THE FUNCTIONS OF PROTOPLASM AND CELLS 41 



Among the lipids are the true fats, such as butter fat, oHve oil, and 

 the fat of beef or pork. True fats are composed entirely of carbon, 

 hydrogen, and oxygen, with the proportion of oxygen much lower than 

 in carbohj^drates. The natural fats have large molecules — around 50 

 atoms of carbon, double as many of hydrogen — but only 6 atoms of 

 oxygen. They are a combination of 1 molecule of glycerol (commonly 

 called glycerin) with 3 molecules of fatty acid. There are a number of 

 different fatty acids characteristic of different fats, some of them used 

 commercially in water emulsions to produce the brushless kinds of 

 shaving cream. 



In other types of lipids there may be more than the three elements 

 which true fats contain. Lecithin, which includes phosphoric acid and 

 another substance in place of one of the fatty acid molecules, is abundant 

 in egg yolk and is probably present in all cells as part of the proto- 

 plasmic structure. Cholesterol, which is foiuid in bile and is a source 

 of gallstones, consists only of carbon, hydrogen, and oxygen, but the 

 carbon in it forms a "skeleton" j^n rings instead of straight chains as in 

 the fats. 



Most significant of the organic compounds are the proteins, because 

 it is they that make one kind of living thing so sharply and definitely 

 different from others. Aside from water, they are the most abundant 

 substances in protoplasm — about 15 per cent of the total mass. Proteins 

 are especially characteristic of lean meat (muscle) but are distributed 

 through all cells. They do not diffuse readily through other substances 

 but alloAV some, though not all, other substances to diffuse readily 

 through them. Chemically they are compounds of the amino acids, a 

 group of 25 different organic acids. A generalized formula of amino 

 acids is R— CH(NH2)-C00H, in which R stands for the "body" of the 

 molecule, different in each of the 25 acids. The rest of the formula 

 applies to all of them. The COOH makes them organic acids, the NH2 

 makes them amino acids. In the simplest amino acid, glycine, R is 

 simply an atom of hydrogen, H; in the next simplest, alanine, R is the 

 radical CHg. These amino acid molecules may be joined with one 

 another, as carbohydrate molecules are joined, with the loss of a molecule 

 of water at each junction. The more complex of these combinations 

 are the proteins. The molecules of proteins are relatively huge, con- 

 taining hundreds or even thousands of atoms. With such large molecules, 

 which may include varying proportions of most of the amino acids, and 

 frequently carbohydrates or lipids, there may be an enormous number 

 of kinds of proteins. 



Enzymes. — Many chemical reactions are greatly hastened by the 

 presence of certain chemical substances which do not enter into the 

 reaction in a definitive way. Hydrogen peroxide (H2O2) is stable enough 



