442 NITROGEN :VlETABOLISAI 



Synthesis of Amino Acids. — Amino acids are synthesized in plant cells 

 from nitrogenous compounds absorbed from the soil and from carbohydrates 

 or their derivatives which have been fabricated in the plant. In the pre- 

 ceding section our attention was focussed on the stages in the reduction 

 of absorbed nitrates. However, on the average about 85 per cent by weight 

 of amino acid molecules is derived from carbohydrates or closely related 

 compounds. It is evident, therefore, that either a shortage of nitrogenous 

 compounds or a deficiency of carbohydrates may retard amino acid synthesis 

 in plant tissues. 



Very little is actually known of the chemical mechanism whereby amino 

 acids are synthesized in plants. It is generally considered that ammonia 

 plays a key role in this process. This compound usually originates in plant 

 tissues from the reduction of nitrates. Except under unusual metabolic 

 conditions ammonia is present in plant cells only in traces, being apparently 

 utilized in the formation of other compounds as fast as it is produced. 



It is also generally believed that certain fatty acids represent an inter- 

 mediate step between carbohydrates and amino acids in the synthesis of the 

 latter. For example aspartic acid, one of the commonest of plant amino 

 acids, might be synthesized as follows : 



HOOC • CH=CH • COOH + NH3 -^ HOOC • CH2 • CH(NH2) • COOH 



Fumaric acid Ammonia Aspartic acid 



Other kinds of amino acids may be built up as a result of similar reactions. 

 It is probable that enzymes play a catalytic role in all such syntheses. The 

 synthesis of amino acids in plants is usually accompanied or preceded by the 

 synthesis of asparagine and (or) glutamine (see later). 



Cystine is the only amino acid containing sulfur which has been obtained 

 by the hydrolysis of plant proteins. This compound therefore plays an 

 important role in the sulfur metabolism of plants. Sulfur always occurs 

 in cystine and other plant sulfur-containing organic compounds in reduced 

 forms. Plants obtain most of their sulfur in the form of sulfates absorbed 

 from the soil, but nothing is known of the chemical mechanism whereby the 

 sulfate ion is reduced in plant tissues. 



Amino acid synthesis apparently can occur in most living plant cells. In 

 some species of plants, however, reduction of nitrates and amino acid synthesis 

 takes place principally in the smaller roots, little if any occurring in the 

 aerial organs of the plant. In other species reduction of nitrates and synthesis 

 of amino acids occurs predominantly in the aerial organs (Nightingale, 1937). 

 Elxamples of plants belonging to the first group are apple, asparagus, and some 

 grasses; examples of species belonging to the second group include peas, 

 soybeans, and tomatoes. 



