42 ELEMENTS OF BIOLOGY 



Plants are, however, able to use nitrogen in the form of nitrites 



potassium nitrite potassium nitrate 



^ -A. 



(KNO2) and nitrates (KNO3). Proteins also contain hydrogen, 

 carbon, and oxygen, which are also the constituents of carbo- 

 hydrates. Plants are able to construct proteins by utilizing the 

 nitrogen from the soil nitrates and the substances in carbohydrates, 

 the energy for the reactions being afforded by the carbohydrate 

 molecules. Animal cells are utterly unable to perform this synthesis 

 and are consequently dependent on plants for the proteins from 

 which animal proteins are formed. 



The chemical processes and changes involved in the building up 

 of complex protein molecules by the metabolism of plants are intri- 

 cate; they may be given here only in their most simple terms. We 

 have seen how in the process of photosynthesis a carbon atom is 

 added to a water molecule to form a simple carbohydrate unit, 

 CHOH. Now if an atom of oxygen is added the result contains a 

 carboxyl radical, COOH, which was shown elsewhere (p. 14) to be 

 present in all amino-acids and hence in all proteins. Thus in the 

 simplest case a fatty acid may be formed by the addition of an 

 oxygen atom and combining of three molecules of CHOH; the re- 

 sult is CH3COOH, acetic acid, the acid of common vinegar. Now 

 suppose that one of the H atoms is replaced by a nitrogen-contain- 

 ing radical, NH2. The result is CH2NH2COOH, known as amino- 

 acetic acid, or glycine, or glycocoU; it is the simplest of all amino- 

 acids. In a similar fashion other and more complex amino-acids are 

 formed and are united to compose protein molecules. Thus the 

 processes of protein construction include the formation of carbo- 

 hydrates by photosynthesis, an oxidation, the addition of NH2, and 

 the union of the resulting amino-acids into protein molecules. 



Plant Nutrition and Animal Nutrition. The nutrition of 

 green plants is therefore an essentially constructive process, includ- 

 ing the synthesis of energy-containing carbohydrates and the build- 

 ing up of proteins; it is known as holophytic nutrition. In con- 



