24 INTRODUCTION 



of the peculiar amphoteric nature of proteids. As long as these 

 two groups are free the proteids can combine with either acids 

 or bases, as they are well known to do, and hence they react as 

 either adds or bases under different conditions. 



It must not be imagined that the structure of the complete 

 molecule is simply a long straight chain of amino-acids joined 

 only in the same way as are the components of glycylglycin. 1 

 The existence of the diamino-acids, of the benzene rings, of 

 hydroxyl groups, (as in serin ortyrosin), of ring compounds, (as 

 pyrrolidintcarboxylic acid), of substances with two acid groups, 

 (as glutaminic and aspartic acid), adds complications to the forma- 

 tion until it is impossible to estimate just how all the various 

 building stones may be arranged. We must bear in mind the 

 size of the proteid molecule, which Hofmeister has estimated (for 

 serum albumin) as having a molecular weight of 10,166, and 

 for hemoglobin the molecular weight has been estimated at 

 16,669. Within such a "giant molecule" there is room for 

 variety almost beyond computation. 



The Proteids of the Cell. By physiological chemists 

 proteids are classified into simple proteids, of which egg and 

 serum albumin are types : and compound proteids, which are 

 characterized by having some special non-proteid group which 

 can be split off, leaving behind a characteristic proteid residue, 

 e. g., nucleo-proteids, glyco-proteids. As primary cell constit- 

 uents the following varieties of proteids may be mentioned : 

 albumin, globulin, nucleo-proteid, nucleo-albumin or phospho- 

 proteid, and coagulated proteids. At one time it was thought 

 that cytoplasm consisted chiefly of albumin, like white of egg, 

 but we now know that this forms but a small part of the cell 

 proteids, often occurring only as traces. It is held by some 

 that true albumin occurs only as a building or intermediate 

 cleavage product of the more complicated forms of cellular 

 proteids, and is itself of relatively slight importance in cell 

 life, not participating in chemical changes except as a food-stuff. 



Albumins are characterized chiefly by their greater solubility 

 in water, and in being less easily precipitated than most proteids. 

 They seem to be a fundamental type of proteids. The three 

 forms of albumin that have been described in animal tissues or 

 products are egg -albumin, lactalbumin of milk, and serum 



1 Fisher and Abderhalden (Ber. deut Chem. GeselL, 1906 (39), 752) have 

 described a polypeptid in which the union of the amino-acids is accomplished 

 in a somewhat different manner, as shown by the formula : 



CH 2 - COOH + CH 3 CH COOH = HN 



CH 2 or 



