146 TEXTBOOK OF BOTANY 



BEEF STEAK 



EGGS 



I WHEAT FLOUR 



J 



CORN MEAL 



I RICE 

 H H POTATOES 

 I ■ ■ 



28.3 ia.4 175 12.7 9.7 7.8 6.8 3.2 1.5 



Fig. 61. Percentage of protein in various foods. 



teins are not clearly understood. The best known of the intermediate 

 products are the amino acids. These are made by oxidation-reduction 

 processes from sugar and salts of nitrogen and in some cases of sulfur 

 also. In this process the carbon, nitrogen, and sulfur are chemically 

 reduced. The energy used in this process comes from the oxidation of 

 sugar or of some of its derivatives. 



Amino acids may be made in both green and non-green cells 

 of the plant and in either light or darkness. Apparently all the condi- 

 tions necessary for the process are present only in the cells of plants. 

 Animals and some of the fungi and bacteria are as dependent upon cer- 

 tain plants for some amino acids as they are for sugar. Some of our 

 common plants, such as corn and wheat, do not contain all the amino 

 acids necessarv for the health and growth of the human body, at least 

 in adequate amounts. This is one of the reasons why mixed diets are 

 desirable and necessary. 



After the amino acids are foraied, more than 100 amino acid molecules 

 become joined together by chemical condensation as a single large mole- 

 cule of protein. During this process phosphorus is sometimes joined in 

 the molecule. All protoplasmic proteins are insoluble in water and are 

 undoubtedly formed in the cells in which they occur. Hence this last step 

 in the chemical condensation of amino acids must occur in all cells of 

 all plants and animals. 



When proteins are digested back to amino acids, these acids may 

 become rejoined in quite a different order and in different proportions, 

 depending upon the enzymes and other conditions within the cells. Pro- 



