144 TEXTBOOK OF BOTANY 



Condensation of amino acids. The last step in the process of protein 

 synthesis, the condensation of amino acids to proteins, is similar to the 

 condensation processes in the formation of complex carbohydrates and 

 fats. More than 100 molecules of amino acids^ become joined together 

 in one large molecule of protein. Since the organic matter in protoplasm 

 is composed largely of proteins or of substances derived from them, the 

 condensation of amino acids to proteins must occur in every young cell 

 of both plants and animals. Animals, however, must obtain amino acids 

 from plants thev eat, or from other animals that have eaten plants. 



Digestion of proteins. The digestion of proteins to amino acids is the 

 reverse of condensation. The process is catalyzed by enzymes in both 

 plants and animals.- The amino acids set free by digestion may be con- 

 ^'erted back to sugar, or they may be broken down by oxidation-reduction 

 to such simple compounds as CO2, Hi-O, NH.., and H^S. That is, the 

 carbon is oxidized, but the reduced nitrogen and sulfur in amino acids 

 and proteins are not oxidized by living organisms, except by special 

 groups of bacteria. 



Recondensation of amino acids. Instead of being oxidized, however, 

 the amino acids set free by the digestion of proteins are often recon- 

 densed to proteins again in both plants and animals. This time they may 

 be combined in a different order or in different proportions, or both, 

 and a different kind of protein is the result. When one considers that 

 there are at least 20 different kinds of amino acids, and that more than 

 100 molecules of amino acids are joined together in each protein mole- 

 cule, it is evident that the number of different kinds of proteins that 

 are mathematically possible is beyond comprehension. 



1 Molecules of amino acids are small in comparison to those of proteins: 



Glycocol ('2H5O2N Lysine C6H14O2N4 



Tryptophane C11H12O2N2 Cystine C6H12O4N2S2 



Amino acids may be converted temporarih to amides: 



Amino acid (aspartic) + Ammonia ^ Amide (asparagine) + Water 



HOOC— CHNH2— CH2-COOH + NH3 ^ H2X()( -CHNH2-CH2-COOH + H2O 



- Perhaps it should be noted that in order to keep this story as simple as possible refer- 

 ence is made only to proteins and amino acids. Some of the other products formed may be 

 indicated briefly: 



Amino a<-ids + NH3;=i Amides + H2O 



Amino acids ;=i Intermediate products ;;=i Proteins + IW) 



In both plants and animals amides may be formed temporarily by the union of amino 

 acids and ammonia. There are also partly digested products between proteins and amino 

 acids in solution in the cells. 



