Molecular Structure in Protoplasm 



61 



scale as the portion of the protein chain in Figure 9D in order that 

 dimensional comparisons may be made. 



The most natural classification of these small molecules is that 

 based on structure. The list may be a bit misleading for it is not 

 known whether all of these occur in all living cells or in what 

 amounts the various forms are associated in a single cell. The list 

 is given on the assumption 

 that many of them occur 

 in every living cell and 

 that the list is not ex- 

 haustive (88, 89). 



They may be sorted out 

 into about six general 

 classes. The first three in- 

 clude the alcohols and car- 

 bohydrates, the aldehydes, 

 and the organic acids, such 

 as ascorbic (90) , succinic, 

 malic, pyruvic, lactic, and 

 amino acids (Fig. 9) . 



The fourth group con- 

 sists of larger molecules 

 called nucleotides, which 

 are composed of phos- 

 phoric acid, a sugar, and a 

 nitrogenous base arranged 

 in the order given (8) . 

 They may occur as single 



forms, such as riboflavinphosphate (91) and adenosine pyrophos- 

 phate; as dinucleotides, such as flavine-adenine dinucleotide (92) 

 and diphosphopyridine nucleotide (93) ; and as tetranucleotides 



(94) , such as nucleic acid. These vary mainly in the nitrogen- 

 containing groups (Fig. 10) . 



The fifth group includes the metal-containing prosthetic groups 



(95, 11) , such as cytochrome, containing iron (96) ; chlorophyll, con- 

 taining magnesium (Fig. 11) ; and certain copper-containing groups 



(95). 



The sixth group consists of molecules containing sulfur, such as 

 glutathione (88) and thiamine pyrophosphate (97, 98) (Fig. 12) . 



This partial listing and brief discussion of the fatty materials 

 and smaller molecules which act as prosthetic groups of enzymes 



Fig. 9. Photographs of models of various 

 types of small organic molecules. A. Ethyl 

 alcohol. B. Ascorbic acid. C. Succinic acid. 

 D. A portion of a protein chain made to 

 same scale as the small organic molecules. 



