CARBOHYDRATES AND FATS. 37 



Amongst the other matters found in protoplasm or closely 

 associated with it those of most frequent occurrence and greatest 

 physiological importance are two groups of less complex sub- 

 stances, viz., carbohydrates and fats. These contain carbon, hy- 

 drogen, 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, espe- 

 cially in nutrition. They are rich in potential energy and mo- 

 bile in molecular arrangement ; hence it is not strange that they 

 figure largely in food, and are often laid by a$ reserve food- 

 materials in the organism. 



B. 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 giycogen. Starch (C 6 H 10 O 5 ) is of 

 very frequent occurrence in plant-cells, where it appears in the 

 form of granules embedded in the protoplasm (Fig. 9). Cel- 

 lulose, having the same chemical formula as starch, but quite 

 different in physical properties, almost invariably forms the basis 

 of the cell-membrane in plants. 



C. FATS. These are of especial importance as reserves of 

 food-materials (e.g., in adipose tissue and in seeds). They con- 

 tain much less oxygen than the carbohydrates ; are therefore 

 more oxidizable, and richer in potential energy.* They com- 

 monly occur in the form of drops suspended in the protoplasm 

 (Fig. 17), and are especially common in animal cells, though by 

 no means confined to them. 



Physical Relations. The appearance, consistency, etc., of 

 protoplasm have already been described ; but it still remains to 

 speak of certain of its other physical properties, and especially 

 of the manner in which its activity is conditioned by various 

 physical agents. 



Relations of Yital Action to Temperature. It is a general 

 law that within certain limits heat accelerates, and cold dimin- 

 ishes, the activity of protoplasm. AVe know that cold tends to 



* According to careful researches, one pound of butter contains 5654 foot- 

 tons, 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. 



