DISCUSSION QUESTIONS 105 



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cations in the water glide past the stationary' anions which adhere to the 

 walls of the tube. Since many membranes are essentially porous in structure, 

 conditions similar to those just described may exist in each capillary of the 

 membrane, and electro-osmosis may occur through such membranes in essen- 

 tially the same way that it occurs through tubes of small bore. Since dif- 

 ferences of electrical potential are frequently present in living organisms it 

 seems entirely probable that electro-osmotic flow of water occurs in living 

 tissues. 



Discussion Questions 



In answering these questions use the theoretical value of 22.4 atmos. at 0° C. 

 for the osmotic pressure of a weight molar solution of an undissociated solute, 

 1.86 as the freezing point depression of such a solution, and assume all membranes 

 to be permeable to water only. 



1. Distinguish between the osmotic pressure and the turgor pressure of a 



solution. 



2. A closed sac-like membrane containing a solution with an osmotic pressure 



of 27 atmos. is immersed in a solution with an osmotic pressure of 16 atmos. 

 In which direction will water move? Why? Assuming dilution of the 

 internal solution to be negligible what will its turgor pressure be at 

 equilibrium? What will it be if the osmotic pressure of the external 

 solution is 22 atmos.? 30 atmos.? 



3. The freezing point depression of a plant sap is found to be 3.72° C. Wliat 



is its osmotic pressure at 25° C. ? 



4. The two arms of a U-tube are separated by a membrane permeable only to 



water. A solution with an osmotic pressure of 15 atmos. is placed in arm 

 A, one with an osm.otic pressure of 20 atmos. in arm B. Which way will 

 water move ? Explain. 



5. If the solution in arm B (question 4) is subjected to a pressure of 10 atmos. 



which direction will water move? If a pressure of 5 atmos. is used? 

 3 atmos.? If the solution in both arms is subjected to a pressure of 

 8 atmos.? Explain. 



6. If the solution in arm B (question 4) is warmed to 30° C. while that in 



A stays at 20° C, in which direction will water move? Why? 



7. When in a flaccid (turgorless) condition a given plant cell has a volume 



of 10 (arbitrary units) and an osmotic pressure of 18 atmos. If in its 

 fully turgid condition its volume is 15, what is its osmotic pressure? 

 (Assume solution volume of solutes to be negligible.) 



8. A 10 per cent solution of sucrose is enclosed in an osmometer with a vertical 



glass tube attached and the osmometer immersed in pure water. Disre- 

 garding dilution of the solution and assuming it has a specific gravity of I, 

 what is the approximate maximum height to which the liquid could rise 

 in the tube? 



9. Assuming 75 per cent ionization and no hydration what is the molar concen- 



tration of a KCl solution which exhibits an osmotic pressure of 22.4 atmos. 

 at 0° C? 



ID. An osmometer containing an 0.5 volume molar solution of sucrose is im- 

 mersed in a beaker of water. After a time its volume is found to have 

 doubled. What will then be the osmotic pressure of the solution inside 

 of the osmometer? 



II. How much sucrose in grams must be added to a liter of water in order to 

 make a solution which has a freezing point of — 4.28° C? 



