ABSORPTION OF NITROGEN COMPOUNDS FROM SOILS 439 



TABLE 42 THE PRINCIPAL AMINO ACIDS 



Glycine CHjCNH,) ■ COOH 



Alinine CHa-CHCNH^) COOH 



Serine CHoOH ■ CHCNH.) • COOH 



Threonine CH3-CH(OH)CH(NH2)-COOH 



Valine (CH3)2-CHCH(NH2)COOH 



Leucine (CH3)2-CH-CHrCH(NH2)-COOH 



CH 

 Isoleucine ' ^CH • CHCNH^) ■ COOH 



C H ^ 



Aspartic acid HOOC-CHa-CHCNH.) -COOH 



Glutamic Acid H00CCHo-CH2CH(NH,)-C00H 



Arcrinine HN=C(NH2) -NHCHsCHjCH.-CHCNHa)- COOH 



Lysine CHo(NH.)CHoCH2CH2-CH(NH2)-COOH 



Cystine SCHo-CHCNH,) -COOH 



S-CH2CH(NH2)-COOH 



.S-CHs 



Methionine CH2\ 



\CH2CH(NH2)-COOH 



Phenylalanine C6H5-CH2-CH(NH2)-COOH 



Tyrosine HO-C6H4-CH2CH(XH2)-COOH 



Histidine HC=^C-CH2-CH(NH2)-COOH 



N NH 



H 



Tryptophane ^CHk 



HC C C-CH2-CH(NH2)COOH 



HC C CH 



"^ch/" ^nh/ 



Proline H2C CH 



2 



H2C chcooh 

 \n/ 



H 



kinds of compounds as sources of nitrogen: (i) nitrates, (2) nitrites, (3) 

 ammonium salts, and (4) organic nitrogen compounds. The mechanism of 

 the absorption of ionic forms of nitrogen is believed to be essentially similar 

 to that involved in the intake of other ions (Chap. XXIV). Nitrates ap- 

 parently can be absorbed by many kinds of plant cells against a concentration 

 gradient. 



Most plants absorb most of their nitrogen in the form of nitrates. Nor- 

 mally metabolizing plants usually contain only relatively small quantities of 



