INTRODUCTION 



BIOLOGICAL Chemistry or the Chemical Reactions 

 occurring in living cells presents a perplexing multi- 

 plicity and complexity of reactions. Not only does it 

 involve a knowledge of Organic Chemistry, but it also requires 

 an acquaintanceship with the laws of chemical reactions in 

 solution : that is, an appreciation of the statics and dynamics 

 of Chemistry. 



On surveying the whole field involved in these reactions 

 we see that the subject can be divided into two portions. 

 The first of these is associated with an accumulation of 

 available energy and is thus concerned with the synthesis of 

 organic material by green plants, under the influence of sun- 

 light. The second is concerned with the liberation of available 

 energy in performing the life processes of plants and animals. 

 The former is the constructive or anabolic phase ; the latter 

 is the destructive or catabolic phase. 



The above two phases constitute a cycle (see Frontispiece). 

 Starting with the simple inorganic substances, water and salts 

 from the soil, and carbon dioxide from the air, the plant cells 

 form carbohydrates, fats and proteins. During the activities 

 of plants and animals carbohydrates, fats and proteins are 

 decomposed, with the liberation of their available energy, 

 so that ultimately the simple inorganic substances are set 

 free once more. 



Examined more closely, the one main cycle can be sub- 

 divided into cycles involving different elements, such as one 

 for nitrogen, one for sulphur, etc., but each of these is dove- 

 tailed into the general description given above. 



The arrangement of the subject matter of this book follows 

 the lines indicated above. 



The first section consists of a few elementary chapters 

 giving a brief outline of the main groups of organic substances 

 found in cells, and a summary of the laws of chemical reactions 



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