PHYSIOLOGY OF CELLS 57 



only in an acid medium and those of the trypsin class operate only in 

 an alkaline medium certain inferences may be made in regard to the 

 kinds of enzymes which are secreted by these cells. 



In plant cells carbohj^drates, fats, or proteins are rendered soluble 

 by the aid of digestive solutions containing enzymes resembling those 

 found in animals. The foods may be in direct contact with the proto- 

 plasm or contained in vacuoles or plastids. 



Digestion in higher animals is of two sorts, that which occurs within 

 cavities of special digestive organs and that which occurs within the 

 tissue cells of the body. Only the latter can be described here. The 

 reserve food of animals is stored in specialized connective tissue cells 

 which may be more or less scattered throughout the body, or localized, 

 as in the so-called fat-bodies. In addition to this major reserve there is 

 always a smaller supply of fat occurring as minute droplets in all tissue 

 cells. Reserve carbohydrates occur in tissue cells, chiefly as granules 

 of glycogen, a material which has the same empirical formula as starch. 

 Reserves of proteins are found in the protoplasm itself, or in an inactive 

 form in special storage bodies. None of these foods can be used for the 

 production of energy or for the up-building of protoplasm until they have 

 undergone digestion, a process which must necessarily occur within the 

 cell. This is accomplished by means of enzymes secreted by the proto- 

 plasm of the cells in which the reserve foods are being digested. Subse- 

 quent to digestion within the storage cells the products of digestion may 

 be transported to other parts of the body, but the account of this transfer 

 belongs in the chapter on organ physiology. 



Assimilation. — After foods have been rendered soluble and diffusible 

 they are absorbed. Absorption occurs through the protoplasm adjacent 

 to the vacuoles in Protozoa or through the protoplasm adjacent to the 

 reserve food in the cells of higher animals or plants. The products of 

 digestion pass through the protoplasm by osmosis, the protoplasm acting 

 as a membrane. Having been absorbed the simple substances produced 

 in the digestive process are recombined and built up into living proto- 

 plasm, perhaps through the agency of enzymes. This synthesis of foods 

 into protoplasm is assimilation. Very little is known as yet about this 

 important process. 



Respiration. — Living things almost Avithout exception require oxygen. 

 Oxygen is used in the organism in the decomposition of the constituents 

 of protoplasm and probably of stored proteins, carbohydrates and 

 fats. Certainly the proteins making up the protoplasmic mixture may be 

 oxidized, and much recent work goes to show that organic substances 

 other than proteins, namely, the carbohydrates and fats, may in like 

 manner be oxidized. This process of combining o?c}'gen with proto- 

 plasmic and other organic substances is true respiration. It occurs in 

 the protoplasm of the cell. Respiration in this sense is not to be confused 



