128 - The Cell 



synthesis of the various organic components 

 of its protoplasm, and the typical plant can 

 live and grow indefinitely so long as these 

 simple foods are available. But an animal 

 cannot do this; an animal's food require- 

 ments are on a higher level of complexity. 

 Compared to plant cells, animal cells have 

 more limited powers of synthesis, and animal 

 metabolism cannot be maintained in the ab- 

 sence of organic foods stub as preformed 

 proteins (or amino acids), carbohydrates, 

 lipids, and vitamins (Table 7-1). 



Animals generally are cjuite similar as to 

 their food requirements, and the same is true 

 for green plants. Accordingly, the nutrition 

 of the typical animal is referred to as holo- 

 zoic nutrition in contrast to the holophytic 

 nutrition ol green plants. 



The holozoic mode of nutrition is associ- 

 ated with the development among animals 

 generally of many structures and activities 

 not found among plants. The animal must 

 obtain organic foods, and these compounds 

 exist mainly in the bodies and remnants of 

 other organisms. Therefore the animal is 

 constantly facetl with the necessity of finding 

 and apprehending other organisms. This 

 necessity has determined an evolution of 

 highly developed sensory, nervous, and mus- 

 cular structures, which are so characteristic 

 of animals, but not of plants. The essential 

 foods of a plant are all inorganic substances 

 that are widely distributed in the environ- 

 ment. The plant does not have to seek them 

 out. Rooted to a single locality, a typical 

 plant remains constantly in direct contact 

 with all its necessary foods. 



Other structures generally present in ani- 

 mals, but absent in plants, are the organs of 

 the digestive system. A digestive system is 

 necessary in animals because most organic 

 foods, such as proteins, are colloidal in their 

 molecular dimensions. Such macromolecules 

 cannot be absorbed into the cells of the ani- 

 mal until they have been digested into the 

 micromolecular units of their structure. 

 Therefore typical animals possess a digestive 

 cavity wherein the colloidal components of 



the food are chemically broken up in prepa- 

 ration for absorption. 



NUTRITION OF UNICELLULAR ANIMALS: 

 HOLOZOIC NUTRITION 



One-celled animals, such as Amoeba, sus- 

 tain themselves in a fashion that is strikingly 

 analogous to the nutrition of larger animals, 

 including man. In obtaining and utilizing 

 food, and in disposing of the waste products, 

 all animals perform essentially the same 

 processes and achieve essentially the same 

 ends. Accordingly it is possible to describe 

 the nutrition of Amoeba, man, or any other 

 typical animal under the same eight head- 

 ings: 1. ingestion, 2. digestion, 3. absorp- 

 tion, 4. egestion, 5. distribution, 6. metabo- 

 lism, 7. respiration, and 8. excretion. 



Ingestion. The amoeba, as seen in a drop 

 ol pond water, slowly approaches its living or 

 nonliving food, engulfing the entire mass. 



PSEUDOPODIUM 



FOOD CUP 

 ,_ FOOD VACUOLE 



Fig. 7-1. Amoeba ingesting a flagellate. 



