OSMOSIS AND OSMOTIC PRESSURE:
Cells have limiting boundary membranes that are called plasma
membranes. These membranes not only keep the cell intact but also allow
the exchange of materials back and forth between the interior of the cell
and its exterior surroundings. Dialysis and osmosis are two ways that
such an exchange of materials occurs. Let us consider osmosis first.
Osmosis can be demonstrated by use of the U-tube shown in figure
5.we place a thin membrane made of cellophane at the bottom of the Utube. Pure water is placed in one arm of the U-tube, and an aqueous
solution of glucose is placed in the other arm. We make sure that the
heights of the columns in both arms of the U-tube are equal (Figure -5a).
Several hours later, we find that the height of the column of glucose
solution is greater than the height of the column of pure water (figure 5b).
Fig 5. A demonstration of osmosis. (a) the levels of the water and the
aqueous glucose solution are the same at the beginning of the
experiment. (b) after some time, the level of the golucose solution
is higher than that of the pure water.
For this change to occur, water must have passed through the
membrane. Materials that allow only certain molecules to pass through
are called semipermeable membranes or osmotic membranes. In our
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experiment, cellophane was the semipermable . In cells, the semi
permeable membrane. In cells, the semipermeable membrane is the
plasma membrane mentioned earlier.
We can now define osmosis. Osmosis is the movement of water
through an osmotic membrane from an aqueous soultuion that is less
concentrated to one that is more concentrated. This is a general
phenomenon that occurs whenever an osmotic membrane separates two
solutions of different concentrations.
We can prevent osmosis from occurring by applying pressure to the
right arm of the U-tube in figure -5. If we apply just the right amount of
pressure, we can keep the height; of the columns in both arms equal and
osmosis does not occur. The pressure needed to prevent osmosis is called
the osmotic pressure of a solution. Notice that a high solute concentration
means high osmotic pressure. Water moves from dilute to more
concentrated solutions. The purpose of this movement of water is to make
the concentrations of the solutions equal.
We must look at the structure of the osmotic membrane at the
molecular level to understand osmosis. An osmotic membrane contains
small holes. The size of these holes is an important property, which
determines what kinds of molecules will pass through the membrane.
Molecules larger than the holes will not pass through. The membrane
therefore acts like a molecular sieve. Certain molecules pass through the
membrane, and others do not. This selectivity of the membrane is
responsible for osmosis, as we will learn from the diagram in Figure 6.
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Fig 6. A molecular view of osmosis
Figure 6 shows a molecular view of two aqueous glucose solutions
of different concentrations separated by an osmotic membrane. The more
concentrated solution is in the right compartment. The holes in the
osmotic membrane are large enough that water molecules can pass in
both directions. But the holes are so small that glucose molecules cannot
get through. All the molecules in both solutions are in continual motion.
As a result, of this motion, water molecules reach the membrane and
collide with it. A water molecule that happens to find a hole in the
membrane passes through it. The amount of water in the concentrated
solution is less than that in the dilute solution, so the number of water
molecules that collide with the membrane is smaller. As a result more
water molecules pass through the membrane from the dilute glucose
solution to the more concentrated glucose solution. The result is a net
movement of water into the more concentrated glucose solution. This is
visible as an increase in its volume.
We can see in figure 6 that water moves to the solution that has the
greater number of dissolved particles (the more concentrated solution).
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This solution also has the higher osmotic pressure. We can conclude that
the greater the number of particles, whether ions or molecules, in a
solution, the greater its osmotic pressure. Any property of a solution that
depends on the number of dissolved particles in the solvent is called a
colligative property.
We can easily show that osmotic pressure is a colligative property.
For example, if we measured the osmotic pressure of a 1 M aqueous
sodium chloride solution, we find that it is exactly twice that of a 1 M
aqueous glucose solution. The reason for this difference in osmotic
pressure is that sodium chloride is an electrolyte, whereas glucose is a
non electrolyte. An aqueous solution containing 1 mole of sodium
chloride actually contains 1 mole of sodium ions and 1 mole of chloride
ions. A 1 M solution sodium chloride contains twice as many particles as
an equal volume of a 1 M solution of glucose, a non electrolyte. As a
result, its osmotic pressure is exactly twice that of a 1 M glucose solution.
The relative osmotic pressures of two solutions are extremely important
in living systems. In fact, they are so important that special terms have
been given to describe their relative osmotic pressure. Two solutions that
have the same osmotic pressure are said to be isotonic. If one solution has
a higher osmotic pressure than the other, it is said to be hypertonic with
respect to the other. One of two solutions that has the lower osmotic
pressure is said to be. Hypotonic compared to the other. Examples of each
of these terms are given in table 5.
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Table 5.
Examples of terms Describing Relative
Osmotic pressure of two solutions
1. Isotonic
A 1 M glucose solution and a 1 M urea (a non electrolyte) solution are
isotonic.
2. Hypertonic
A 1 M NaCl solution has a higher osmotic pressure than a 1 M glucose
solution. Therefore, it is hypertonic compared to a 1 M glucose solution.
3. Hypotonic
A 1 M NaCl solution has a lower osmotic pressur than a 2 M LiBr
solution. Therefore, it is hypotonic compared to a 2 M LiBr solution.
The plasma membranes of red blood cells behave as osmotic
membranes. The cells contain an aqueous fluid made up of dissolved
compounds. This fluid has an aasomtic pressure determined by the
concentration of dissolved molecules and ions in the fluid. Osmosis
occurs when a red blood cell is placed in water. The solution inside the
cell is hypertonic compared to pure water, so water enters the cell. So
much water enters that the cell is ruptured. The rupture of red blood cells
in this way is called hemolysis. We say that the cells are hemolysed.
