Osmosis helps biological cells maintain their structure. Cell membranes are semipermeable and
allow water, small molecules, and hydrated ions to pass, while blocking the passage of
biopolymers synthesized inside the cell. The difference in concentrations of solutes inside and
outside the cell gives rise to an osmotic pressure, and water passes into the more concentrated
solution in the interior of the cell, carrying small nutrient molecules. The influx of water also keeps
the cell swollen, whereas dehydration causes the cell to shrink. These effects are important in
everyday medical practice. To maintain the integrity of blood cells, solutions that are injected into
the bloodstream for blood transfusions and intravenous feeding must be isotonic with the blood,
meaning that they must have the same osmotic pressure as blood. If the injected solution is too
dilute, or hypotonic, the flow of solvent into the cells, required to equalize the osmotic pressure,
causes the cells to burst and die by a process called hemolysis. If the solution is too concentrated,
or hypertonic, equalization of the osmotic pressure requires flow of solvent out of the cells, which
shrink and die.
Osmosis also forms the basis of dialysis, a common technique for the removal of impurities from
solutions of biological macromolecules. In a dialysis experiment, a solution of macromolecules
containing impurities, such as ions or small molecules (including small proteins or
is placed in a bag made of a material that acts as a semipermeable membrane and the fi lled bag is
immersed in a solvent. The membrane permits the passage of the small ions and molecules but not
the larger macromolecules, so the former migrate through the membrane, leaving the
macromolecules behind. In practice, purification of the sample requires several changes of solvent
to coax most of the impurities out of the dialysis bag.
Osmometry is the determination of molar mass by the measurement of osmotic pressure. The
simple form of van’t Hoff equation is:
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simple form of van't Holl equalion is:
II = [B]RT
In this equation the [B] is the molar concentration of solute. So:
m
= Cg/MA
MAV
[B] =
Where CR the mass concentration of the solute is in the total volume of solution and M, is the
molar mass of the solute. This equation can be replaced in the previous one to get:
RT
II =
M, CB
In this equation molar mass of given solute can be determined from the slope of the II vs Ce plot.
This equation applies only to solutions that are sufficiently dilute to behave as ideal-dilute
solutions. In the case of non-ideal solutions, however, the extended formula is:
II = [B]RT{1+ k. [B] + n. [B]? + ……}
Biological macromolecules dissolve to produce solutions that are far from ideal, but we can still
calculate the osmotic pressure by assuming that the van't Hoff equation is only the first term of a
lengthier expression:
II × [B]RT(1+b. [B])
II
- = RT + BRT. [B]
[B
= RT + bRT. "/M.
TB/MA
RT bRT
. CB
CB
MA' MA
In this equation molar mas
given biomolecule can
determined from the intercept of
vs.
Св plot.
The osmotic pressures of solutions of a protein at 25°C were as follows:
CB (g/L) | 0.5
1.0 1.5 2.0 2.5
| П (Ра)
40.0
110 200 330 490
What is the molar mass of the protein?](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2Fc9542b58-8062-458c-8070-8c4b8ad633b0%2F7177bd77-a5b9-4eea-a265-10ee92f49968%2Fl63wmow_processed.png&w=3840&q=75)
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