Diffusion
All matter consist of minute particles(Atoms or Molecules) in constant motion. In solids the particles vibrate quickly but do not move from their fixed positions, in fluids they move freely and in gases very rapidly. Heat increases the intensity of these movements. This movement, or energy, ensures that in time all the particles in a given liquid,gas or mixture of gases, will be distributed evenly throughout the whole volume of the material. This process of intermixing in the absence of an intervening membrane is called diffusion. For example, when a bottle of perfume is opened in one corner of a room, the odour gradually spreads to every part of the room. In this case, the liquid perfume evaporate and the molecules of air in the vicinity of the bottle at first. Eventually as time goes on, they spread to all parts of the room.
Diffusion is responsible for the movement of substances in solutions of different strengths. For example, if a concentrated solution of sugar is placed in contact with water, there would be net movement of the solute from the region of higher concentration to that of a lower concentration. Diffusion occurs " Down a concentration gradient; the higher the difference in concentration between two system; the more rapid the rate of diffusion.
In case of diffusion between cells and their extra cellular fluid, the cell membrane acts as a barrier. However, as long as there are pores in the membrane large enough to allow a given size of molecules to pass through, diffusion of that substance will occur.
Diffusion of molecule across the membrane depends on molecular volume and the liquid solubility of the substance. Diffusion of ions depends on both the concentration and the electric gradients across the membrane.
Biological Significance of Diffusion:
Diffusion plays an important role in living cells. Principles inside a cell are distributed unevenly. Diffusion tends to equalize their distribution so that material in cells can be transported from one place to another.
Some important examples are:
- Absorption of glucose and amino acids from the intestine into blood.
- Exchange of O2 and CO2 between air and blood, in the lung capillaries.
- Exchange of anions between plasma and red cells during chloride shift.
- Exchange of materials( food stuffs, O2 etc. ) between the capillary bed and tissue fluid.