Animal Cells and Their Nutrition - 131 



the food vacuole consists of an aqueous so- 

 lution of simple sugars, amino acids, glycerol, 

 fatty acids, etc. These substances can pene- 

 trate the vacuole membrane, and as a result 

 of digestion, each of the end products tends 

 to reach a relatively high concentration in 

 the vacuolar fluid. But these substances are 

 constantly consumed or altered by metab- 

 olism, and therefore they remain at a rela- 

 tively low level in the protoplasm surround- 

 ing the vacuole. Accordingly, each food 

 substance tends to diffuse spontaneously from 

 the vacuole into the cell. In some cases, how- 

 ever, absorption may be accelerated by active 

 transport mechanisms. 



The water and inorganic salts included in 

 the vacuole at the time of its formation are 

 also absorbed into the protoplasm, the former 

 very rapidly and the latter very slowly. Ini- 

 tially the pond water that is engulfed along 

 with the organic food is distinctly hypo- 

 tonic. Thus a newly formed vacuole tends to 

 shrink for several minutes, by losing water 

 to the surrounding protoplasm. But later the 

 vacuole enlarges, because the soluble end 

 products of digestion accumulate, making 

 the vacuolar fluid hypertonic to the proto- 

 plasm. Finally, however, the vacuole under- 

 goes a permanent shrinkage, owing to the 

 slow absorption of organic solutes together 

 with a quantity of water sufficient to keep 

 the vacuolar fluid isotonic with the proto- 

 plasm. Eventually just a small vestige of the 

 vacuole remains, and such an "old" vacuole 

 contains only a few residual granules of non- 

 digestible material. These digestive wastes 

 cannot be absorbed and must be ejected from 

 the cell. 



In higher animals, food substances are ab- 

 sorbed into the blood and lymph rather than 

 directly into the protoplasm; and among 

 animals generally, diffusion and osmosis are 

 important in the absorption of substances 

 from the digestive cavity, although other fac- 

 tors may modify the situation. In the higher 

 animals also, there is always a residuum of 

 nonabsorbable material that must be elimi- 

 nated from the digestive cavity. 



Egestion. Egestion represents the elimina- 

 tion of nonabsorbable materials from the 

 digestive cavity. In the amoeba, egestion is 

 essentially the reverse of ingestion. The 

 nearly empty food vacuole comes into con- 

 tact with the cell membrane at any point, 

 and suddenly the vacuolar granules are 

 spilled into the outside medium. In the Para- 

 mecium, an old vacuole circulates in the 

 cytoplasm until it reaches a fixed point, 

 called the anal spot (Fig. 7-3). At the anal 

 spot, which is situated near the posterior end 

 of the paramecium, the pellicle is relatively 

 weak, or perhaps absent; and egestion can 

 occur as it does in the amoeba. Higher ani- 

 mals generally possess a tubular digestive 

 passage, and egestion, or defecation, occurs 

 through the anal opening. 



The composition of the egestive wastes, or 

 feces, varies according to the food habits of 

 the particular animal. Herbivorous animals 

 always have large remnants of cellulose from 

 the woody parts of ingested plant tissues. 

 This potential source of glucose (from the 

 hydrolysis of cellulose) is partly lost, how- 

 ever, because cellulase is not included among 

 the digestive enzymes of most animals. Car- 

 nivorous diets leave smaller residues of in- 

 digestible materials, such as bone and gristle; 

 and in all animals small quantities of salts 

 and water escape absorption. 



Distribution. Distribution represents the 

 transportation of substances throughout the 

 body of the organism. In animals, absorbed 

 foods must be carried from the digestive 

 cavity to the other parts at a rate commen- 

 surate with the metabolic needs. Also, meta- 

 bolic wastes must be transported to the site 

 of elimination, and hormones and other sub- 

 stances must be carried from part to part in 

 the body. 



Distribution is not a very serious problem 

 in one-celled animals, because diffusion is 

 adequate to disseminate the foods and other 

 substances within such narrow limits. But 

 larger animals have developed circulating 

 fluids, such as blood and lymph, which accel- 

 erate the distribution. 



