In this article we will discuss about:- 1. Subject-Matter of Diffusion 2. Factors Influencing Rate of Diffusion.
Subject-Matter of Diffusion:
Diffusion is the net movement of molecules or ions of a given substance from a region of higher concentration to that of lower one by virtue of their kinetic energy. The movement continues till their respective concentrations become uniform and a state of dynamic equilibrium is achieved between the two phases.
If a crystal of copper sulphate is placed in a breaker full of water it goes into the solution and gradually the coloured solution diffuses equally throughout the solution. The molecules and ions of copper sulphate diffuse from a region of higher concentration to a region of lower concentration.
The process continues till the concentration becomes uniform throughout. At the same time water also diffuses towards the space occupied by the crystal where diffusion pressure of water is less. In an imperfectly mixed solution, water molecules would diffuse from the gradient from region of more dilute solution to the region of more concentrated solution.
In the former the water molecules have higher potential while in the latter they have lower Ψ. Likewise the solution molecules would also diffuse down their concentration gradient i.e., from a region of higher concentration to a region of more dilution.
The diffusion rates are proportional to the kinetic energy of the molecules, their size, and the density of the medium and the gradient of concentration over which they diffuse. When uniform distribution of molecules takes place the net movement of molecules ceases and a dynamic equilibrium is established.
The direction of diffusion of one material is independent of the direction in which the other materials are diffusing. However, there could be collision between the molecules of the two substances causing slowing down of the rate of their diffusion.
However, the direction of their diffusion is dependent upon the concentration. At any given time some substances diffuse out of the cell whereas others diffuse in. Diffusion should be distinguished from the mass movement.
If a rubber balloon is placed in a closed jar containing CO2, balloon would tend to distend since rubber wall is readily permeable to CO2. It will exert pressure on the wall and the pressure is directly proportional to the number of molecules per unit volume.
The net diffusion of gases results from a difference in gas pressure in the two areas under a given condition. When the pressures are equal, there is a state of dynamic equilibrium. This pressure is called diffusion pressure.
Factors Influencing Rate of Diffusion:
Diffusion may be regarded in terms of differences in free energies. Thus, the net diffusion of gases or a substance e.g., water, is from a region of their greater free energy to a region of their lesser free energy. Clearly the factors which affect or influence changes in free energy also affect diffusion.
Temperature:
The speed of diffusion increases with an increase of temperature since the activity of molecules is increased with raised temperature. In fact free energy increases directly with temperature, and molecules of a substance move towards the region of lower temperature.
External forces:
This is especially true of water movement. Suppose we exert a greater force on water, it will move briskly or penetrate the membrane more rapidly. In other words, the activity of the water enhances, as the energy of the water subjected to a pressure is increased.
Size of molecules:
The rate of diffusion is also dependent upon the size of the molecule; larger the molecules slower are the rate of diffusion.
Presence of other molecules:
The rate of diffusion decreases when molecules other than the substance are added. This decrease is possibly due to the collision of the molecules of a given and ‘foreign’ substance. However, the direction of the diffusion remains unaffected.
Concentration Gradient:
Increasing the concentration of the molecules also increases free energy. The net diffusion of molecules will be more rapid if the difference in free energy between two regions is highly pronounced.
However, the distance between the two regions also determines the rate of diffusion; it decreases with an increase in distance. Concentration gradient indicates the application of concentration differences over a specific distance.
Living membranes are differentially permeable i.e. for any given membrane some molecules and ions penetrate quite readily, others slowly, and some fail to do so.
In fact there are gradations in the rate of penetration. Structures of the membranes possibly contribute towards their differential behaviour. Cell membranes control the entry or exit of different substances. In addition several internal and external factors also bring about changes that regulate the penetration of molecules from one point to another.
Thus, individual rates may change but the general order of penetration remains constant. The cell wall is porous having several micro-channels between the cellulose fibres. These channels contain water.