THE CHEMISTRY AND PHYSICS OF TH& CELL 29 



composition is evidently too uncertain to permit of surmises as 

 to its special purpose. 



Jecorin, which is generally considered as a combination of 

 lecithin and glucose, is probably also not a definite compound, 

 according to the most recent observations. 1 



CARBOHYDRATES 



The third great class of food-stuffs, the carbohydrates, is 

 represented in the cell by pentoses and hexoses combined with 

 proteids and with lipoids, and also by glycogen, which exists 

 free. Glycogen is a rather difficult substance to isolate, and, 

 therefore, although it is not found in all cells by our present 

 methods, yet it may well be that it is a constant constituent of 

 the protoplasm. There is no evidence, however, that it is any- 

 thing more than a source of heat and energy to the cell. Its 

 properties and occurrence will be considered more fully in the 

 discussion of glycogenic infiltration. Since glycogen is formed 

 from dextrose and is constantly breaking down into dextrose, it 

 is probable that the latter is also constantly present in the cells. 



INORGANIC SUBSTANCES 



Up to this point the substances of the cytoplasm that have 

 been discussed have all been organic compounds which do not 

 naturally exist independent from living or once living cells, yet 

 the inorganic substances of the protoplasm are also of vital 

 importance. As Mann says, " so-called pure ash-free proteids 

 are chemically inert, and, in the true sense of the word, dead 

 bodies. What puts life into them is the presence of electro- 

 lytes." The various salts of potassium, sodium, calcium, mag- 

 nesium, and iron which all cells contain do not exist merely 

 dissolved in the water of the cell, but in part they are combined 

 with the organic constituents of the protoplasm. They are not 

 combined as simple additions of the salts to the proteids ; but 

 ions, both anions and kations are united in chemical combina- 

 tion to the large proteid molecule (ion-proteids). Possibly 

 the proteids participate in vital chemical processes only as ion 

 compounds with inorganic elements. It is extremely difficult, 

 indeed almost impossible, to secure proteids entirely free from 

 inorganic substances (ash-free proteids). The fact that the in- 

 organic substances are held in the cells chemically rather than 

 by simple diffusion into them from the surrounding fluids is 

 shown by the great difference in the proportions of various salts 



Meinertz, Zeit. physiol. Chem., 1905 (46), 376; Siegfried and Marx, Ibid, 

 1906 (46), 492. 



