mined by sugar plasmolysis ? Calculate the isotonic coefficient (i) 

 for KNO 8 at the isotonic concentration. Then calculate the degree 

 of dissociation (a) of the KNO,, using the formula: 



i 1 -)- (K l)a, where K = number of ions from one 

 molecule, and a the proportion of molecules split into ions. 



121. Perform the same experiment as above, and make the 

 same calculations, using instead of Spirogyra the leaves of the 

 water plant, Elodea, only two cell layers in thickness : a layer of 

 large cells above and of smaller cells below. 



122. Place blood corpuscles of the frog in m/2, 111/4, m/6, 111/7, 

 m 8, ni/io, and m/i6 NaCl solutions. In which solution do the 

 corpuscles retain their volume unchanged? What is the osmotic 

 pressure of this solution if the isotonic coefficient is 1.83? 



123. Try plasmolysing the cells of a marine plant (Ulva) using 

 2m, i.^ni and /;/ sugar solutions. In which solution does plasmolysis 

 take place? Why are stronger solutions needed? 



B. CELL PERMEABILITY 



Dead membranes such as parchment, or collodion or cellulose are 

 readily permeable to crystalloidal substances in general, but not to 

 many colloids. The membrane surrounding living cells is permeable 

 to some crystalloids but not to others, thus exhibiting a ''selective 

 permeability", which is characteristic of both animal and plant cells. 

 Some of the permeability relations of living cells are brought out 

 in the following experiments. 



124. Permeability to non-electrolytes. Place Elodea leaves in 

 the following solutions of both 111/2 and 111/3 concentrations in watch- 

 glasses : Cane sugar, grape sugar, urea, glycerine, alcohol. Which 

 of these solutions have the same osmotic pressure? Do they all 

 plasmolyse? Note carefully whether plasmolysis is permanent (till 

 the end of the laboratory period) when it occurs, or only temporary. 

 How do you explain any differences noted? Which substances 

 penetrate the cells most rapidly ? 



125. Pcnncabilit\ to salts. Place Elodea leaves in 111/2 and 111/3 

 solutions of common salt and KNO 3 . Have these solutions the same 

 osmotic pressures? Have they the some osmotic pressures as the 

 above mentioned solutions? Is plasmolysis permanent in both salt 

 and KNO 3 ? Leave in the solutions for i hour or longer. What does 

 this indicate? ^i 



126. Permeability to alkalies. Elodea leaves stained in neutral 

 red are to be used. Note if they show protoplasmic rotation. If the 



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