60 



LIFE: ITS BEGINNINGS AND NATURE 



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amino acid 



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cell repair 

 "^and qrowth 



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carbon dioxide 

 woter 



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■^ ammonia 

 urea 



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 water 



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Fig. 3-3. Schematic drawing showing how food is metabolized in a cell. 



Nutrition. In the broad sense, nutrition 

 includes all activities that have to do with 

 securing, consuming, and ultimately me- 

 tabolizing food ( Fig. 3-3 ) . This is essential 

 for cells because it is the only way they can 

 obtain the energy with which to carry on 

 their life processes (such as reproduction, 

 motion, and generating heat). Plant cells 

 require only simple inorganic material, 

 whereas animal cells demand complex or- 

 ganic compounds in addition to certain 

 inorganic substances, such as sodium and 

 iron. As was pointed out earlier, animals 

 depend on the plants for much of their food 

 directly and all of it viltimately. This differ- 

 ence in food requirements between plants 

 and animals has profoundly influenced the 

 course each has pursued in evolution. The 

 plant can remain serenely rooted in one 

 spot throughout its lifetime, and under 

 ordinary circumstances *all of its needs in 

 the way of food are in its immediate sur- 

 roundings. The essential inorganic salts and 

 water are in the soil at its "feet," and the 

 carbon dioxide and sunshine are in the air 

 overhead. It is not so with the animals. 

 They must pursue and apprehend complex 

 organic diets, necessitating a much more 



highly specialized organism, endowed with 

 sense organs, coordinating systems, and 

 machinery for rapid movement such as 

 skeletons and muscles. The food thev secure 

 is not in a state for immediate utilization, 

 but requires an elaborate digestive system 

 to break the large insoluble molecules into 

 smaller utilizable ones. This is the price ani- 

 mals have had to pay for being unable to 

 satisfy their needs from simple inorganic 

 food sources. Perhaps there are otlier com- 

 pensations for being an animal. For man, 

 there is the joy of a rare steak smothered 

 with mushrooms, for example. 



First of all, individual animal cells must 

 receive their foods from the outside world, 

 whether the cell is within the body of a 

 complex animal or a single isolated cell. In 

 the latter case, entire microorg-anisms are 

 engulfed (Fig. 7-5), digestion proceeds 

 within the cell body, and the products of 

 digestion are absorbed into the surrounding 

 protoplasm, ultimately to be utilized as a 

 source of energy or for growth and repair. 

 In the multicellular animal the individual 

 cells receive their food in utilizable form so 

 all they need to do is to use it. Each cell is 

 bathed in a fluid (lymph in man) which 



