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PHYSIOLOGY OF CELLS 45 



and against other portions of the gas itself are matters of common observa- 

 tion. In hke manner, when a substance is dissolved in water, the mole- 

 cules or ions of the dissolved substance spread through the liquid until 

 the limits of the liquid are reached. The liquid in this case sets the limit 

 of dispersion of the diffusing substance. As in the case of the gas, pressure 

 is exerted by the molecules and ions of the solute at all points within the 

 solution. This pressure is what is called osmotic pressure. At a given 

 temperature, the intensity of osmotic pressure depends upon the number 

 of molecules and ions present per volume of the solution. Diluting 

 a solution thus decreases the osmotic pressure. All the dissolved 

 particles, regardless of size, whether molecules or ions, exert the same 

 osmotic pressure, so that a substance which ionizes (that is, one whose 

 molecules in solution divide into ions) to a large extent 

 at a given concentration exerts a greater pressure than 

 does another substance which ionizes to a lesser extent at 

 the same concentration. 



In the sugar solution referred to above and in Fig. 23, 

 the molecules of the sugar exert a certain osmotic pres- 

 sure. Were they free to pass into water surrounding the 

 funnel they would do so, and the concentration (and 

 hence the osmotic pressure) would be everywhere re- 

 duced; but the bladder, being approximately a semi-per- 

 meable membrane, largely prevents the passage of the p^^ 24 — 

 sugar. However, water passes into the sugar solution. Diagram show- 

 and dilution of the solution with the consequent lowering a"^a\ag^of pro- 

 of its osmotic pressure results just as if the sugar mole- topiasm encased 

 cules had passed out of the funnel. The passage of water endoTin^a soiu- 

 and other substances through membranes is known as tion in a vacuole 

 osmosis. Osmosis occurs in all living cells, and is an im- protoplasm-' V, 

 portant feature of life processes. In higher animals, as vacuole; TF, wall, 

 is pointed out in a later chapter, it has much to do with 

 the absorption of nutrient materials, with respiration, and with the 

 eUmination of wastes in excretion. 



Isotonic or Isosmotic Solutions. — Since in a plant cell the protoplasm 

 occurs in the form of a bag encased in a cell wall and enclosing a solution, 

 as shown diagrammatically in Fig. 24, such a cell may be used to compare 

 the osmotic pressures of two solutions. The cell wall is permeable, 

 but the protoplasm acts like a semi-permeable membrane. The effect 

 of the solution on the cell can be observed under the microscope. If the 

 cell contents shrink it is known that water has been withdrawn from the 

 cell. If such cells be immersed successively in a series of solutions of 

 very slightly different osmotic pressures, some of which cause shrinkage 

 of the cells and others do not, the solution with the greatest pressure 

 which does not cause shrinkage exhibits the same osmotic pressure as 



