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

energy

Diffusion

• It is a process, which involves movement of a substance from a region of its higher concentration to a region of its lower concentration.

• 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 membranes.

• 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.

Osmosis

• 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 hypotonic.

• 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 experiment.

• 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.

Demonstration of Osmosis

Examples of Osmosis

• Example: 1Cell 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 drinking water.

• 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 polymers.

• Reverse osmosis finds use in water purification

Tonicity

• Tonicity is a measurement gradient used to measure or compare osmotic pressures of two solutions separated by a semipermeable membrane.

• Accordingly, there are three types of tonic measurements

• Isotonic SolutionsSolutions 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 another.

• 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 solutions.

• 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 each other.

• 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 Diffusion

• 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 Diffusion

• In osmosis, only solvent particles move. The solvent particles move from higher concentration to lower concentration. 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

Osmosis and Diffusion Venn Diagram

Examples of osmosis n diffusion

• Osmosis• 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 of osmosis.

• 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.

• Diffusion• 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 concentration.

• 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.