^54 



FINE-STRUCTURE OF PROTOPLASM 



II 



increased and the cytoplasm becomes negatively charged, i.e., it acts 

 like a weak anion. This occurs as a rule in neutral nutrients, since the 

 I.E. P. of protoplasm is usually lower than 7 (Table XVII). 



TABLE XVII 

 ISOELECTRIC POINT (i.E.P.) OF CERTAIN PROTOPLASTS 



(according to Pfeiffer, 1929) 



PH 



Bacteria: 



Fungi : 

 Algae: 

 Angiosperms : 



Bacteriutn coli .... 

 grampositive bacteria 

 gramnegative bacteria 



Fusarium 



Nitella 



Hyacinthus (root tip) . 

 Lupinus, Pisum . . . 



Rheum 



Solarium 



12-13 

 about 5 



2-3 



5-4 



4.4-9.6 



4-3 



4-3 

 4.5-4.8 



6.4 



The isoelectric state determines the acidity at which the heteropolar 

 junctions of the salt bonds are most effective. Any deviation of the p^ 

 from this state results in a loosening of this type of bond. 



Up to a certain point esterifications, i.e., bridges formed between 

 alcoholic OH and acid groups of neighbouring polypeptide chains 

 (Fig. 96III, p. 145), can Hkewise be reckoned among the heteropolar 

 valency bonds. Their firmness is dependent also on the pjj of the 

 medium, since hydrogen ions are capable of hydrolyzing and hydroxyl 

 ions of saponifying these ester bonds catalytically. 



IV. Homopolar valency bonds are formed either by elimination of 

 water (ether, glucoside and peptide bridges. Fig. 96 IV, p. 145) or by 

 splitting off hydrogen, i.e., dehydrogenation (methylene and sulphur 

 bridges. Fig. 102). The former still possess a certain polarity and can 

 be hydrolyzed under suitable conditions. Without the aid of enzymes 

 this can now only be effected at temperatures above the physiological; 

 compare, for instance, the hydrolysis of glucosides and proteins by 

 boiling acids. This is of particular importance for the stability of the 

 carbohydrates and the peptide bonds. The purely homopolar valency 

 bridges (-CH2-CH2- -S-S-) can no longer be hydrolyzed at all. Here 



