the
lipid bilayer, constituting an alternative pathway through
the cell membrane. Most of these penetrating proteins,