30 INTRODUCTION 



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. Phos- 

 phoric 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-proteids. These matters will be taken up in 

 greater detail in considering the physical chemistry of the cell. 



THE PHYSICAL CHEMISTRY OF THE CELL AND ITS CON- 

 STITUENTS 



From the standpoint of physical chemistry 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 semipermeable membrane, and per- 

 haps subdivided by similar membranes. Physical chemical 

 processes, as we shall see later, play an all-important part in 

 the life phenomena of the cell, and therefore some space may 

 profitably be occupied 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 condi- 

 tions 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 col- 

 loids. The chief representatives of this group that are found 

 usually or constantly in the cell are the inorganic salts, sugar, 

 and the innumerable decomposition products of the proteids, 

 including particularly urea, creatin, purin bases, ammo-acids, 

 etc. Most of these are by no means so characteristic of living 

 things as are the colloids, sometimes occurring quite independ- 

 ently of a cellular origin, which the proteids never do. The 

 inorganic salts in particular seem quite foreign to livingprocesses r 

 and as they enter and leave the body practically unchanged they 

 are evidently not a source of energy through chemical change. 

 Their importance to the cell lies almost entirely in their physi- 

 cal or physico-chemical properties. The organic crystalloids, 

 although of nutritional value, also have physical properties in 

 some respects similar to those of the inorganic crystalloids, and 

 therefore to this extent they exert similar influences, but the 

 essential difference between the organic and the inorganic 

 crystalloids is that all the latter are electrolytes, while many 

 of the organic crystalloids that occur in cells are non-electro- 

 lytes. The importance of this distinction lies not in the utility 



