78 



Cellular Structure and Activity 



fashion. It is now known that the synthesis 

 is linked to other processes and must be 

 formulated in stepwise fashion. 



The reaction starts with the linkage of 

 some group to one sugar. In bacterial cells, 

 at least, this is apparently by phosphoryla- 

 tion, the phosphate group being furnished 

 by ATP: 



Glucose + ATP ^ Glucose— P + ADP 



anisms to give a high energy bond; (2) 

 transphosphorylation to an intermediate takes 

 place; (3) transphosphorylation to a sugar 

 occurs; (4) a "transhexose" reaction now 

 takes place with the phosphate acting as 

 donor rather than transferee, and the glucose 

 is linked to another sugar, fructose, to form 

 the end product of the synthetic reaction, 

 sucrose. 



FATS, 



PR0TE(NS 



ETC. 



SUGAR 



Fig. 7. Schematic flow diagram, assuming that the total activity of a cell may be regarded as a chemical 

 manufacturing plant composed of unit processes. Preliminary stages such as glycolysis provide raw materials 

 which are burned to CO2 and H2O in the citric acid and oxidase units, energy being collected in the "pool" 

 of coenzymes and wastes eliminated. The materials of the energy pool, with other end products or raw 

 materials, then are used for synthetic reactions or the "work of the cell." A major purpose of the diagram is 

 to illustrate the interrelationships of unit processes and to emphasize that control could reside either in 

 distribution of materials (extrinsic factors of control) or in the machinery of each unit (intrinsic control). 



The transphosphorylation once accomplished, 

 the ester linkage is there (roughly equivalent 

 to a glucosidic linkage) and the phosphate 

 can, so to speak, with the proper enzyme 

 toss the glucose to the waiting fructose: 

 Glucose — P + fructose ;=i sucrose + Phosphate 

 Since the ATP, in intact cells, is regener- 

 ated by the oxidative uptake of inorganic 

 phosphate, the over-all reaction in respiring 

 cells may be written as 



sucrose ;i± glucose + fructose 

 and the catalytic components, ADP and phos- 

 phate, have not changed in amount. 



The pattern of sucrose synthesis thus 

 emerges as follows: (1) phosphate is linked 

 to an organic molecule by oxidative mech- 



It is probable that this series of reactions, 

 like those of glycolysis and oxidation, offers 

 a general model for various types of syn- 

 theses taking place in cells. For example, 

 polysaccharide may be built up by trans- 

 ferring glucose from the glucose — fructose 

 linkage rather than from glucose — P link- 

 age (Hassid and Doudoroff, '50). Nucleotides 

 appear to be formed from pentose and nitrog- 

 enous base by similar mechanisms (Kalc- 

 kar, '45). Once a group needed for synthetic 

 processes is linked to any compound in proper 

 fashion, it appears probable that the cells 

 possess the essential enzyme systems to trans- 

 fer the group to other compoimds, the main 

 energy expenditure being in the formation of 



