GENERAL ZOOLOGY 



followed by direct conversion to certain of the intermediates of the metabolic 

 sequences of carbohydrate metabolism. The — NHg groups are converted 

 into ammonia (NH3), the nitrogenous waste product of cellular metabolism. 

 On the other hand, the fatty acids derived from lipid foods are initially 

 handled in a more complex manner. The fatty acid, which is a long-chain 

 hydrocarbon molecule (Fig. 2. Si:), is degraded in the fatty acid cycle which 

 can be compared to a spiral staircase. First, the fatty acid must be "acti- 

 vated" by combining with coenzyme A in the presence of ATP. Then, on 

 each turn of the staircase, a two-carbon fragment complexed with coenzyme A 

 and known as acetyl coenzyme A is removed from the chain. The remaining 

 part of the fatty acid molecule is again activated by another molecule of 

 coenzyme A, and another molecule of acetyl coenzyme A is removed. This 

 process is repeated until the entire fatty acid is degraded. Each of the 

 molecules of acetyl coenzyme A enters into the TCA cycle for final oxida- 

 tion. 



These reactions of cellular metabolism which have been described occur in 

 animals from protozoa to man, and in plants as well. Compounds resulting 

 from different preparatory reactions of the carbohydrates, lipids, and proteins 

 eventually enter the TCA cycle. Here oxidation occurs, and the waste 

 products carbon dioxide and water are formed. 



Cellular metabolism includes the synthetic mechanisms of anabolism as well 

 as the degradations of catabolism. In the cell, some of the reactions described 

 for the degradation of food compounds may be reversed, under appropriate 

 conditions, with the formation of a new compound necessary for the living 

 organism as the end result. Enzymes, secretions, nucleic acids, characteristic 

 amino acids and fatty acids as well as the proteins, lipids, and compound 

 carbohydrates are built up or synthesized in cells. Particularly important for 

 many synthetic reactions is coenzyme A (p. 33) and, of course, ATP. 



Cell Division 



When cells were first discovered, it was thought that they arose sponta- 

 neously by a sort of crystallization. The nucleus was interpreted by some 

 early investigators as a new cell in the process of formation. As the micro- 

 scope was perfected and more observations were made, new cells were found 

 to be formed as a result of the division of previously existing cells, and in 

 no other way. Periods of division alternate with periods during which the 

 cell is said to be in the vegetative or nutritive stage, that is, when it is 

 either growing or maintaining itself as a functional unit of the organism. 

 The cell is sometimes referred to, at this time, as a resting cell, but no 

 designation could less adequately describe it during this period of metabolic 

 activity. After a cell has reached a certain size, it may divide. Whether 

 or not cell size is the only factor conditioning cell division, it is certainly 

 a very important one. The division of the cytosome is always preceded by 



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