Common Pathways of Cellular Metabolism 

 ENERGY TRANSFORMATIONS IN THE CELL 



143 



The cell, of course, must have a source of 

 energy for supporting not only its move- 

 ments, active transport processes, electrical 

 activities, and so forth, but also the construc- 

 tive phases of its metabolism. One major 

 source, which is available to every cell, is the 

 energy content of organic molecules present 

 in the protoplasm. Generally speaking, the 

 energy content of an organic molecule is 

 much higher than that of the inorganic end 

 products of its catabolism. Moreover, the 

 larger and more complex the molecule, the 

 greater is its fund of potentially available 

 energy. All cells derive energy through ca- 

 tabolism. In addition, the cells of green 

 plants and autotrophic (p. 182) organisms 

 generally can utilize other forms of energy 

 for the replenishment of their organic fuels; 

 but even these cells depend upon organic 

 molecules for current, moment to moment, 

 expenditures. An important area of inter- 

 mediary metabolism, therefore, deals with 

 the step-by-step breakdown of organic mole- 

 cules and the manner in which the cell de- 

 rives energy at each step of this downward 

 path. Some of the energy liberated in catabo- 

 lism may be utilized for constructive pur- 

 poses, however. In fact catabolism and anab- 

 olism in the cell are intricately interwoven 

 and beautifully integrated. 



It is important to realize that the same 

 thermodynamic laws govern all chemical and 

 physical reactions, whether they occur inside 

 or outside of the cell. According to the first 

 law, the sum of mass and energy remains 

 constant as any reaction proceeds. Significant 

 mass changes do not occur in biological sys- 

 tems, however, and therefore the first law, in 

 essence, states that energy does not diminish 

 or increase. But there are two general forms 

 of energy; free energy — free in the sense that 

 this energy is available for the performance 

 of work — and dissipated energy, or entropy, 

 which is not available for work. The second 

 law of thermodynamics states that during 

 any reaction in which energy is being trans- 



formed there is always some loss of free 

 energy. In other words, the entropy of a sys- 

 tem tends to increase. This means, of course, 

 that no energy transaction can be 100 per- 

 cent efficient — whether it is carried out by 

 the cell for the performance of its work, or 

 by an engineer in operating an engine. Some 

 energy is dissipated, usually in the form of 

 heat, which is lost to the environment. The 

 cell, nevertheless, is a chemical machine that 

 operates with remarkable efficiency. In trans- 

 ferring the energy liberated during the oxida- 

 tion of carbohydrate fuel (glucose) to the 

 high-energy phosphate reserves (p. 144) of the 

 cell, more than 50 percent of the available 

 free energy may be conserved. Yet engineers 

 have not been able to devise a steam engine 

 that can transform more than 30 percent of 

 the combustion heat into useful mechanical 

 work. 



High-Energy Phosphate Compounds; Ade- 

 nosine Triphosphate (ATP). Many cellular ac- 

 tivities — the beating of cilia, the twitching of 

 muscle, the electrical discharges at cell sur- 

 faces, and so forth — are very rapid. They re- 

 quire an almost instantaneous mobilization 

 of energy. A natural question is therefore: 

 how do cells develop energy so quickly? 



Previously it was thought that cells get 

 quick energy directly from the oxidation of 

 glucose and other fuels. Now, however, it is 

 known that oxidations are more concerned 

 with storing energy in a form that is ready 

 for instantaneous use. Certain compounds 

 are used as repositories for this stored energy. 

 Especially significant in this regard is a group 

 of high-energy phosphate molecules. And 

 chief among the high-energy phosphates is 

 adenosine triphosphate, or ATP (see p. 90). 



ATP and other high-energy phosphate com- 

 pounds possess a unique distinction. All 

 possess one or more special chemical struc- 

 tures, called high-energy phosphate bonds. 

 Ordinary phosphate bonds, like that of glu- 

 cose-phosphate, represent the binding of 

 considerable energy (about 2 Cal per gram 

 molecule). But high-energy phosphate bonds, 

 for which a special symbol (~) has been de- 



