. 



NATURE AND ORIGIN OF LIFE. 15 



katabolic processes leads to increase of protoplasm, to 

 growth, and ultimately to reproduction. An excess of 

 katabolic over anabolic processes leads, on the contrary, 

 to reduction and to death. 



Like all physico-chemical processes, metabolism is 

 limited by definite conditions. The essential elements 

 must be present, and a sufficiency of food to balance 

 the waste. Water is another essential, since all the 

 chemical reactions really take place in solutions. Free 

 oxygen is also necessary for the burning of the food 

 material, except in the rare case of certain parasitic 

 organisms and bacteria, which can obtain it from com- 

 pound substances. Moreover, metabolism can only take 

 place at all within a somewhat narrow range of tem- 

 perature, the exact limits of \^hich vary with different 

 organisms. No metabolism is, of course, possible at 

 a temperature so high as to coagulate or destroy the 

 proteins, or so low as to stop chemical action. 



Most, perhaps all, of the processes of metabolism take 

 place with the help of special proteins, known as ferments 

 or enzymes, which have the property of facilitating and 

 hastening chemical actions. Just as a small trace of 

 platinum black will cause an indefinitely large amount of 

 hydrogen peroxide (H 2 O 2 ) to decompose into oxygen and 

 water, so a small quantity of ferment will cause an inde- 

 finitely large amount of carbohydrate, fat, or protein to 

 break up into simpler substances. Such ferments, which 

 are not themselves affected, which are not involved in the 

 end products of the actions they facilitate, are called 

 catalytic, and play a most important part in the mechan- 

 ism of life. 



In the foregoing pages we have seen that living organ- 

 isms contain no special vital elements differing from those 

 of non-living matter, and are actuated by no special vital 

 force. From the physico-chemical point of view life is 

 a process involving perpetual change in a complex of 

 elaborate compounds continually being built up and con- 