Osmosis also occurs when a red blood cell is placed in a concentrated
saline (sodium chloride) solution. But in this case, the solution inside the
cell is hypotonic compared to the saline solution and osmosis occurs in
the reverse direction. Water leaves the cell and passes into the solution.
This causes the red blood cell to shrivel and shrink. This process is called
crenation.
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A 0.95% saline solution is isotonic compared to the solution inside
red blood cells. Consequently, red blood cells placed in such a solution
undergo neither crenation nor hemolysis.
There is very important practical reason for worrying about the
osmotic pressure of the fluid inside a red blood cell compared to that of
the cell's environment. Patients often must be fed intravenously. To
prevent damage to their red blood cells, the concentration of the solution
must be controlled so that neither hemolysis nor crenation occurs.
Therefore, the concentration of the solution must match closely the
concentration of all of the particles within the red blood cells. In other
words, the solution to be given a patient intravenously must be isotonic
with blood.
The fluids in living systems carry not only dissolved ions and
molecules, but also larger particles called colloids.
COLLOIDS AND COLLOIDAL DISPERSIONS:
We learned that solutions are homogenous mixture of solute and
solvent molecules. The particles in a solution are the size of atoms and
molecules. That is, their sizes range from 0.05 to 0.25 nm (1nm = 10 -9m).
Sometimes intermolecular attractions between molecules cause several
hundred to several thousand of them to cluster together. The sizes of
these clusters range from 1 to 100 nm. Matter containing particles of this
size is called a colloid.
A uniform dispersion of a colloid in water is called a colloidal
dispersion. This dispersion is a similar to a solution in that the particles
do not settle out on standing. However, a colloidal dispersion usually
appears cloudy, and its particles are large enough to be photographed
with the aid of an electron microscope. The colloid in a colloidal
dispersion is called the dispersed substance. The continuous matter in
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which the colloid is dispersed called the dispersing substance many
compounds of higher molecular weight in living systems from colloidal
dispersions rather than solutions in water. Starch and proteins are
examples of such compounds.
Is colloids are clusters of molecules. Why don’t the clusters increase
in size until they get large enough to settle out? The reason is that the
particles in the most stable colloidal dispersions all have the same
electrical charge. These charges can be caused by adsorption of ions to
the surface of the particles, or the large particles themselves can be
charged. As a result, the particles repel each other and cannot from
particles large enough to settle out. This repulsion between colloids in
water is shown in figure 7.
Fig 7. Colloids formed by attractions between complex molecules. One
end of each individual molecule has a negative charge (balanced by
a sodium ion), and the other end is a long nonpolar tail. The long
tails are held toghter by hydrophobic attractions. The negatively
charged ends form the surface of sphere. Adjacent colloids are
replled by their identical charges.
Other colloids are stabilized in water by the action of a third
substance called an emulsifying agent. An example is a mixture of oil and
water. Oil is immiscible with water. However, if we add soap to the
mixture, the oil is emulsified by the soap. The soap is the emulsifying
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agent. The soap breaks up the oil into small drops. The soap molecules
from a negatively charged layer on the surface of each oil drop. This
causes the oil drops to repel each other, and they disperse throughout the
water. Bile salts are another example of an emulsifying agent. These salts
break up the fats we eat into small globules that can be more effectively
digested. The fluids of living systems are a complex mixture of colloids
and dissolved ions and molecules. The behavior of these fluids in the
body is vital to life. A particularly important property of these fluids is
dialysis.
DIALYSIS AND LIVING SYSTEMS:
We learned that an osmotic membrane allows water molecules, but
not solute particles, to pass through. Cell membranes must be able to do
more than this, because cells need to take in nutrients and discharge waste
products. Membranes that allow small molecules and ions to pass while
holding back large molecules and colloidal particles are called dialyzing
membranes. Plasma membranes are examples of such membranes. The
selective passage of small molecules and ions in either direction by a
dialyzing membrane is called dialysis. Dialysis deffers from osmosis in
that osmotic membranes allow only solvent molecules to pass.
The process of dialysis is shown by the figure 8. the apparatus
consists of a bag made of a dialyzing membrane such as an animal
bladder. The bag contains a mixture of colloids and dissolved molecules
and ions. The bag is placed in a container of pure water and water is
continually passed through the membrane. The water carries the ions and
molecules through the membrane, leaving the colloids behind. The ability
of dialyzing membranes to allow the passage of only selected substances
is extremely important to living systems.
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Fig 8. Dialysis apparatus.Dissolved moluces and ions pass through the
dialyzing membrane, but colloids do not
The kidneys are an example of organs in the body that use dialysis to
maintain the solute and electrolyte balance of the blood. The main
purpose of the kidneys is to cleanse the blood by removing the waste
products of metabolism and control the concentrations of electrolytes.
The kidneys do this job very efficiently. Approximately 180 L of blood
are purified daily in a 68-kg (150-lb) adult. Approximately 99 percent of
the total volume processed is retained, and the remaining 1 percent is
eliminated as urine. Part of the purification of blood occurs by dialysis.
In recent years, kidney machines have been built that purify the blood
of patients with kidney failure. Each machine contains a series of tubes
that are, in effect, dialyzing membranes. These tubes are chosen so that as
the blood flows through, waste products pass through the membrane, but
the larger cells and other molecules needed by the body do not. The
machine thus removes many of the waste products of the blood and
allows the patient to live a relatively normal life.
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