CHAPTER 4 



Cell Metabolism 



An examination of the properties of living things reveals that chemical 

 reactions are basic to all of them. These chemical activities of proto- 

 plasm, called metabolism, provide for the irritability and movement of 

 protoplasm and for its growth, maintenance, repair and reproduction. 

 Modern biochemical research has shown that the metabolic activities 

 of animal, plant and bacterial cells are remarkably similar, despite the 

 apparent differences of the organisms themselves. In all cells, sugars and 

 related substances are continually being metabolized, via a large num- 

 ber of intermediate compounds, to water and carbon dioxide with the 

 release of energy which is made available to the cell for further use. 



Green plants differ from animals in their ability to photosynthesize, 

 that is, to capture the energy of sunlight and to use it to synthesize 

 complex, energy-rich substances from simple raw materials— water, car- 

 bon dioxide, nitrates and phosphates. Animal and bacterial cells have 

 the ability to "fix" carbon dioxide, to incorporate it into any one of a 

 number of organic compounds and thus build a new compound with 

 one more carbon atom in the chain. Only green plants and a few bac- 

 teria, however, can utilize radiant energy to fix carbon dioxide; animals 

 and the rest of the bacteria must get energy for the reaction from some 

 energy-releasing process such as the metabolism of glucose. 



19. Chemical Reactions 



A chemical reaction is a change involving the molecular structure 

 of one or more substances; matter is changed from one substance, with 

 its characteristic properties, to another, with new properties, and energy 

 is released or absorbed. Hydrochloric acid, HCl, for example, reacts 

 with the base, sodium hydroxide, NaOH, to yield water, H2O, and the 

 salt, sodium chloride, NaCl; in the process energy is released as heat. 

 The chemical properties of HCl and NaOH are very different from those 

 of NaCl and HoO. In chemical shorthand a plus sign connects the sym- 

 bols of the reacting substances, HCl and NaOH, and the products, NaCl 

 and H2O. An arrow indicates the direction of the reaction: 



HCl + NaOH^NaCl + HoO 



Most chemical reactions are reversible and this reversibility is indicated 

 by a double arrow: :;=i. 



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