CELLS AND TISSUES ^\ 



fluid of a higher animal. In general, only dissolved substances can pass 

 through the plasma membrane, but not all dissolved substances pene- 

 trate the plasma membrane with equal facility. The membrane behaves 

 as though it had ultramicroscopic pores through which substances pass, 

 and these pores, like the holes in a sieve, determine the maximum size 

 of molecule that can pass. Factors other than simple molecular size, such 

 as the electric charge, if any, of the diffusing particle, the number of 

 water molecules bound to the diffusing particles and its solubility in 

 fatty substances, may also be important in determining whether or not 

 the substance can pass through the plasma membrane. 



A membrane is said to be permeable if it will permit any substance 

 to pass through, impermeable if it will allow no substance to pass, and 

 semipermeable, or differentially permeable, if it will allow some but 

 not all substances to diffuse through. The plasma membranes of all 

 cells and the membranes surrounding food and contractile vacuoles are 

 semipermeable membranes. Permeabdity is a property of the membrane, 

 not of the diffusing substance. 



The diffusion of a dissolved substance through a semipermeable 

 membrane is known as dialysis. If a pouch made of collodion, cello- 

 phane or parchment is filled with a sugar solution and placed in a beaker 

 of water, the sugar molecules will dialyze through the membrane (if the 

 pores are large enough) and eventually the concentration of sugar mole- 

 cules in the water outside the pouch will equal that within the pouch. 

 The molectdes then continue to diffuse but there is no net change in 

 concentration for the rates in the two directions are equal. 



A different type of diffusion is observed if a membrane is prepared 

 with smaller pores, so that it is permeable to the small water molecules 

 but not to the larger sugar molecules. A pouch may be prepared of a 

 membrane with these properties and filled with a sugar solution, then 

 the pouch is fitted with a cork and glass tube and placed in a beaker of 

 water so that the levels of fluid inside and outside of the pouch are the 

 same. The sugar molectdes cannot pass through the membrane and so 

 must remain inside the pouch. The water molectdes diffuse through the 

 membrane and mix with the sugar solution, so that the level of ffuid 

 within the pouch rises. The liquid within the pouch is 5 per cent sugar, 

 and therefore only 95 per cent water; the liquid outside the membrane 

 is 100 per cent water. The water molecules are moving in both directions 

 through the membrane but there is a greater movement from the region 

 of higher concentration (100 per cent, outside the potich) to the region of 

 lower concentration (95 per cent, within the pouch). This diffusion of 

 water or solvent molecules through a membrane is called osmosis, and 

 is illustrated diagrammatically in Figure 3.10. 



If an amount of water equal to that originally present in the pouch 

 enters, the solution in the pouch will be diluted to 2.5 per cent sugar 

 and 97.5 per cent water, but the concentration of water outside the 

 pouch will still exceed that inside and osmosis will continue. An 

 equilibrium is reached when the water in the glass tube rises to a 

 height such that the weight of the water in the tube exerts a pressure 

 just equal to the tendency of the water to enter the pouch. Osmosis then 



