258 PROTOPLASM 



Molecular chains that have an unsatisfied valence bond at each 

 end may readily join, one to another. Linear protein molecules 

 may show such an affinity for each other because of ionization at 

 their unlike ends. A polypeptid illustrates the type of linear 

 and polar protein molecule likely to occur in protoplasm: 



H2N— CH2— CO— NH— CH2— CO— NH— CH2— COOH 



Ionization of the terminal groups (NH2 and COOH into NH3+ 

 and C00-) leaves the ends free to unite. Where there exist 

 unsatisfied valences, as in the cellulose molecules, or ionized 

 radicals, as in proteins, the ends may become saturated {e.g., 



with OH radicals), or the molecule 

 may form a closed ring (page 485), 

 or one chain may join to another and 

 form a continuous intertwining thread 

 (Fig. 137). J. A. Wilson regards a 

 similar structure as probably occurring 

 in gelatin. He says that we may 

 look upon a plate of gelatin as a con- 

 tinuous network of chains of amino 

 acids, there being no individual 

 molecules, unless one wishes to regard 

 "fi^^stT— An^r^a^'n'^iwit the entire plate of gelatin as one huge 

 of a continuous thread, iiius- molecule. Another possible picture, 



trating a possible structure of i • i i j ^ j^- n x • 



jellies. {From V. Cofman.) and One which better satisfies certam 



physical properties of lyophilic systems 

 such as gelatin, is the following. If the structural configuration of 

 protein molecules is that of long chains with lateral, rather than, 

 or in addition to, terminal unsatisfied bonds (as shown on page 

 475), then we have possibilities of weak unions along the chain. 

 Such a situation meets the structural requirements of a brush 

 heap of loose construction. 



A brush heap (Fig. 126) is, as already suggested, a mechanism 

 permitting of a satisfactory interpretation of the properties of 

 jellies — lyophilic colloidal systems which, whether of high or of 

 very low viscosity, possess elastic qualities. The brush heap is 

 likewise a suitable mechanical basis upon which to interpret the 

 thixotropic properties of certain gels which, though they may 

 contain but little solid matter, set to a firm mass (page 149). 

 Long structural units permit the formation of an open yet rigid 



