PART II. 

 PHYSIOLOGICAL CHEMISTRY. 



INTRODUCTION. 



PHYSIOLOGICAL Chemistry, Chemical Physiology, or Bio-chemistry, is 

 the subject which treats of the chemical processes connected with life. 

 It comprises a study of the chemical constitution of the various tissues 

 and of the chemical nature of the interchanges undergone by the food- 

 stuffs in their passage through the organism. 



The chlorophyll in the leaves of green plants absorbs certain of the 

 spectral rays of sunlight (the red, yellow and orange) and utilizes the 

 absorbed energy to bring about a reduction of carbon dioxide and 

 water. In this process oxygen is evolved and there is formed a carbo- 

 hydrate in which the energy absorbed from the light becomes locked 

 up in potential form, as the carbohydrate can again combine with 

 oxygen with the liberation of energy. A chemical synthesis is said 

 to occur, and although simple carbohydrates are invariably the first 

 products of this synthesis that we can isolate, yet, by further chemical 

 transformations of the same nature, more complex carbohydrates, fats 

 and proteins are evolved. 



Animals eat the products of plant life and decompose them so as 

 to liberate the potential energy, that is to say, to convert it into 

 kinetic energy, which is then used in the functions of the animal 

 body. The ultimate source of animal energy is, therefore, certain of 

 the sun's rays. In thus decomposing the large molecules supplied 

 them by the plant animals absorb oxygen and evolve carbon dioxide 

 which again the plants absorb and thus complete the cycle. 



Plants with no chlorophyll such as the parasites and saprophytes, 

 etc. cannot perform these syntheses, but like animals they absorb 

 oxygen, decompose complex molecules and liberate carbon dioxide, 

 Even green plants exhibit this latter process, although in day light 

 it is masked by the more active synthetic changes. In the daik, 

 however, green plants behave like chlorophyll-free plants. 



