Diffusion

Diffusion: Plant-water relations

Diffusion is the movement of gas, liquid and solute molecules or ions from their region of higher concentration to the region of their lower concentration.

The exchange of carbon dioxide, oxygen and water vapour between leaf and external atmosphere is the process of diffusion. It continues till the differences in concentration of substances disappear.

Diffusion is more rapid when the difference in concentration is larger.  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. For example, 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 the 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.  The diffusion of oxygen molecules depends upon the concentration of oxygen molecules and not the carbon dioxide molecules.  Similarly the diffusion of carbon dioxide is not affected by the concentration of oxygen molecules.

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.

Simple Diffusion:

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.

Facilitated Diffusion:

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.).

Simple and facilitated diffusion both are passive transport mechanism, they do not involve expenditure of energy.  The direction of transport in both the cases is always from the region of higher concentration to the lower concentration.

Diffusion Pressure (D.P.)

Diffusion pressure can be defined as the physical force of a substance by virtue of which its molecules or ions move from one place to another, if other conditions do not resists the movement.

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:

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 larger is the molecules, the slower is the rate of diffusion.

When the surrounding medium of a cell is having pure water, rather than having a solution, diffusion of water molecules is more rapid.  If the concentration of solute inside the cell increases then also the rate of diffusion of water molecules into the cells increases.

This indicates that density of the solvent molecule in the surrounding influences the rate of diffusion.

Temperature:

The rate of diffusion increases with increase in temperature.  This is because free energy increase with rise in temperature and the molecules move towards the region of low temperature.

Sugar crystals do not dissolve easily in ice cold water while they do so easily in warm water.  It is because a rise in temperature increases the rate of diffusion.

Permeability of medium:

Rate of diffusion decreases with density of the medium.  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.

Diffusion pressure gradient (DPG):

It is difference in the concentrations 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.

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.

Size of the molecules:

The rate of diffusion decreases with the increase in the size of the molecule. The smaller the molecules the more is the rate of diffusion. For example the oxygen molecules are heavier  than the hydrogen molecules and therefore, diffusion of the oxygen molecules into the cells is lesser than the hydrogen molecules.

Significance 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.