CELLULAR RESPIRATION 129 



events should be required. From the equation given above it is 

 apparent that burning a single molecule of glucose yields 

 690,000 calories of energy per gram-molecular weight (i.e. 180 

 grams glucose) in the form of heat. But the animal does not 

 require such large amounts of heat for, unlike a steam engine, it 

 does not have the machinery available for converting it into 

 work. Furthermore, the liberation of such quantities of heat 

 would have serious consequences on the body temperature! 

 What the body requires is a system whereby this total chemically- 

 bound energy is made available in a series of small packets, each 

 of which can be utilised to synthesise the fuel or currency that is 

 used by the body to perform work of various kinds. This 

 currency takes the form of so-called energy-rich phosphate 

 bonds ( ^ P), notably those attached to the molecule of ATP 

 (Adenosine Triphosphate). 



So far our discussion has been in terms of the liberation of 

 small packets of energy from the complex molecules but a more 

 functional way to think of it is in terms of the creation of body 

 energy (as ~ P) which can be used for a variety of purposes. The 

 formation of each gram molecule of ATP from inorganic 

 phosphate and ADP involves the capture of 10,000 calories of 

 energy. 



During aerobic respiration the total output of ATP from 

 the degradation of a single molecule of glucose amounts to 38 

 molecules. These molecules of ATP incorporate in their energy- 

 rich phosphate bonds 380,000 of the 690,000 calories contained 

 in the original gram molecule of glucose. Consequently, the 

 energy-capture efficiency of the whole process of cellular respira- 

 tion is at least 50% . This is a good performance when it is 

 remembered that engineers are reasonably satisfied when they 

 are able to convert one-third of the heat of combustion of a fuel 

 into mechanical work. In this comparison we have, of course, 

 not considered the change from the ATP to the performance of 

 mechanical work but apparently this is a very efficient process. 



Thus far, then, we have seen that the cytochrome system is the 

 vital link between the metabolic pathways of the body involving 

 the degradation of large molecules and that the stepwise nature 



