8 BIOLOGY 



ical composition of any proteid. The following are the best- 

 known proteids: albumen, the white of an egg; myosin, the 

 lean part of the meat; casein, the curd of the milk; gluten, 

 the sticky substance in flour; legumen, a similar sticky material 

 present in peas and beans. Besides these, there are many 

 other proteids present in animal and plant tissues. Living 

 tissue is almost entirely proteid in character. 



Sources of proteids. Since living things are made up 

 largely of proteids, we next inquire into the source of these 

 proteids. As will be noticed later, green plants can combine 

 the gases of the air with the water and certain minerals obtained 

 from the soil, and thus manufacture their own proteids. Animals 

 and colorless plants (fungi) are totally unable to manufacture 

 proteids from inorganic compounds. Hence it follows that 

 animals and the colorless plants depend upon the green plants 

 for their proteids, which is simply another way of stating the 

 fact that animals require plants for their food. Although unable 

 to manufacture proteids, colorless plants and animals are, how- 

 ever, able to modify them more or less, having the power to 

 transform one kind of proteid into another. If, for example, 

 an animal is fed with the white of an egg, it can transform 

 this proteid into the proteid of muscle, thus changing albu- 

 men into myosin. Since animals are unable to manufacture 

 muscles from any substances but proteids, it follows that they 

 are obliged to have proteids in their diet. 



Carbohydrates. Starches and sugars are the best-known 

 examples of carbohydrates. They are much simpler than 

 proteids, consisting of only three chemical elements: carbon, 

 oxygen, and hydrogen. These elements are combined in mole- 

 cules with the following formulas: C 6 Hi 5 (starch) and C 6 Hi 2 O 6 

 (sugar). There is quite a large number of starches and sugars, 

 differing from each other in some respects, but these formulas 

 are typical of their general nature. It will be seen from the 

 formulas that the difference between the molecules of starch 

 and sugar is in the presence, in sugar, of H 2 in addition 



