THE FUNCTIONS OF PROTOPLASM AND CELLS 49 



which is just the reverse of that by which ghicose is formed in photo- 

 synthesis, namely, 



CeHizOe + 6()2->6H20 + 6(X^2 + energy 



This equation says, in words, that one molecule of glucose and six 

 molecules of oxygen are recombined (in combustion) to form six molecules 

 of water and six molecules of carbon dioxide, with the release of energy. 

 Even some of the transitory steps involved in this reaction are reversals 

 of those occurring in photosynthesis. 



If new protoplasmic structure is required, the soluble products of 

 digestion are available for this purpose. If the digested foods are in 

 excess of the requirements for these two purposes, they may be stored; 

 but in this case they must be rendered insoluble again, for otherwise they 

 could not be retained. If carbohydrates are to be stored in animals, the 

 glucose is commonly converted into animal starch or glycogen. Glycerol 

 and fatty acids are again converted into fats, although the fats are likely 

 to be of different kinds from those which were taken as food. The pro- 

 duction of these insoluble storage products is done by enzymes, and the 

 same enzyme may work in both directions, that is, either break down 

 substances (starches, for example) or build them up. 



Little is known about the construction of new protoplasm out of 

 digested foods. The name assimilation is given to the process, and it 

 seems certain that enzymes are engaged in the work, but of its nature 

 we are mostly ignorant. 



Respiration.— To provide energy or new protoplasm, all living things 

 require oxygen. Land animals and plants get it from the air, submerged 

 aquatic ones from the oxygen which is dissolved in water. There 

 are, however, some kinds of animals and plants that normally live in 

 situations devoid of oxygen, and some of these organisms would die 

 if brought into contact with free oxygen. Such organisms require 

 oxygen in their metabolism, but they secure it from compounds in 

 which it occurs. 



The combination of oxygen with protoplasm and foods results finally 

 in the formation of water and carbon dioxide, as indicated by the equation 

 in the preceding section. The carbon dioxide must be eliminated. The 

 absorption of oxygen and the elimination of carbon dioxide are together 

 called respiration. 



In simple animals and plants, dissolved oxygen diffuses directly 

 through the surface of the organism into the protoplasm. Thence by 

 diffusion and protoplasmic currents it is carried to all parts of the cell. 

 In many small multicellular animals and plants with few layers of cells the 

 oxygen may readily diffuse through the intervening cells to those which 



