CARBOHYDRATES 29 



large part of his nourishment directly from plant sources. The synthesis 

 of food from simi)le compounds is, therefore, a most fundamental opera- 

 tion, and it is the peculiar function of plants. The formation of glucose 

 may be taken as typifying this synthesis, although recent investigations 

 reveal that various sugar phosphates and sucrose are formed before 

 glucose (see p. 397). 



Carbon dioxide from the air and water from the soil are converted 

 in the leaves of plants into the various carbohydrates. Since sunlight 

 furnishes the energy required for the synthesis of carbohydrates, this 

 process is known as photosynthesis. The net result of the process is 

 often represented by the following equation : 



6C0o + 6H0O + 717.6 Cal. = CgHioOg + 60. 



In this equation C6H12O6 stands for a hexose sugar such as glucose. 

 Additional details are given in Chap. 15. 



Tiie most important point to note in connection with photosynthesis is 

 that energy, 717.6 Cal. for a gram molecule of hexose sugar (180 g.), is 

 required to cause the reaction to take place. The energy thus stored 

 becomes available to man and other animals when the carbohydrate is 

 oxidized in the body: 



Oxidation or respiration 



CgHioOe + 6O2 = 6CO0 + 6H2O + 683 Cal.i 



The supply of carbon dioxide furnished by animals and microorgan- 

 isms enables the plant to continue its life processes. Plants, animals, 

 and microorganisms are so interdependent that no one class could func- 

 tion in its normal manner without the activities of the other two. 



Preparation of Glucose. Commercial glucose is made from starch, 

 corn starch in the United States and potato starch in Europe. To pre- 

 pare a glucose sirup the starch is suspended in water containing a small 

 amount of hydrochloric acid (0.6 per cent) and is heated under pressure 

 until the solution fails to give a red color with iodine, at which point 

 the solution still contains a large proportion of partially hydrolyzed car- 

 bohydrate. The acid is neutralized, and the liquid is decolorized with 

 powdered adsorptive charcoal and concentrated to a thick sirup contain- 

 ing about 80 per cent solids. Large quantities of this flat sirup are 

 used in making candy. For table use, cane sirup is added to the fiat 

 sirup to give it a better flavor; this is the so-called "corn" sirup. Owing 

 to certain peculiarities that it possesses, glucose is, in certain respects, 



^ The ai)paront discfppancy between 717.6 and GS3 Cal. is due to differences in the 

 concentration and state of the reactauts and products in the two processes (see pp. 

 395 and 414). 



