STRUCTURE AND ACTIVITIES OF CELLS 



of digestion which render food usable in cellular metabolism will be discussed 

 in the following chapter. 



Cellular Metabolism 



Food is the fuel of the cell; the energy stored in its chemical bonds must 

 be released by chemical reactions in order that the work of the cell can be 

 done. The release of energy during cellular metabolism does not occur as the 

 result of a vigorous reaction of burning, as in a calorimeter, during which the 

 energy is dissipated as heat. Instead, the release of free chemical energy 

 during metabolism occurs by the gentle degradation of organic molecules in 

 an orderly and conservative manner. The handling of food molecules in this 

 way is a very complex process. Long chains of reactions, each catalyzed by 

 a different enzyme, are necessary to break down, bit by bit, the organic com- 

 pounds of food. We shall consider, in as simplified a way as possible, some 

 of the essentials of dynamic cellular chemistry in order that you may gain a 

 better understanding of the metabolic processes common to all living cells. 

 Some of you will not have sufficient knowledge of chemistry to appreciate all 

 the implications of the following account. You should, however, be able to 

 follow the main line of the presentation and gain some conception of the 

 intricate precision which characterizes life at the cellular level. If the ma- 

 terial is too difficult for you, your teacher may suggest that you omit this 

 section. 



It is necessary to recall that in some chemical reactions energy is liberated 

 when a bond is broken and in others energy must be used to break a bond. 

 There are a few compounds that carry unusually large amounts of energy in 

 certain of their bonds; the breaking of such high-energy bonds releases free 

 energy which can then be used to drive reactions requiring that work be done. 

 This relationship implies that the compound carrying or storing energy must 

 constantly be replaced in order that the chemical sequences of cellular metab- 

 olism can continue. There is one exceedingly reactive compound that sup- 

 plies the great amounts of free chemical energy required to do the work of 

 the cell. This is adenosine triphosphate, known as ATP (Fig. 2.8A/); when 

 its terminal (third) phosphate bond is broken, nearly 12,000 calories of free 

 energy become available for use, per mole of ATP. Such a bond is known as 

 a high-energy phosphate bond. 



Carbohydrate has already been identified as the chief source of fuel for the 

 cell. However, carbohydrates do not react directly with oxygen in the cell, 

 nor do they immediately break down into carbon dioxide and water. The 

 production of these two waste products of cellular metabolism occurs in the 

 final phase of the metabolism not only of carbohydrate but of lipid and pro- 

 tein, as well. Either glycogen, the stored polysaccharide carbohydrate made 

 up of many glucose molecules, or glucose itself can be used by the cell. The 

 steps in carbohydrate metabolism are subdivided into two groups, dependent 



35 



