400 



PLANT METABOLISM 



branch points in the amylopectin structure. The Q-enzyme, in conjunc- 

 tion with the P-enzyme, will form amylopectin: 



P-enzyme .-, 



glucose-1-phosphate < — amylose ■ ^'""' — > amylopectin 



Lipides. Lipides are the chief storage materials in seeds. Much more 

 energy is stored in a gram of lipide than in a gram of protein or carbo- 

 hydrate (see p. 423). When oily seeds mature, their fat content builds 

 up rapidly, chiefly at the expense of carbohydrate. Upon maturation, 

 the fat increases in unsaturation, and the constituent fatty acids increase 

 in average chain length. These changes in seeds are reversed when the 

 seeds germinate. The fact that almost all the fatty acids have an even 

 number of carbon atoms suggests that they originate from condensation 

 of C2 units. 



NITROGENOUS COMPOUNDS AND THEIR METABOLISM 



The animal requires an external supply of several amino acids (or 

 their keto acid analogues) to survive, but the plant can synthesize its 

 needed amino acids from inorganic sources of nitrogen, carbon dioxide, 

 and water. Lack of an adequate supply of nitrogen in the soil, more 

 frequently than the lack of any other element, limits the growth of 

 plants. As a result, nitrogenous fertilizers, and crop rotations including 

 nitrogen-fixing legumes, are of great importance in our agricultural 

 economy. 



Nitrogen nutrition 



Nitrate has been used as a fertilizer for centuries. Because it can 

 be employed in higher concentrations than ammonia, it often has been 

 considered inherently superior to ammonia for the nutrition of plants. 

 Actually, when ammonia is supplied in nontoxic concentrations, it nor- 

 mally supports a rate of growth in plants equal to or superior to that for 

 nitrate. The conditions for optimum utilization of the two compounds 

 differ, however; nitrate is used best at a pH below, and ammonia above, 

 neutrality. 



Urea and calcium cyanamide (CaCN2) are widely used organic fer- 

 tilizers which readily yield ammonia. Manures and composts serve as 

 sources of nitrogen for plants, and it is generally suggested that their 

 utilization by the plant is preceded by ammonifi cation and nitrification} 

 In the process of ammonification, bacteria convert the nitrogen of pro- 

 teins and other compounds to ammonia. This is followed by the action 



^ Nitrification is the process of converting ammonia to nitrites, or nitrites to nitrates, 

 or botli. 



