58 



ELEMENTS OF BIOLOGY 



FOOD MOLECULE 



ENERGY- 



OCOp 



that the endoplasm is constantly streaming (cyclosis) is o£ material 

 assistance in the distribution of the digesting substances throughout 

 the cell. 



Respiration. After the food has been digested and assimi- 

 lated, its utilization either in building up protoplasm or in provid- 

 ing energy for cell activities is dependent on a very common chem- 

 ical reaction between the assimilated food and a common gas, 

 oxygen. The most common reaction occurring in the non-living 



world is oxidation. A wide variety 

 of common substances when exposed 

 to the oxygen of the air unite with 

 it. The union of oxygen with any 

 substance takes place with the evo- 

 lution of energy in some form; in a 

 fire, for example, this energy is ap- 

 parent in the form of heat. It is of 

 interest that not only does living 

 protoplasm basically consist of the 

 more common elements and that the 

 most prominent of its substances is 

 water, but also that the fundamental 

 energy releasing chemical reaction in 

 the system is the reaction most common in the non-living world. 

 In protoplasm the union of oxygen with the various substances 

 that are oxidizable releases energy which has several fates (Fig. 13). 

 Some of it is degraded into heat. Some of it is utilized in work. 

 Some of it is employed in the building up of protoplasmic com- 

 pounds that require energy in their synthesis. The most common 

 chemical end-product of oxidation in protoplasm is a completely 

 oxidized carbon atom, carbon dioxide. This substance is a chem- 

 ically inert gas which in the protoplasm is dissolved in water. In 

 water its presence results in the ionization of hydrogen from water 

 molecules, and consequently such a solution is acid. It does not 



Fig. 13. — A scheme to represent 

 the principles of the process of 

 respiration in a cell. By osmosis 

 oxygen molecules enter the cell, 

 there to react with food molecules. 

 Carbon dioxide is one of the prod- 

 ucts of the chemical action; by 

 osmosis it passes out of the cell. 

 Energy, inactive in the food mole- 

 cule, is released by the oxidative 

 reaction and is transformed into 

 various forms in the protoplasm. 



