74 THE VERTEBRATE ANIMAL: METABOLLSM 



found in butter, lard, tallow, olive oil, and cottonseed oil. Fats are 

 readily oxidized in protoplasm, with the production of much heat. 

 Lipins differ chemically from fats chiefly in the fact that they con- 

 tain phosphorus and nitrogen, or nitrogen only, in addition to 

 carbon, hydrogen, and oxygen. The structural units in the 

 majority of cases are glycerol, fatty acid, phosphoric acid, and 

 some nitrogen base. Examples of lipins are lecithin, which is 

 abundant in the yolks of eggs, and the cerebrosides found in 

 nervous tissue. 



Proteins are very complex compounds of carbon, hydrogen, 

 oxygen and nitrogen, with traces, in almost all cases, of sulphur 

 and phosphorus, and sometimes of magnesium and iron. The 

 protein molecule is verj^ large, and is composed of structural 

 units known as amino acids, which are organic acids containing 

 nitrogen in an amino group (XHo). There are at least twenty 

 different amino acids, which are combined in different amounts 

 and groupings with one another and with other molecules, to form 

 many different types of proteins. Chemists have not yet suc- 

 ceeded in determining the exact composition of any protein mole- 

 cule, but by certain methods it is possible to estimate the empirical 

 formulae of some of them. For instance, fibrin, the protein of 

 clotted blood, has the approximate formula 



C640H1004O207N178S5; 



while casein, the characteristic protein of milk, is 



C708H1130O224N1S0S4P4. 



Gelatin and the albumin of eggs are examples of protein. In 

 animal protoplasm, proteins are the most abundant of the organic 

 constituents. 



Enzymes are substances whose chemical composition has not 

 been determined, but which act in the nature of organic catalysts. 

 Catalysts are substances that affect the rate of a chemical reaction 

 but do not appear among the end products of the reaction. The 

 classic example of inorganic catalysis is the effect of a minute 

 quantity of finely divided platinum upon the rate at which hydro- 

 gen and oxygen combine to form water. Enz\Tnes are produced 

 in all cells and are responsible for the autocatalysis which is char- 

 acteristic of protoplasmic reactions. Some cells, known as gland 



