6 PHYSICAL CHEMISTRY 



hydrogen and hydroxyl ions is nearly the same as in pure water. 

 A nearly neutral reaction is very necessary to the life of proto- 

 plasm. The fluids that bathe the majority of body cells, and the 

 waters in Which aquatic organisms live are very nearly neutral 

 in reaction, and are maintained in this condition by the presence 

 of carbonates and phosphates (buffers), as will be described later. 



The decomposition products of cells are proteins,' carbohydrates, 

 fats and fatlike bodies and neutral salts, as well as other sub- 

 stances that occur in small amount or which may arise from the 

 decomposition of the above. The proteins are composed of 

 amino acids. Amino acids have the general formula NH 2 .... 

 COOH, in which the COOH group has acid and the NH 2 group 

 (after hydration) has alkaline properties, so that the amino acid 

 may be nearly neutral. The proteins are formed by the union 

 of the COOH group of one amino acid with the NH 2 group of 

 another with the elimination of one molecule of water. The 

 number of amino acid molecules in one molecule of protein is 

 very large and hence the molecular weight of proteins is enor- 

 mous and is not very definitely settled in many cases. Before 

 the proteins of the food are taken into the cells they are broken 

 down into amino acids. Each cell builds up its characteristic 

 proteins, irrespective of the proteins given as food. 



The carbohydrates are aldehydes or ketones of polyhydric 

 alcohols. Because of their aldehyde character, they are more 

 easily burned in 'the body, and furnish most of the energy for 

 muscular work. Some of them undergo a molecular rearrange- 

 ment, giving rise to the lactone form without free aldehyde or 

 ketone groups, but these lactone forms are changed back to the 

 original type in digestion or metabolism. The carbohydrates are 

 stored in insoluble form, and transported in soluble form (sugar). 



The fats are esters (salts) of glycerine or other polyhydric 

 alcohols, and fatty acids. The fats in adipose tissue are saturated 

 and are insoluble in water. The fats of other cells are to a large 

 extent unsaturated, and many of them contain a choline and 

 phosphoric acid radical, which makes them capable of forming 

 a colloidal solution with water. These phosphorized fats aire 

 perhaps more correctly called phospholipids, but together with 

 cholesterine (a polyhydric alcohol) they have been called lipoids 

 by many physiologists. 



