Physiology 467 



However, such effects have been minor ones and cannot, with any assur- 

 ance, be attributed to utilization of sugars as substrates. This situation 

 has been puzzhng in view of the fact that these flagellates store carbo- 

 hydrates and evidently utilize such reserves. Perhaps the difficulty lies in 

 some fundamental deficiency such as the lack of an adequate phos- 

 phorylating mechanism for utilizing exogenous carbohydrates. Or pos- 

 sibly the permeability of the body wall is too low for effective absorption. 

 Low rates of absorption presumably would not be a hindrance in holozoic 

 species. Consequently, the investigation of polysaccharides and disaccha- 

 rides as substrates for holozoic Euglenida and Chrysomonadida might 

 yield significant information. 



Utilization of monosaccharides (Table 8. 7) has been demonstrated by 

 fermentation reactions, by measuring stimulation of growth or oxygen 

 consumption, and by quantitative sugar determinations. The decom- 

 position of a monosaccharide^^ involves a series of reactions catalyzed by 

 a number of enzymes (Fig. 8. 3), the initial step being phosphorylation 

 of the sugar to glucose-6-phosphate. This reaction precedes the dissimila- 

 tion of exogenous sugar and often its storage as polysaccharide. In this 

 connection, it is interesting that hexokinase has not been found in Poly- 

 tomella caeca (356), and also that hexosediphosphate but not glucose can 

 be oxidized by Astasia klebsii (83). Although utilization of glucose also 

 has not been demonstrated for Euglena gracilis, this species does contain 

 the following compounds which appear in glycolysis: glucose- 1-phosphate, 

 fructose-6-phosphate, fructose- 1,6-diphosphate, and glycerophosphoric 

 acid (1). 



In organisms equipped with hexokinase, glucose-6-phosphate is pro- 

 duced and also may be stored, presumably by conversion into glucose- 1- 

 phosphate and thence into polysaccharide. Or, glucose-6-phosphate may 

 undergo dissimilation, the next step being conversion into fructose-6- 

 phosphate. Phosphorylation of this ester yields fructose- 1,6-diphosphate. 

 The diphosphate then undergoes cleavage into two interconvertible 

 triose-phosphates. Later reactions are traced to pyruvate in Figure 8. 3. 

 The series of reactions up to this point yields a certain amount of utiliz- 

 able energy. Aerobically, pyruvate may be oxidized through the tricar- 

 boxylic acid cycle, with more efficient utilization of the original free 

 energy in the glucose molecule. Anaerobically, pyruvate may be converted 

 into lactate or into ethanol. 



Glycolysis has been traced, at least to some extent, in a number of 

 Protozoa. Trypanosomes apparently vary in their methods of attacking 

 glucose. In the earlier work, no evidence was obtained for phosphoryla- 

 tion. More recently, Trypanosoma equiperdum has been shown to phos- 

 phorylate glucose to fructose- 1,6-diphosphate, which is split into triose- 



^■"' For details of glycolysis, discussions by Baldwin (10) and Lardy (308) may be 

 consulted. 



