110 - The Cell 



B 



Fig. 5-5. Digestion of starch by saliva. A, two beakers 

 were filled with an opaque suspension of starch in 

 water. Some saliva (which contains ptyalin) was added 

 to the left beaker only. B, later, the saliva has digested 

 the starch, converting the large molecules into smaller 

 molecules of sugar, rendering the solution transparent 

 in the left beaker, while the solution in the right beaker 

 remains unchanged. (From Digestion of Foods, Encyclo- 

 pedia Britannica Films, Inc.) 



The thermal behavior of enzymes imposes 

 serious limitations upon organisms generally. 

 Most cells lose their capacity to carry on me- 

 tabolism at temperatures above 40° C. A 

 few organisms possess enzymes that are espe- 

 cially resistant to heat, and only such organ- 

 isms are able to survive in exceptionally hot 

 places. In most cells, the rate of metabolism, 

 and hence the intensity of the life processes, 

 changes as the temperature varies from day 

 to day, and from season to season. Thus the 

 winter metabolism of most organisms sub- 

 sides to a point where dormancy is inevi- 

 table. Only "warm-blooded" organisms, such 

 as man and a few other vertebrates, have 

 evolved a method of controlling their body 

 temperature. In man, for example, the tem- 

 perature seldom fluctuates more than a few 

 degrees above or below 37.2° C. Accordingly 



the numerous enzymes of our tissues operate 

 at optimum efficiency, and our digestive and 

 metabolic reactions proceed on schedule, de- 

 spite fairly drastic changes in the environ- 

 mental temperature. 



ENZYMATIC NATURE OF GENES; 

 AUTOCATALYSIS 



Fundamentally the character of each cell 

 must be determined, at least in large meas- 

 ure, by its metabolism; and the metabolism, 

 in turn, must be determined by a particular 

 set of enzymes, which are inherited by the 

 cell. In some fashion, therefore, the genes of 

 an organism must determine the nature of 

 its intracellular enzymes. These considera- 

 tions, of course, are tremendously important 

 and they will be approached more closely 

 later (Chap. 28). 



It has been assumed for many years that 

 genie substances possess a unique property, 

 namely autocatalysis. This is the capacity of 

 a substance to catalyze the production of 

 itself. Such self-replication, it was thought, 

 provided a basis for understanding how each 

 cell, via the mechanisms of mitosis, could 

 perpetuate its own characteristics. However, 

 the mechanism of genie action — how a par- 

 ticular set of genes achieves the production 

 of a particular set of enzymes — remained ob- 

 scure. 



Currently, however, much progress in this 

 field is taking place. Now it is possible to 

 identify the genes as DNA proteins, each 

 characterized by a distinctive pattern in the 

 arrangement of its nucleotide constituents (p. 

 134). When self-replication occurs, moreover, 

 the pattern of each gene provides a template 

 (p. 134) for the production of an equivalent 

 unit. But in addition, each genie pattern 

 provides a code for the production of sim- 

 ilarly patterned RNA proteins, which have a 

 site of action in the cytoplasm (p. 525). And 

 finally, the pattern of each different RNA 

 component carries the code for the synthesis 

 of some one of the specific enzyme proteins 

 in the cell (p. 529). The genes, accordingly, 



