NUTRITION 79 



78. Enzymes. — For a long time it has been known that 

 during photosynthesis plants take in carbon dioxide and 

 water and give off oxygen, but the intermediate steps 

 have never been clearly understood. The appearance of 

 starch in green tissues is an evidence that photosynthesis 

 has taken place, but it was early recognized that starch 

 was not the first organic substance to be formed. It is 

 now known that some of the various steps in the process 

 are accomplished by means of certain substances called 

 enzymes, formed in every cell. Enzymes have the re- 

 markable power of transforming other substances, with- 

 out being thereby used up or permanently changed them- 

 selves. They belong to the class of substances known as 

 ferments J but their real nature and mode of action are not 

 well understood. Each enzyme is commonly named from 

 the particular substance in the transformation of which 

 it takes part, and this name usually ends with the termi- 

 nation, -ase. Thus we have oxidase, which acts upon 

 substances to oxidize them, maltase, which acts upon mal- 

 tose (a form of sugar), protease, which acts upon protein, 

 and so on. 



79. The Steps in Starch-formation. — Careful experi- 

 ments have suggested that the first step in the formation 

 of starch may be the interaction of water and carbon 

 dioxide, under the agency of an oxidase, resulting eventu- 

 ally in formaldehyde (CH2O). The subsequent steps 

 may be something as follows : 



2. Condensation of the formaldehyde molecules into 

 a simple sugar, dextrose (C6H12O6), by aldehydase. 



3. Transformation of dextrose into a more complex 

 sugar, maltose, by maltase. 



4. The changing of maltose into dextrine by dextrinase. 



