20 GENERAL BIOCHEMISTRY 



come equal on both sides. A cell using oxygen, for example, keeps the 

 conceniration ol oxygen low in its interior by consuming oxygen mole- 

 cules in one oi more reactions. Tlien additional oxygen diiluses in 

 from the outside as long as the external concentration exceeds the in- 

 ternal concentration. 



This diffusion process transfers gases in all biological systems. Gases 

 diffuse through porous barriers, from one part of a solution to another, 

 and through solid barriers in which the gases dissolve. In this last case 

 a gas reaches the barrier by diffusion, some molecules strike it and dis- 

 solve, a part of these diffuse in solution to the other surface of the 

 barrier, and some diffuse away into the adjoining system. As long as 

 the concentrations differ on the different sides of the barrier, more gas 

 passes in one direction than the other and there is a net transfer. In 

 actual cells it is common for more than one gas to be transferred or 

 exchanged. Since gases move independently of one another, different 

 ones may be diffusing simultaneously in the same or in different direc- 

 tions. 



Many cells are not in contact with a gaseous system. Part of these 

 are immersed in soil solutions, liquid cultures, natural bodies of water, 

 and the like, which are in turn in contact with a gas phase. In this 

 situation gases are exchanged by diffusion between gas and liquid 

 phases and between liquid phases and the cells. Here more stages are 

 involved in the overall gas exchange, and the complete process takes 

 somewhat longer. Yet it is efficient enough for many organisms, in- 

 cluding most of the small forms. 



To promote more rapid gas transfer, diffusion has been supple- 

 mented, especially in the larger organisms, by auxiliary mechanisms. 

 One of these is protoplasmic streaming, common among the larger 

 single-celled species and among cells of multicelltdar organisms. The 

 protoplasm of these cells consists partially of a viscous liquid in which 

 currents produce a circulation that supplements diffusion in the 

 transfer of gases and of other dissolved materials. Outside the cells 

 there are specialized circulatory systems in the higher plants and ani- 

 mals. Such systems are specifically adapted in part to aid in the 

 transport of gases in either gaseous or solution states. In the former 

 case, pores provide air passages among the tissues, bringing more cells 

 into contact with the gas phase. This development is elaborated still 

 further when volume control is provided in respiratory chambers, 

 pumping gases in and out and thus bringing them into contact with 

 specialized cells. Gases in solution are circulated by means of flowing 

 blood and lymph in animals and by the movement of solutions 



