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, 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 (CH 2 O). The subsequent steps 
may be something as follows : 
2. Condensation of the formaldehyde molecules into 
a simple sugar, dextrose (CeH^Oe), by aldehydase. 
3. Transformation of dextrose into a more complex 
sugar, maltose, by maltase. 
4. The changing of maltose into dextrine by dextrinase. 
