Animal Cells and Their Nutrition - 129 



Gradually the pseudopodia extend around 

 the food, forming a food cup (Fig. 7-1), and 

 finally, when the lip of the cup closes, a food 

 vacuole is formed. This primitive digestive 

 cavity now contains a quantity of pond water, 

 together with the organic food, which is to 

 be digested (Fig. 7-2). Essentially, in short, 

 ingestion by the amoeba represents a type of 

 phagocytosis, such as was described previ- 

 ously (p. 121). 



Fig. 7-2. Amoeba ingesting a living ciliate (dark oval 

 body). The pseudopodia are about to close around the 

 prey. The magnification is so great that only a small 

 part of the amoeba can be seen. Photograph retouched 

 to clarify the outlines. (Encyclopedia Britannica Films, 

 Inc.) 



Ingestion is the process by which an ani- 

 mal takes food into its digestive cavity. Al- 

 though the method of ingestion varies in dif- 

 ferent animals, the end result is always the 

 same. A paramecium (Fig. 7-3) swims about 

 by means of its cilia, which also serve to 

 sweep bacteria and other finely suspended 

 particles of organic food along the oral 

 groove, down the gullet, and into the food 

 vacuole (Figs. 7-3 and 7-4). Or a frog catches 

 a fly with a swift thrust of the tongue and 

 carries its prey into the mouth for swallow- 



ing. Or a man manipulates his food with im- 

 plements and introduces it into the upper 

 part of the digestive tract. Whatever the 

 method, the result is quite the same. Food is 

 taken into some special cavity where diges- 

 tion will begin. 



Digestion. The crystalloidal components 

 of the amoeba's food, such as water or salt, 

 do not require digestion. Such foods are 

 absorbed directly from the digestive cavity 

 into the surrounding protoplasm, without 

 preliminary preparation. But the colloidal 

 molecules of proteins, polysaccharides, fats, 

 etc., cannot penetrate the vacuolar mem- 

 brane; and such molecules must be frag- 

 mented before they can be absorbed into the 

 protoplasm. In every typical animal, diges- 

 tion is a series of hydrolytic decompositions, 

 promoted by enzymes secreted into the di- 

 gestive cavity, whereby the initially nonab- 

 sorbable (colloidal) components of the food 

 are converted into absorbable (crystalloidal) 

 end products. 



Digestion in the amoeba begins as soon as 

 digestive enzymes are secreted by the proto- 

 plasm into the food vacuole. Only traces of 

 these enzymes are necessary. The digestive 

 enzymes of amoebae and other animals in- 

 clude several proteases. These proteases hy- 

 drolyze the proteins of the engulfed food, 

 liberating amino acids that dissolve in the 

 vacuolar water. Likewise there are carbohy- 

 drases, which act upon carbohydrates, digest- 

 ing them mainly into glucose, and lipases, 

 which convert the fats into glycerol and 

 fatty acids. All these end products of diges- 

 tion are quite soluble in the aqueous content 

 of the vacuole and all are capable of pene- 

 trating the vacuole membrane. 



The digestive enzymes of lower animals, 

 such as the amoeba, and those of higher ani- 

 mals, including man, display many similari- 

 ties. In the amoeba, during the first 10 min- 

 utes of digestion, the content of the food 

 vacuole displays a distinctly acid reaction, 

 indicating that some acid is secreted into the 

 vacuole along with the enzymes. This acid 

 phase of digestion in the amoeba has its 



