EARLY HISTORY OF LIFE 

 H H O H H 



\ I I! \ I II 



N— C— C N— C— C 



43 



H 



H 



H 



H OH 



H 



H OH 



glycine 



H 



N— C— C 



H 



/ 







H 



glycine 



H 



N— C— C 



H 



/ 



O 



+ H,0 



OH 



glycyl-glycine 



In this manner the various amino acids 

 are tied together to form molecules that 

 range in size from several hundred to 

 several thousand atoms. Their size can 

 be estimated by the relative molecular 

 weights. For example, glycine has a molec- 

 ular weight of 75, whereas that of the 

 oxygen-carrying protein of the crayfish's 

 blood (hemocyanin) is over 5 million. Ob- 

 viously, by combining the 25 amino acids 

 in various ways, limitless numbers of dif- 

 ferent proteins can be formed. That is why 

 the proteins of every animal or plant difFer 

 from those of every other animal or plant. 

 It is a well-known fact that proteins of one 

 species of animal cannot be exchanged with 

 those of another. For example, it is impos- 

 sible to transplant skin, let us say, from the 

 back of a dog to the face of a man because 

 the proteins that go to make up the skin 

 are different in both man and dog. Think 

 what startling surgery could be performed 

 if this were possible! 



Proteins are the most characteristic and 

 most abundant material (exclusive of wa- 

 ter) in protoplasm. Besides forming the 

 actual supporting structure of protoplasm, 

 proteins afford excellent material in which 

 the large variety of chemical reactions es- 

 sential for life take place. 



Enzymes. Enzymes have been mentioned 

 from time to time in the foregoing pages 

 without an explanation of what they are; 

 that must be clarified now. Enzymes are 

 frequently spoken of as organic catalysts, 

 which may be defined as substances that 



hasten a chemical reaction but are not 

 themselves consumed by the reaction. The 

 numerous enzymes present in all proto- 

 plasm are responsible for the complex re- 

 actions that go on so smoothly within every 

 cell. Without enzymes protoplasm loses its 

 ability to start and maintain the multitude 

 of activities that go on within it. They are 

 absolutely essential in the business of liv- 

 ing but the details of their operation are 

 only now beginning to be understood. 



A simple experiment will demonstrate 

 how an enzyme works. A watery starch so- 

 lution is placed in two test tubes, to one of 

 which is added saliva, to the other, nothing. 

 Any attack on the starch can be detected 

 by using an appropriate test for maltose, 

 one of the products of polysaccharide 

 breakdown. Within a few minutes this 

 sugar can be demonstrated in the tube con- 

 taining saliva, whereas in the tube without 

 it there will be no maltose for many hours. 

 The enzyme ptyalin brings about this reac- 

 tion without fuss or furor. The same break- 

 down can be accomplished without the 

 enzyme, but it requires drastic treatment 

 with strong acids at high temperatures, 

 conditions that could not be tolerated by 

 protoplasm. It is obvious, then, that en- 

 zymes can bring about a difficult chemical 

 change at body temperatures and in a very 

 short period of time. All of this is essential 

 if the many reactions that go on in proto- 

 plasm are to occur as quickly and smoothly 

 as they do. 



In order that some appreciation may be 



