FROM SINGLE CELLS TO MANY CELLS 



81 



FOOD 

 O 



UREA 



Fig. 4-12. The problem of consuming food, digesting it, burning it, and excreting the waste products is essen- 

 tially the same at all levels of animal organization. Here it is compared in the single-celled animal, amoeba, 

 the simple metazoan, hydra, and in a highly complex form such as man. The problem is always reduced to the 

 level of the cell and must be solved at that level in all animals. The letters EP mean end products and E means 



enzymes. 



then is forced to develop a breathing sys- 

 tem in combination with a transportation 

 system, which would make it possible for 

 oxvsen and carbon dioxide to ^et directly to 

 and from each indi\'idual cell. In aquatic 

 forms such as fish, the breathing organs are 

 gills; in land forms such as man, they are 

 lungs. Both must meet certain requirements 

 if they are to function as breathing organs. 

 They must be constructed so that oxygen in 

 the surrounding medium (water or air) 

 can pass readily into a transporting medium 

 (blood) which will convey the gas to each 

 cell in the body. This means that the blood 

 must flow in thin-walled tubes ( capillaries ) 

 very close to the external environment in 

 order that the exchange can be readily 

 accomplished. Microscopic examination of 

 a gill filament or a lung sac will reveal that 

 this situation is met beautifully. Blood flow- 

 ing through thin-walled capillaries passes 

 within two cells of the outside world ( Fig. 

 18-19), which makes the gaseous exchange 

 possible and simple. 



The transporting system delivers the oxy- 



gen to the cells of the body and collects the 

 carbon dioxide from them. Here again the 

 circulating blood must come in close prox- 

 imity to every single cell of the body in 

 order that at no time wfll the cell be short 

 of oxygen or have a surplus of carbon di- 

 oxide. When the blood reaches the cells it 

 is no more than one cell away so that 

 gaseous exchange can be accomplished 

 quickly. 



Nutrition. At the cellular level the busi- 

 ness of securing food, digesting it, absorb- 

 ing it, and ultimately metabolizing it is also 

 a relatively simple procedure. The amoeba 

 surrounds its food, thus forming food vacu- 

 oles which mio;ht be thouo;ht of as miniature 

 intracellular "stomachs." Digestion is car- 

 ried on in these tiny structures and the final 

 end products (amino acids, glucose, fatty 

 acids, and glycerol) are absorbed into the 

 surrounding cytoplasm (Fig. 4-12). The 

 food is burned and the locked up energy 

 released to be utilized by the amoeba in the 

 many ways that are essential for its life. 



The simple metazoan obtains and digests 



