38 THE CELL 



C. DIGESTION 



The solids taken up in this way by ihe cells must undergo various changes 

 in order that they may be of use to the cell. Often this is true also with 

 the dissolved foods, gill such processes by which the foods are changed so 

 that they may be assimilated or further elaborated by the cell are included 

 under the term digestion. 



The digestion accomplished by the cell is either extra- or intracellular. 

 In the former case digestion takes place under the influence of special sub- 

 stances, the enzymes, formed by the cell. These substances, like catalytic 

 agents, have the property when present in very small quantity of producing 

 chemical changes in great quantities of complex molecules, thereby splitting 

 them into simpler compounds, which in their turn reduce the activity of the 

 enzymes. By the action of enzymes proteid is split into albumoses and pep- 

 tones, starch into sugar, and fat into glycerin and free fatty acids (cf. Chapter 

 VII). It should be noted further that every special enzyme acts only upon 

 a definite compound or group of compounds. The proteid-splitting en- 

 zyme therefore does not act upon starch, nor the starch-splitting enzyme 

 on fats, etc. 



It is quite possible that the enzymes can produce their effects inside the 

 cell just as well as in the surrounding medium, and it is very doubtful whether 

 an intracellular digestion takes place anywhere without the help of enzymes, 

 though such participation cannot be definitely asserted. 



Enzymes which have the property of dissolving proteids just like the 

 corresponding enzymes in the digestive fluids have been obtained from finely 

 minced organs spleen, lymph glands, kidneys, liver, heart, etc. It is assumed 

 that they are present in the living cells, though this has not been finally 

 proved. Nothing definite can be said at this time with regard to the impor- 

 tance of such enzymes in the normal processes of the body. It is possible that 

 in starvation they may effect the solution of the tissue proteids, also that the 

 autolytic processes which take place after death are initiated and carried on 

 by such enzymes. 



The enzymes are products of cell- activity., but once they are formed they 

 act entirely without the help of the cell and are nonliving substances. In 

 general it is supposed that they are proteid in nature, and in fact Pekelharing 

 has prepared from the stomach of the dog a very pure enzyme (pepsin) which 

 was free of phosphorus and had the constitution of proteid. But it would 

 be premature to draw any general conclusions from a single observation. 



Enzymes occur in the cells for the most part in the form of precursors, 

 known as zy mo gens. Often the zymogen is changed into the active enzyme 

 in the act of secretion; or its activation may be brought about under the 

 influence of another enzyme. 



Enzymes are only slightly diffusible, but they pass through a porcelain 

 filter and can be separated in this way from the cell fragments of an extract 

 or juice. In the dissolved state they withstand heating up to 70 C. ; in the 

 dry state many enzymes are not destroyed by a temperature of over 100 C. 

 In general they are most powerfully active at 35 to 45 C. ; at a lower tern- 



