21 THE CHEMISTRY AMJ PHYSICS OF THE CELL 



dence, however, that it is anything more than a source of heat and energy to the 

 cell. Its properties and occurrence will be considered more fully in the discus- 

 sion of glycogenic infiltration. Since glycogen is formed from dextrose and is 

 constantly breaking down into dextrose, it is probable that the latter is also con- 

 stantly present in the cells. 



Inorganic Substances 



Up to this point the substanf es 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 proteins are chemically 

 inert, and, in the true sense of the word, dead bodies. What puts life into them 

 is the presence of electrolytes." The various salts of potassium, sodium, calcium, 

 magnesium, and iron which all cells contain do not exist merely dissolved in the 

 water of the cell, but in part they are comlnned with the organic ccmstituents 

 of the protoplasm. They are not combined as simple additions of the salts to 

 the proteins; but io)is, both anions and cations, are united in chemical combina- 

 tion to the large protein molecule (ion-proteins)." Possibly the proteins partici- 

 pate in vital chemical processes only as ion compounds with inorganic elements. 

 It is extremely difficult, indeed almost impossible, to secure proteins entirely free 

 from inorganic substances (ash-free proteins). The fact that inorganic substances 

 are held in the cells cliemically rather than by simple diffusion into them from the 

 surrounding fluids is shown by the great difference in the proportions of various 

 salts in the cells and in the extra-cellular fluids. Thus potassium is nearly always 

 much more abundant in the cells than in the tissue fluids, while sodium is more 

 abundant in the fluids. Piiosphoric acid is also more abundant in the cells, and 

 chlorin in the plasma. In cells iron seems to exist chiefly in combination with the 

 nucleo-protcins. '^ 



THE PHYSICAL CHEMISTRY OF THE CELL AND ITS CONSTITUENTS'" 



From the standpoint of physical chemist ly the cell consists of a 

 collection of colloids and crystalloids, electrolytes and non-electrolytes, 

 dissolved in water, in lipoids, and in each other, surrounded by a semi- 

 permeable membrane, and perhaps subdivided by similar membranes 

 or surfaces. Physical chemical jjrocesses, as we shall see later, 

 play an all-important part in the life phenomena of the cell, and there- 

 fore some space may be occupied profitabl}^ in explaining the nature 

 of these changes and of the .substances that participate in them. 



Crystalloids and their Properties 



Crystalloids, or substances that tend under favorable conditions 

 to form crystals, and which diffuse readily through most diffusion 

 membranes, form a relatively small part of the total mass of the cell, 

 but they are fully as essential as the colloids. The chief representa- 

 tives of this groui) that are found usually or constantly in the cell are 

 the inorganic salts, sugar, and the innumerable decomposition products 

 of the proteins, including particularly urea, creatine, purine bases, 

 amino-acids, etc. Most of these are by no means so characteristic 

 of living things as are the colloids, sometimes occurring (juite inde- 



"SeeJ. Loci), Science, 191't (SO), 4:{<). 



" See Macallum on Microchemistry, lMgei)nisse Piiysiol., 1908 (7), 552. 

 "See Hayliss, "Principles of (Jciieral Physiology," London, 1915, for a more 

 extensive discussion of these topics. 



