132 



FINE-STRUCTURE OF PROTOPLASM 



§ I. Cytoplasm 



II 



a. Molecular Constituents of the Cytoplasm 



The chemical composition of the cytoplasm is described here only 

 from the point of view of the molecular shape of its compounds 

 (Sponsler and Bath, 1942). The molecular structures concerned are 

 known in principle, but an attempt at morphological synthesis of 

 cytoplasm with the aid of these structural units is impossible. Never- 

 theless, this morphological point of view enables us to explain the 

 physico-chemical behaviour of cytoplasm to a certain extent. 



Proteins. The basic substances of the proteins, isolated by means of 

 hydrolysis and paper chromatography, are a-amino acids which possess 

 the structure given in Fig. 87a, where R represents a group of C-atoms. 



a) R CHNHz-COOH 

 cc 



rRChf- 



To be exact, the NH2- and COOH- 



^NH 



,NH2 basic 



RCH 



•^ 



b) 



'^CQOH O'^'cf 



/vrt?;. 



\ 



CHR 



NH 



CO 



-RCH 



XO ^o 



i 



XHR- 



^NH 



".CO 



i 



HOOC 



Fig. 87. Molecular structure of 

 amino acids, a) Overall formula; 

 h) principle of chain formation; 

 c) polypeptide chain. 



groups should be bound to the C-atom 

 as individual atom groups, as shown 

 in Fig. 87b. It can easily be seen that 

 two amino acids can form a so-called 

 dipeptide by eliminating water. If this 

 process is repeated many times, a long 

 polypeptide chain is formed, the ends 

 of which have been left open in Fig. 87c. 

 Like the paraffin chain, it is kinked. The 

 distance between two equivalent groups 

 is 3 . 5 A, as has been ascertained by means 

 of X-ray analysis of crystalline fibre 

 proteins. Only the >CO and >NH 

 groups are similar along the whole length ofthe chain, while R differs ac- 

 cording to the kind of protein and thus is responsible for the great 

 variety in this class of substances. The zig-zag chain drawn in Fig. 87c 

 can be considered as a relatively indifferent frame, which cannot be 

 responsible for the chemical lability which we know the cytoplasm 

 to possess. Its unusual reactivity is due to the side chains R. 



In chemical text books the amphoteric character of the proteins is 

 often explained by the fact that amino acids possess both an acid and a 

 basic group (Fig. 87b). However, it follows from the structural picture 

 of the polypeptide chain that these groups disappear in the condensa- 

 tion process, thus losing their capacity for dissociation. If in spite of 



