DEFINITION. 



THE substances composing the organic material connected with the 

 phenomena of life, and the great majority of the products of vital 

 activity, are mixtures of compounds of the element Carbon. 



From these substances the chemist has isolated numerous pure 

 carbon compounds and prepared others ; he has also synthesised carbon 

 compounds from their elements. 



About 1 50,000 carbon compounds are now known. The possibility 

 of their existence is due to the unique property which the element 

 carbon possesses of being able to combine with itself; compounds are 

 known which contain in their molecules from one up to sixty atoms 

 of carbon directly joined together. 



Of these 150,000 carbon compounds only a small number are 

 directly concerned in vital processes. 



The chemistry of all the carbon compounds is termed organic 

 chemistry. 



The chemistry of those carbon compounds which are the con- 

 stituents of living matter and are concerned in vital processes is 

 physiological or biological chemistry ; the term physiological chemistry 

 more frequently refers to the compounds and their functions in 

 animals ; the term biological chemistry comprises the compounds and 

 their functions in both plants and animals. The changes which they 

 undergo and the functions which they fulfil in the living plant or 

 animal form the subject of chemical physiology. 



Though a distinction can be made between biological chemistry 

 and chemical physiology, the two subjects are so closely interrelated 

 that they are essentially only different aspects of the same subject. 

 No biological change can be followed until a knowledge of the chemical 

 properties of the substances involved has first been acquired. Chemi- 

 cal physiology is thus dependent on biological chemistry, or Bio- 

 chemistry. Bio-chemistry is the branch of organic chemistry which 

 deals with the natural organic compounds and with the functions of 

 these compounds in nature. 



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