Define diffusion. Mention the characters, significance and the factors affecting the rate of diffusion. Differentiate between diffusion and osmosis.

DIFFUSION

Diffusion is the movement of molecules or ions of a solute or solvent from the region of higher concentration to that of its lower concentration.  The exchange of carbon dioxide, oxygen and water vapour between leave and external atmosphere is the process of diffusion.  Gases diffuse more rapidly than liquids.  Solids are the slowest to diffuse.

CHARACTERISTICS OF DIFFUSION

The following are the important characteristics of diffusion:

1. The molecules or ions diffuse from region of their higher concentration to region of their lower concentration (e.g. if a bottle of perfume is opened in one corner of a room, soon the odour of the perfume is felt throughout the room. Similarly, if a crystal of copper sulphate is placed in a beaker containing water, an intense blue colour is seen around the crystal).
2. The diffusing molecules move randomly towards all the region of their lower concentration. This continues till the molecules are evenly distributed in the space available.  The movement of the molecules is due to their kinetic energy.
3. The direction of diffusion of one substance is independent of movement of another substance. Two gases can diffuse in the same or opposite directions at the same time independently without affecting the movement of another.  The diffusion of carbon dioxide and oxygen through the stomata takes place simultaneously in opposite direction.
4. The rate of diffusion of molecules is proportional to their kinetic energy, their size, the density of medium through which they move and the gradient of concentration over which they diffuse.

 

Simple and facilitated diffusion:  Diffusion of molecules or ions through a membrane is of two types – simple and facilitated.  The simple diffusion refers to the type of transfer in which the diffusing molecules or ions do not combine with the constituents of the membrane.  For example, gases like oxygen, carbon dioxide etc., and water molecules diffuse readily through gas created by the random movement of fatty acyl chain of lipids.  The facilitated diffusion is the movement through the membrane with the help of certain transport proteins.  Membranes have several such proteins which facilitate the diffusion of solutes (Cl^–, HCO3^— etc.)  Plasma membranes of some organisms have protein pores called, porins which are water-filled transmembrane channels.

 

DIFFUSION PRESSURE (D.P.)

The pressure exerted by the tendency of the particles to diffuse from the area of its higher concentration to the region of its lower concentration is called, diffusion pressure.  The diffusion pressure is directly proportional to the concentration of diffusing particles.  Therefore, greater the concentration of diffusing molecules in a system, their diffusion will be greater.  Hence, pure water would have more diffusion pressure than a sugar solution.

 

FACTORS AFFECTING THE RATE OF DIFFUSION

The factors affecting the rate of diffusion are as follows:

1. Density: The rate of diffusion of a substance is inversely proportional to the square root of its relative density (Graham’s law of diffusion), the large is the molecules, the slower is the rate of diffusion.
2. Temperature: The rate of diffusion increases with increase in temperature.  This is because free energy increases with rise in temperature and the molecules move towards the region of low temperature.
3. Permeability of medium: Rate of diffusion decreases with density of the medium.  The rate of diffusion would be slower if the medium is concentrated, i.e., increase in the number of foreign molecules causes the rate of diffusion to decrease.  Thus, a gas would diffuse more rapidly in vacuum than in air.
4. Diffusion pressure gradient (DPG): It is difference in the concentration of the diffusing molecules between one area and another over a specific distance.  The steeper is the diffusion pressure gradient, the faster is the rate of diffusion.  Thus, the rate of diffusion is directly proportional to the difference of diffusion pressure at the two ends of a system and inversely proportional to the distance between the two.
5. Solubility of solutes: Diffusion of solute molecules in a particular solvent depends upon their solubility in that solvent.  The greater is the solubility of the molecule in a solvent, the higher would be the rate of diffusion.

 

Importance of diffusion

1. Exchange of gases such as carbon dioxide and oxygen between the plant and outside air occurs through diffusion.
2. Transpiration or loss of water in vapour form is a diffusion process.
3. Osmosis is a special type of diffusion in which water diffuses through a semipermeable membrane.
4. It is a means of spreading of ions and other substances throughout the protoplast.
5. Diffusion keeps the cell wall of the internal plant tissues moist.
6. Aroma of flowers is due to diffusion which helps to attract pollinating animals.

 

DIFFERENCE BETWEEN DIFFUSION AND OSMOSIS

DIFFUSION	OSMOSIS
1.  Diffusion is the movement of molecules or ions of a solute or solvent from the region of higher concentration to that of its lower concentration.	1.  It is the movement of solvent (water) from its higher concentration to its lower concentration when the two are separated by a semipermeable membrane.
2.  Diffusion occurs in solids, liquids and gases.	2.  Osmosis occurs only in solvent (water)..
3.  Diffusion does not require semipermeable membrane.	3.  The process of osmosis takes place through a semipermeable membrane.
4.  It is only dependent on the free energy of the diffusing substance.	4.  Osmosis is dependent upon the degree of free energy of one solvent over that of another.
5.  Diffusion helps in equalizing the concentration of the diffusing substance throughout the available space.	5.  It does not equalize the concentration of solvent on the two sides of the system.
6.  Diffusion of a substance is largely independent of the presence of other substances.	6.  Osmosis is dependent upon the number of particles of other substances dissolved in liquid.
7.  Hydrostatic or turgor pressure does not normally operate in diffusion.	7.  Osmosis is opposed by turgor or hydrostatic pressure of the system.
8.  Diffusion is not influenced by solute potential.	8.  Osmosis is dependent upon solute potential.