130 COLLOIDS IN BIOLOGY AND MEDICINE 



Like plants, the animal organism has the power of changing carbohy- 

 drates into crystalloids. Ferments change the starches into sugar, 

 in fact cellulose which is so resistant to chemical attack is made 

 soluble in the intestine of vegetarians, so that it can enter the animal 

 body. As soon as the crystalloid forms of carbohydrate have passed 

 the intestinal wall they are transferred to the main depot, the liver, 

 where they remain in the stable colloidal condition as animal starch, 

 glycogen. We also find glycogen in most of the other organs, where- 

 as the mobile state of carbohydrate, grape sugar, occurs only in 

 minimal quantities (0.08 to 0.12 per cent), in fact only just so much 

 as is necessary for the production of energy. 



Fats, too, are found in the truly soluble form (e.g., soaps) in plants, 

 only in the germs of seeds; and in animals, probably only at the 

 moment when they pass through the wall of the intestines. They have 

 hardly passed the intestine when they immediately regain their colloidal 

 condition of emulsion, and are carried in that condition to their depots. 



The same statements hold for proteins. Crystalloid cleavage prod- 

 ucts are found in germinating seeds and in minimal quantities in the 

 vascular paths; in plants, asparagin; in animals, among others, 

 urea, uric acid and ammonia salts. The organism strives its utmost 

 to retain the colloidal condition. Hardly have the crystalloid 

 cleavage products of albumin which have been formed in the stomach 

 and intestines passed through the intestinal wall, than they are 

 straightway changed back into the colloidal form, so that their re- 

 turn may be cut oft 7 . The crystalloid combustion products are given 

 an avenue of escape through the kidneys. 



Physiological chemistry deals with the role of the carbohydrates, 

 fats and proteins apart from water and the inorganic salts. In the 

 study of biocolloids, water and salts cannot be neglected, because 

 water and salts are an indispensable part of the colloids; no colloid 

 can exist in the organism without them, because they condition the 

 turgescence which is characteristic of living colloid. 



In the case of cells with true membranes, salts may determine at 

 times the balance of osmotic pressure within and without the cell. 

 This general fact does not explain the necessity of the various 

 kinds of anions and cations (K, Na, Ca, Mg, Cl, SO 4 , PO 4 , CO 2 ); 

 the balance in osmotic pressure may be maintained by any non- 

 electrolyte (e.g., sugar) and yet a cell cannot be kept alive in an iso- 

 tonic sugar solution. Inorganic salts have specific relations to certain 

 organs to which we shall refer later; they are the expression of 

 characteristic sharply defined physical states assumed in the presence 

 of given quantities of water and salt, by the proteins, carbohydrates, 

 etc., of which the organs consist. 



