88 Introductory Biology 



organs as the kidneys help to hold them in place. Fats are not utilized 

 as a source of body energy as readily as are the carbohydrates. Fats 

 can be formed from carbohydrates by the body, and, to a limited extent, 

 fats can be converted into usable glucose. Fats of the animal body are 

 derived from the carbohydrates and fats consumed as foods. Fats occur 

 in butter, cream, oils, meats, seeds, and nuts. 



C. Proteins 



Proteins (pro'tein) (Gr. protos, first) are complex organic com- 

 pounds which contain carbon, hydrogen, oxygen, and nitrogen, and 

 usually sulfur and phosphorus. Proteins are present in all protoplasm and 

 are characterized by the element nitrogen. It is thought that there are 

 specific proteins for each species of living organism and that each living 

 organism probably has several specific and unique types. The theory of 

 species specificity states that due to the constituent proteins, the proto- 

 plasm of each species of living organism is specific for that species and 

 differs less slightly from that of related species and markedly from that 

 of more distantly, or unrelated, species. Studies along these lines have 

 substantiated the evidence of evolutionary relationships between certain 

 organisms which has been derived from other facts. 



Proteins are the most varied and complex of all the constituents of 

 protoplasm, each molecule being made of hundreds of atoms. Proteins 

 contain such elements as: 



The units of which proteins are made are amino acids, which contain 

 an amino group (NH2) and an acid (carboxyl) group (COOH). There 

 are over thirty different amino acids known. The great variety of pro- 

 teins is made possible by the various combinations and proportions of 

 the amino acids used in their construction. The various proteins are 

 formed by joining the acid group of one with the amino group of an- 

 other amino acid. The protein can act as an acid, and thus com- 

 bine with alkalis, because of its acid group (COOH) and can act as 

 an alkali, and thus combine with acids, because of its amino group 



*Present only in certain types of proteins. 



