STRUCTURE AND ACTIVITIES OF CELLS 



acid. This four-carbon acid is an essential component of the TCA cycle and 

 must be reformed to prime the system for the oxidation of a second two- 

 carbon molecule. In the series of reactions occurring during each turn of 

 the TCA cycle, two more molecules of carbon dioxide are split off. Such 

 reactions are called decarboxylations and are coupled with dehydrogenations. 

 The hydrogen atoms removed in these reactions become associated with what 

 are known as carrier compounds which relay them through the flavoprotein 

 (p. 32) and cytochrome tran.sfer .system to a final reaction with oxygen to 

 form water. These reactions are referred to as biological oxidation, or 

 cellular respiration, because in order to proceed they require oxygen. Dur- 

 ing the reactions leading to complete oxidation of one two-carbon compound 

 in the TCA cycle, free energy is stored in at least 16 molecules of ATP. 



A better appreciation of the efficiency of the cellular mechanisms for the 

 release and subsequent storage of energy can be gained by consideration of 

 the following balance sheet for carbohydrate metabolism. Combustion of 

 glucose in a bomb calorimeter produces the end products carbon dioxide and 

 water and releases energy in the form of heat: 



1 glucose (CgHj^Og) + 6 oxygen (O2) ^ 6 carbon dioxide (CO2) 



+ 6 water (HgO) + 673,000 calories of energy (heat) 

 In the cell, as a result of anaerobic sequences: 



Enzymes 



1 glucose — — ^ 2 pyruvic acid + 58,000 calories of free energy, 



.\TP 



of which 48,000 calories are stored in four gram molecules of ATP. The 

 aerobic reactions, on the other hand, release a far greater amount of energy: 



Enzymes 



2 pyruvic acid + 10 oxygen (Og) ^ 6 carbon dioxide (CO,) 



ATP 



+ 4 water (H2O) + 420,000 calories of free energy, 

 of which 384,000 calories are stored in 32 gram molecules of ATP. 



During the metabolism of a mole of glucose, approximately 480,000 calories 

 of its total of 673,000 calories of energy are stored in ATP and thus made 

 available for the work of the cell. The operating efficiency of the cell is, 

 therefore, about 71 per cent; for comparison, the efficiency of modern steam 

 turbines approaches 50 per cent. 



The basic units of protein and lipid foodstuffs are also metabolized in the 

 cell in such a way that their intrinsic energy is progressively released in 

 small amounts and much of it stored in ATP until needed. In both cases, the 

 derivatives of initial breakdown reactions are channeled into the TCA cycle 

 for complete oxidation. The processing of amino acids, the units of protein 

 foods, involves their deamination (loss of their — NH.^, groups. Fig. 2.8/) 



37 



