144 - The Cell 



vised, represent the binding of a much 

 greater amount. In fact, the high-energy 

 phosphate bond holds some four tunes more 

 energy, or about 8 Cal per gram molecule. 

 Thus if we write: 



Adenosine Monophosphate (AMP) 



(Adenosine— Phosphate) 



it indicates that the compound AMP pos- 

 sesses just an ordinary phosphate bond; 

 whereas if we write: 



Adenosine Diphosphate (ADP) 



(Adenosine— Phosphate— Phosphate) 



it indicates that ADP has one energy-rich 

 bond, in addition to the ordinary one: and if 

 we write: 



Adenosine Triphosphate (ATP) 



(Adenosine— Phosphate — Phosphate— Phosphate) 



it shows that ATP has tiuo high-energy bonds, 

 as well as one ordinal) linkage. 



The ATP accumulations of a cell represent 

 a very important fund of quickly available 

 energy which can be used for a wide variety 

 of purposes. When needed, the energy is lib- 

 erated by the splitting ol the high-energy 

 bonds. This is a hydrolysis reaction. It re- 

 quires an appropriate enzyme (an ATP-ase); 

 and it produces one molecule of inorganic 

 phosphate for each of the energy-rich bonds 

 that is broken. It may be written: 



ATP + HoO 



ATP-ase 



ADP -f- inorganic phosphate -f- £(8 Cal) 



Fritz A. Lipmann, working at Harvard 

 University, shared a Nobel Prize (1953) partly 

 on the basis of his pioneer work on the 

 energy-rich bonds of ATP; and since then the 

 importance of the ATP system has been 

 demonstrated in many kinds of cells. It now 

 is known that ATP provides energy for such 



diverse cellular activities as muscle contrac- 

 tion, ciliary beating, amoeboid movement, 

 bioluminescence, and cell division. But 

 equally important, ATP provides energy for 

 synthesizing complex organic compounds, 

 and it serves to spark a number of important 

 catabolic reactions. Moreover, many meta- 

 bolic activities are mainly concerned with re- 

 storing, maintaining, and expanding the in- 

 tracellular reserves of ATP. 



Oxidative Metabolism. The over-all oxida- 

 tion ol an organic compound such as glucose 

 may be stated very simply, by specifying the 

 initial and end products of the reaction for 

 example: 



CgH 12 6 



glucose 



over-ail 

 oxidation 



60 2 



oxygen 



6H 2 + 6C0 2 + E(4 Cal/g) 



carbon 

 dioxide 



energy 



But such an equation merely shows that, 

 when the oxidation is complete, all the hy- 

 drogen of the original compound has united 

 with oxygen, forming water, and all the car- 

 bon has been liberated as carbon dioxide. 

 Nothing is shown of the complex inter- 

 mediary stages. In the cell, however, these 

 intermediary metabolic reactions are of great 

 importance. Each step liberates a certain 

 quantity of energy, which must not be wasted. 

 Frequently these bursts of energy go into the 

 formation of energy-rich phosphate bonds 

 and thus they maintain the energy reserves 

 of the cell; or they may assist in the synthesis 

 of other compounds. 



Biological Oxidation: an Oxidation-Redudion 

 Series. Fundamentally, when a substance 

 undergoes oxidation, it loses one or more of 

 its electrons. Such electrons do not remain 

 free, however. Instantaneously they are 

 picked up, or gained, by some other sub- 

 stance; and this process of gaining electrons 

 is called reduction. Consequently, each oxi- 

 dation must occur concomitantly with an 



