In the living organisms, each cell is bounded by cell membrane. The cell membranes are able to regulate the movement of materials across them.
The cells constantly exchange materials with their environment through the cell membranes. But the movement of materials, in and out of the cells, across the cell membranes is not an uncontrolled traffic, because certain materials can pass through the membrane, others cannot. Thus the primary function of the cell membranes is to regulate the passage of substances between the interior of the cell and its external environment.
Many different molecules can move across the cell membrane. Of all these, water is the most important one. Many inorganic salts can move across the cell membrane in solution i.e. dissolved in water. One of the processes by which the exchange of materials between the cells and their environment occurs is called diffusion. For instance, water and inorganic salts dissolved in it pass into the plants by diffusion through the root hairs into the cells of the roots.
It is a process in which the random movement of the molecules or ions takes place down a concentration gradient i.e. from region of higher concentration to a region of lower concentration. Water, CO2, O2 and some other simple molecules can diffuse across the cell-membranes in this way. It is also the main process by which other substances move within the cells. During photosynthesis and respiration in the plants, the exchange of carbon dioxide and oxygen gas, between the cell and the atmosphere also takes place by diffusion. Although diffusion is a slow process, yet it is efficient and rapid enough to fulfill all the gaseous requirements of the plant.
“The molecules of some substance because of their size or polarity cannot pass readily, into and out of the cell through the cell membrane. This is brought about by certain proteins called carrier proteins. But here too the movement would be according to the principle of concentration area. This type of diffusion which takes place with the help of carrier proteins, is called facilitated diffusion. Like simple diffusion, in this case also expenditure of energy is not involved.”
Diffusion is a well known phenomenon. It can be clearly demonstrated by a simple example i.e. when a colored crystal like (KMn O4) is dropped into a bowl of water, initially the color or pigment will remain concentrated at its molecules will diffuse evenly in all directions throughout the water, resulting in uniform distribution of color in all regions of the water in the bowl (fig. 12.2). similarly, if we spray perfume (air-freshener) in one corner of a room, initially its fragrance will be strongest in that corner and will gradually spread by diffusion in all areas of the room.
The efficiency of diffusion depends on two factors, (a) a distance (b) a concentration gradient difference. The speed of diffusion also depends on the kind of molecules, e.g., diffusion of gases is faster than of liquids; and similarly the diffusion of solids is the slowest.
Diffusion plays a key role in the transport of substances into and out of cell in all living organisms.