CHEMICAL STRUCTURE AND METABOLISM 89 



One molecule of glucose combines with six molecules of oxygen (obtained 

 from the air through respiration). The products obtained are six molecules 

 of carbon dioxide, six molecules of water, and the energy "locked up" in 

 the process of building the glucose molecule in the first place. Since energy 

 was required to build this molecule, energy is released when the mole- 

 cule is finally broken down. Part of the free energy can then be used by 

 the animal cells for carrying on their life processes. Thus in a round- 

 about way each cell of the animal body receives energy from the sun. 



Our account of the transfer of energy from sun to animal cell is greatly 

 oversimplified. Every stage of the process is enormously complex. The 

 manufacture of glucose by green plants involves many steps, including in- 

 termediate products having complex chemical composition. The 6 CO2 + 

 6 H:.0 forms the beginning of a series of substances; C;H]._.0,; is the cul- 

 mination of this series. Included in the series are substances which do not 

 appear in the final product. Of these we may mention particularly enzymes 

 and coenzymes (see below), without which the synthesis could not be 

 accomplished. Formation of energy-rich carbohydrate from energy-poor 

 carbon dioxide and water is complex and difficult business. 



Complexity also characterizes the release in animal cells of energy stored 

 in the carbohydrate molecules. The simple formula given above conveys 

 the impression that the process is much like burning coal in a steam 

 engine. In the engine coal unites with oxygen (burns), with resuhant pro- 

 duction of heat (energy). Similarly, our formula shows glucose uniting 

 with oxygen, with release of energy. While this is true, it is again an over- 

 simplification. Between the glucose-plus-oxygen on the one hand and the 

 carbon dioxide-plus-water on the other at least twenty-five intermediate 

 steps occur. In a muscle cell, for example, free energy is transferred 

 from the glucose to complex organic constituents of the muscle cell it- 

 self. Many intermediate products are involved in the chemical transforma- 

 tions by which, in a series of steps, free energy from the glucose is trans- 

 mitted to the contractile mechanism of the muscle cell, making possible 

 contraction of the latter. It is important to note that each of the steps in- 

 volves the action of an enzyme. Previously we noted enzymes concerned 

 with the digestion of food. But enzyme action is of much more general oc- 

 currence. So far as we can tell, every chemical change in metabolism is 

 activated — catalyzed — by an enzyme. Enzymes are usually, if not always, 

 protein in nature and hence in themselves extremely complex substances. 

 Not only is an enzyme needed for each chemical transformation, but fre- 

 quently one or more complex substances necessary for the activity of an 

 enzyme must also be present. These are called coenzymes. 



