The Physical Processes
The physical processes involving
movement of materials in and out of a
cell are diffusion and osmosis. Both
these movements involve movement
along the concentration gradient.
Hence, there is no expenditure of
• It is a process, which involves movement of a
substance from a region of its higher
concentration to a region of its lower
• Molecules of any substance are in constant
random movement in all fluids. This movement is
called Brownian movement.
• Apart from the three states of matter, diffusion
can also occur through semipermeable
• The molecules of the substance undergoing
diffusion exert a pressure in the available space.
This pressure is known as diffusion pressure.
Higher the diffusion pressure, higher is the rate of
diffusion. The rate of diffusion is decided by
factors like concentration of the molecules
undergoing diffusion, space available for diffusion
and temperature of the medium.
• It is a process involving net diffusion of water
molecules from a region of higher water
concentration to a region of lower water
concentration through a semipermeable
membrane. The osmotic movement of water
from a surrounding medium into a cell is called
endosmosis. Such a medium is described as
• The diffusion pressure exerted by water
molecules on the semipermeable membrane is
called as osmotic pressure
Physiological Experiment to demonstrate Osmosis
• Osmosis in a cell can be demonstrated by a potato osmoscope
• Peel the skin of a potato and remove the inner contents to form a
cup shaped hollow. Place the potato in a container with water. Pour
5% sugar solution in to the potato cup up to a particular level.
Pierce a pin to mark the initial level. Leave the experimental set up
undisturbed for about 30 minutes.At the end of the duration check
the level of sugar solution in the potato cup. You will be able to
record an increase in level caused by osmotic movement of water.
• You can repeat the experiment by reversing the situation. Take
sugar solution in the container and water in the potato cup. Mark
the initial level. At the end of 30 minutes note the difference. This
time you will be able to record a decrease in the level of water
inside the potato cup.
Demonstration of Osmosis
• A thistle funnel, covered at the broad end by a
differentially permeable membrane, contains
a 10% sugar solution. The beaker contains a
5% sugar solution as shown in fig. a. The
solute is unable to pass through the
membrane, but the water passes freely
through in both directions. The net movement
of water towards the inside of the thistle
funnel occurs because the thistle funnel has a
lower water concentration.
Examples of Osmosis
• Example: 1
Cell membranes act as semipermeable
membranes. For example, if a cell is placed in
pure water it will swell. The cytoplasm inside the
cell contains dissolved ions and some of the
water outside the cell passes through the
membrane owing to osmosis.
But, if the cell is placed in a concentrated solution
of salt, the cell shrivels. This time the water
passes out of the cell into the more concentrated
solution around it
• Example: 2
When a camel takes in large quantity of water,
its blood cells swell owing to water molecules
passing into the cells. After many days without
water, the cells shrink as water passes out of
the cells into the blood stream.
Importance of Osmosis
• Osmosis is a very important process in all
biological systems as well as in purification of
• Osmosis process is also used to desalinate ocean
water for its usage in industrial applications.
• Osmosis has also been of great use in
determining molar masses, especially those of
• Reverse osmosis finds use in water purification
• Tonicity is a measurement gradient used to
measure or compare osmotic pressures of two
solutions separated by a semipermeable
• Accordingly, there are three types of tonic
• Isotonic Solutions
Solutions which have the same osmotic
pressure as the same temperature are said to
be isotonic. When two solutions having the
same osmotic pressure are put into
communication with each other through a
semipermeable membrane, there will be no
transference of solvent from one solution to
• Hypertonic Solutions
If one solution has a higher osmotic pressure than the
other, and are connected together by a semipermeable
membrane, such solutions are called as hypertonic
solutions. Usually, this is used in connection to the fluids
inside and outside red blood cells.
If the fluid inside the red blood cell has lower osmotic
pressure and that outside the red blood cells have higher
osmotic pressure, such solutions are said to be hypertonic
• Hypotonic Solutions
If one solution has a lower osmotic pressure
than the other, the solution is hypotonic.
Is Osmosis a Type of Diffusion?
• In diffusion, the solute molecules, and the solvent
molecules move to equalize the concentration.
• diffusion can occur when the solutions are in contact with
• osmosis, we see the movement of solvent particles, though
not the solute particles.
But, they again move through a semipermeable membrane,
again to achieve an equilibrium concentration. So, we can
safely say that, though there is a difference, osmosis can be
called a type of diffusion.
Osmosis is a type of diffusion, because here too, solvent
move and an equilibrium concentration is reached.
Compare and Contrast Osmosis and
• Osmosis and diffusion are similar in many
respect. Both the phenomenon leads to
equalizing the concentration of two solutions,
in contact with each other. Thus, osmosis is a
type of diffusion.
Difference Between Osmosis and
• In osmosis, only solvent particles move.
The solvent particles move from higher concentration to
Also, the movement of solvent particles takes place with
the help of a semipermeable membrane, which does not
allow the solute to pass through.
In diffusion, both solute and solvent molecules move.
They strive to achieve an equal distribution of solute and
solvent molecules throughout.
Diffusion does not involve a semipermeable membrane.
Though diffusion also involves movement particles from
area of high concentration to lower concentration, it
involves a two way process
Examples of osmosis n diffusion
• We know that plants need both sunlight and water to
grow. When we water the plants, the roots absorb
water. This absorption is only due to the phenomenon
• The plant roots are semipermeable in nature. When we
water the plants, it tries to withdraw water from the
soil. This is only possible, if the water inside the roots
are lesser than the water in the soil. Thus, through
osmosis, plants absorb water from the soil.
• There are many examples of diffusion :
• When we add sugar inside water, and do not stir it and
keep it aside, after sometime, we can see that the
sugar particles from the bottom of the glass are trying
to spread towards the top, to equalize the
• Diffusion is spreading evenly. SO, when a gas escapes
the chimney of a building, it is visible. But after a
certain distance, the gas dissipates. This is due to the
fact that the gaseous particles get diffused with the air
and spreads out.