UNICELLULAR FORMS 59 



accumulate in any considerable quantity in the cell but passes out 

 by dififusion (osmosis) through the cell boundary. Its presence in 

 the protoplasm, however, has a great deal to do with the acidity 

 of the system; since H ions are positively charged, its presence 

 influences the whole series of protoplasmic reactions that are con- 

 trolled by electrical conditions. 



The source of oxygen for the amoeba, and in fact for all but a 

 few organisms that live in water, is the oxygen dissolved in the 

 water. At ordinary room temperature water contains nearly 6 cubic 

 centimeters of oxygen per liter, or approximately 0.6 per cent. This 

 is materially lower than the concentration of oxygen in air, which 

 amounts to approximately 20 per cent, but it suffices as the supply 

 for aquatic forms. The cell boundary of the amoeba is permeable 

 to oxygen and to carbon dioxide. The oxygen that enters the cell is 

 being constantly bound into compounds, so there is a constant flow 

 of free oxygen inward from the higher concentration in the sur- 

 rounding water to the lower concentration inside the cell. Like- 

 wise, the oxidative reactions within the cell are constantly producing 

 carbon dioxide, so there is a constant outward flow of this dissolved 

 gas. The various steps of entrance of oxygen into the cell, its union 

 with oxidizable substances there, and the outward flow of carbon 

 dioxide are collectively called respiration (Fig. 13). The processes 

 involved are not greatly diflferent in all cells of all organisms, strictly 

 comparable so far as their fundamental nature is concerned, with 

 the processes as described for amoeba. We commonly term the 

 mechanical act of breathing respiration. There are many kinds of 

 breathing mechanisms, but their common and fundamental func- 

 tion is to afford a supply of oxygen for the cells of the organism 

 and as a means of escape for carbon dioxide. Calling breathing 

 respiration has the virtue of common usage, but it must be remem- 

 bered that the fundamentals of respiration are events occurring in 

 the several cells, not the mechanical act of inspiring air, or causing 

 water that contains oxygen to flow over gills. The fact that water 



