38 LIVING MATTER OB PROTOPLASM. 



The molecular instability of proteids is proven by the ease 

 with which they may be decomposed into simpler compounds ; 

 their complex constitution by the numerous compounds, them- 

 selves often highly complex, which may thus be derived or split 

 off from them. 



Amongst the other matters found in protoplasm those of 

 most frequent occurrence and greatest physiological importance 

 are two groups of less complex substances, viz., carbohydrates 

 and fats. These contain carbon, hydrogen, and oxygen, but no 

 nitrogen ; they do not appear to be closely related to proteids 

 in chemical constitution, but they occur to some extent almost 

 everywhere in living organisms, and in many instances are known 

 to be of great importance, especially in nutrition. They are rich 

 in potential energy and mobile in molecular arrangement ; hence 

 it is not strange that they figure largely in food, and are often 

 laid by as reserve food-materials in the organism. 



Carbohydrates. These substances are so called because, besides 

 carbon, they contain hydrogen and oxygen united in the same 

 proportions as in water. They include starch, various kinds of 

 sugar, cellulose, and glycogen. Starch (C 6 H 10 O 5 ) is readily con- 

 verted into glucose or grape-sugar (C f H 1B O 6 ) in the tissues of 

 plants and animals by the more or less direct influence of the 

 protoplasm a metamorphosis which, as will hereafter appear, is 

 of paramount importance in the process of nutrition. 



Fats, These are of especial importance as reserves of food- 

 materials (e.g., in adipose tissue and in seeds). They contain much 

 less oxygen than the carbohydrates ; are therefore more oxidizable, 

 and richer in potential energy.* They are insoluble in water, 



the agency of structures essentially proteid in nature that the chemical and 

 mechanical processes of the body arc effected. It is true that the proteids are 

 not the only organic constituents of the tissues and organs, and that there are 

 others, present in minute quantities, which probably are almost as widely dis- 

 tributed, such as for instance phosphorus- containing fatty bodies, and glyco- 

 gen, yet avowedly we can (at the most) only say probably, and cannot, in ref- 

 erence to these, affirm that which we may confidently affirm of the proteids 

 that they arc indispensable constituents of every living, active, animal tissue, 

 and indissolubly connected with every manifestation of animal activity." (Gam- 

 gee, Phyxiological Chemistry, Chap. I.) 



* According to recent researches, one pound of butter contains 5654 foot. 

 Ions, and a pound of sugar 2755 foot tons, of energy. A pound of proteid is 

 nearly equivalent in this respect to a pound of carbohydrate. 



