Movement of Molecules Across Cell Membranes
entered in the ﬁ rst place; that is, extracellular sodium behaves
as a nonpenetrating solute. Any chloride ions that enter cells
are also removed as quickly as they enter, due to the electrical
repulsion generated by the membrane potential and the action
of secondary transporters. Like sodium, therefore, extracellu-
lar chloride ions behave as if they were nonpenetrating solutes.
Inside the cell, the major solute particles are potassium
ions and a number of organic solutes. Most of the latter are
large polar molecules unable to diffuse through the plasma
membrane. Although potassium ions can diffuse out of a cell
through potassium channels, they are actively transported
back by the Na
-ATPase pump. The net effect, as with
extracellular sodium and chloride, is that potassium behaves
as if it were a nonpenetrating solute, but in this case one con-
ﬁ ned to the intracellular ﬂ uid. Thus, sodium and chloride
outside the cell and potassium and organic solutes inside the
cell behave as nonpenetrating solutes on the two sides of the
The osmolarity of the extracellular ﬂ uid is normally in
the range of 285–300 mOsm (we will assume a value of 300
for the rest of this chapter). Because water can diffuse across
plasma, water in the intracellular and extracellular ﬂ
come to diffusion equilibrium. At equilibrium, therefore, the
osmolarities of the intracellular and extracellular ﬂ uids are the
same—approximately 300 mOsm. Changes in extracellular
osmolarity can cause cells, such as the red blood cells shown
in the chapter opening photo, to shrink or swell as water mol-
ecules move across the plasma membrane.
If cells with an intracellular osmolarity of 300 mOsm
are placed in a solution of nonpenetrating solutes having an
osmolarity of 300 mOsm, they will neither swell nor shrink
because the water concentrations in the intra- and extracel-
lular ﬂ uid are the same, and the solutes cannot leave or enter.
Such solutions are said to be
ing any solution that does not cause a change in cell size.
Such solutions have the same concentration of
solutes as normal extracellular ﬂ
uid. By contrast,
solutions have a nonpenetrating solute concentration lower
than that found in cells, and therefore water moves by osmo-
sis into the cells, causing them to swell. Similarly, solutions
containing greater than 300 mOsm of nonpenetrating solutes
solutions) cause cells to shrink as water diffuses
out of the cell into the ﬂ uid with the lower water concentra-
tion. Note that the concentration of
a solution, not the total osmolarity, determines its tonicity—
isotonic, hypotonic, or hypertonic. Penetrating solutes do not
contribute to the tonicity of a solution.
Another set of terms—
—denotes the osmolarity of a solution relative
to that of normal extracellular ﬂ uid without regard to whether
the solute is penetrating or nonpenetrating. The two sets of
terms are therefore not synonymous. For example, a 1-L solu-
tion containing 300 mOsmol of nonpenetrating NaCl and 100
mOsmol of urea, which can rapidly cross plasma membranes,
would have a total osmolarity of 400 mOsm and would be
hyperosmotic. It would, however, also be an isotonic solu-
tion, producing no change in the equilibrium volume of cells
immersed in it.
cells placed in this solution would
shrink as water moved into the extracellular ﬂ
urea would quickly diffuse into the cells and reach the same
concentration as the urea in the extracellular solution, and
thus both the intracellular and extracellular solutions would
soon reach the same osmolarity. Therefore, at equilibrium,
there would be no difference in the water concentration across
the membrane and thus no change in ﬁ nal cell volume, even
though the extracellular ﬂ uid would remain hyperosmotic.
Changes in cell volume produced by hypertonic,
isotonic, and hypotonic solutions.
Blood volume must be restored in a person
who has lost large amounts of blood due to
serious injury. This is often accomplished
by infusing isotonic NaCl solution into the
blood. Why is this better than infusing an
isoosmotic solution of a penetrating solute,
such as urea?
Answer can be found at end of chapter.
Intracellular fluid 300 mOsm
No change in cell volume
Normal cell volume