Before cooking dried fruits such as prunes and apricots it is customary to lea soaking in cold water for some hours so that they swell up and become rest far as possible, to their original condition. During this time water passes through cell walls of the fruit by a process called osmosis.
Osmosis can be demonstrated by the following experiment (Fig. 13.10). glass tube, more than a meter long, ends in a large thistle funnel having i – covered with parchment. Care must be taken to tie the parchment very s tightly with stout thread so that the joint is entirely free from leaks. The f is then nearly filled with strong sugar solution and the whole is supported stem vertical and the funnel mouth downwards in a beaker of water. Very liquid level in the stem of the funnel begins to rise, showing that water – through the parchment into the sugar solution. The osmotic pressure of the is measured by the hydro static pressure, h, of the liquid column in the tu Parchment is described as a semi-permeable membrane, i.e., it pe molecules to pass through its pores but obstructs the passage of the 1 :: molecules. Water molecules therefore pass through in both directions, b – the space occupied by the sugar molecules the concentration of water less on the sugar side of the membrane than on the water side. Consequent rate of passage of water molecules is greater from the water side to the reason why a prune swells in water now becomes apparent. The cells of the fruit
contain a high concentration of sugar and are surrounded by a semi-permeable membrane.
Most animal and vegetable membranes are semi-permeable, and so osmosis plays an important part in the general functioning of living things. For example, water is absorbed through the roots of plants by this process. Osmosis also plays a part in the elimination of waste products frotn the blood through our kidneys. Cellophane, which is another semi-permeable material, has been successfully employed in the construction of an artificial kidney. This is connected between an artery and a vein and maintains the essential purification of the blood while the natural kidneys are out of action through illness.