VII. 

 PHYSIOLOGICAL CHEMISTRY. 



53. PROTEINS. 



General remarks. Physiological chemistry is that part of chem- 

 istry which has more especially for its object the various chemical 

 changes which take place in the living organism of either plants or 

 animals. It considers the chemical nature of the different substances 

 used as "food/' follows up the changes which this food undergoes 

 during its absorption and assimilation in the organism, and treats, 

 finally, of the products eliminated by it. The chemical changes tak- 

 ing place in the organism are either normal (in health) or abnormal 

 (in disease). The abnormal products formed under abnormal condi- 

 tions are generally termed " pathological " products. 



Of the three classes of organic compounds, viz., fats, carbohydrates, 

 and proteins, from which our food-supply is chiefly derived, the first 

 two have been considered in the part on Organic Chemistry, while the 

 study of proteins is taken up in this chapter. 



Occurrence in nature. Proteins form the chief part of the solid 

 and liquid constituents of the animal body ; they occur in blood, 

 tissues, muscles, nerves, glands, and all other organs ; they are also 

 found in small quantities in nearly every part of plants, and in larger 

 quantities in many seeds. They have never yet been formed by arti- 

 ficial means, but are almost exclusively products of vegetable and 

 animal life. 



General properties. The word protein (formerly proteid) re- 

 fers to a member of that group of substances which consist, so 

 far as is known at present, essentially of combinations of a-amino- 



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