DIFFUSION

DIFFUSION:

DEFINITION:

“The tendency on the part of molecules, atoms, ions, etc., of gases, liquids and solids to get evenly distributed throughout the available space on account of their ran­dom kinetic motion is called diffusion.”

Random kinetic motion of particles is due to kinetic energy present in them.

Diffusion is also defined as the movement of the particles of different substances from the region of their higher concentration, free energy or diffusion pressure to region of their low concentration, free energy or diffusion pressure.

Diffusion is dependent upon the number of particles per unit volume, density of medium, distance through which diffusion is to occur, temperature and pressure.

Diffusion will be more rapid when the difference in concentration is larger. Gases diffuse more rapidly than liquids. Solids are the slowest to diffuse. When the particles of diffusing substance get evenly distributed, a state of equilibrium is reached. It stops further movement.

Diffusion of particles of one substance is independent of the diffusion of particles of another substance, provided the two do not react. It is known as independent diffusion. The diffusion pressure of the individual substance is then known as partial pressure.

Thus in photosynthesizing leaf, water vapours and oxygen diffuse out while carbon dioxide enters the leaf depending upon the differences in their partial pressures in the leaf interior and the outside. Diffusion is also responsible for uptake and distribution of water and solutes.

(i) Open a bottle of perfume in one comer of room. Soon odour of the perfume will be felt throughout the room,

(ii) Place a crystal of copper sulphate in a beaker containing water.

An intense blue colour will be seen around the crystal. It decreases with the increase of distance from the crystal. It shows that copper sulphate molecules are diffusing into water (Fig. 11.3). Water molecules will also move towards copper sulphate crystal in order to occupy that space. Ultimately the molecules of the two, water and copper sulphate, will be evenly distributed throughout the solution.

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. It is proportional to the concentration of diffusing particles. A porous pot is fitted to a glass tube, one end of which dips in water.

A gas jar filled with hydrogen is inverted over the porous pot. Hydrogen has the ability to diffuse more rapidly into porous pot than the ability of air to diffuse out of the porous pot. This develops a pressure inside the porous pot which pushes the air from the tip of glass tube into water in the form of air bubbles (Fig. 11.4).

Factors Influencing Diffusion:

(i) Density:

Rate of diffu­sion of a substance is inversely proportional to square root of its relative density (Graham’s Law).

(ii) Permeability of Medium:

Rate of diffusion decreases with density of the medium.

(iii) Temperature:

A rise in temperature increases the rate of diffusion with Q10 = 1-2-1-3. Because of it sugar crystals do not dissolve easily in ice cold water while they do so easily in warm water.

(iv) Diffusion Pressure Gradient:

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.

Importance of Diffusion in Plants:

(i) Diffusion keeps the cell walls of the internal plant tissues moist.

(ii) It is a means of spreading of ions and other substances throughout the protoplast.

(iii) Tran­spiration or loss of water in vapour forms is a diffusion process.

(iv) Exchange of gases (CO2 and O2) between the plant interior and outside air occurs through diffusion.

(v) Osmosis is a special type of diffusion in which water diffuses through a semi-permeable membrane.

(vi) Aroma of flowers is due to diffusion of volatile aromatic compounds to attract pollinating animals.

Significance of Diffusion:

1. The process of diffusion is involved in the transpiration of water vapours.

2. Gaseous exchange during the process of respiration and photosynthesis takes place with the help of diffusion.

3. During passive salt uptake the ions are absorbed by the process of diffusion.

4. It also helps in translocation of food materials.

5. Gas exchange in submerged hydrophytes takes place by general body surface (epidermis) through diffusion.

6. Aroma of flowers is due to diffusion of volatile aromatic compounds to attract insects.