36 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



basis of differences in coulombic forces between the cations and fixed negative 

 binding sites within the cell. According to his hypothesis an active transport 

 mechanism is not only unnecessary but indeed impossible. 



The results of the present investigations on Ulva and Valonia suggest strongly 

 that two independent mechanisms are operative in regulating cellular sodium 

 and potassium concentrations. 



At the present time certain important findings are cogent to this general 

 problem. The presence of a dynamic equilibrium between the cell and its 

 environment has been indicated by the use of radioactive potassium and sodium 

 as tracers. 



Such investigations have revealed that a large fraction, if not all the potas- 

 sium and sodium in those cells studied, is constantly exchanging with environ- 

 mental cations (14-16, 22). Potassium and sodium ions are continuously diffus- 

 ing across the cell membrane with their concentration gradients, outward and 

 inward respectively. But the ionic composition of a particular type of cell is 

 relatively constant; hence there must be cellular mechanisms which compensate 

 for this continual flux of ions across the cell surface. These mechanisms are 

 apparently energetically coupled with carbohydrate metabolism, for carbo- 

 hydrate metabolism has been found to be of great importance in the mainte- 

 nance of potassium and sodium balance in a large variety of cells (8, 10, 16, 



25, ii^ 38, 39)- 



This paper represents a summary of our investigations of the general prob- 

 lem of sodium and potassium balance in the cells of the marine alga Ulva 

 lactuca var. latissima. In addition, certain studies carried out by the senior 

 author on Valonia macrophysa during the spring of 1954 at Bermuda are 

 included for comparative purposes. 



Evidence will be presented which indicates the following: a) an influence of 

 metabolism on the exchange rate of potassium ion in Ulva; b) an association of 

 carbohydrate metabolism with cation-regulating mechanisms; c) an active 

 transport of cations; d) the presence of independent mechanisms for the trans- 

 port of sodium and potassium in Ulva. 



The green alga Ulva lactuca, like most cells living in a high-sodium, low- 

 potassium environment, normally accumulates potassium and partially excludes 

 sodium. Consisting of large membranous fronds two cell layers in thickness, 

 thus presenting a large surface area for interchange with the environment, this 

 alga is particularly well suited for investigations of this nature. Furthermore, 

 it is possible to prepare the material for analysis in such a way as to remove 

 completely contamination with electrolytes in the extracellular fluid. Evidence 

 for this will be discussed in the next section. 



Valonia has the unique distinction of great cell size, as much as i cc. in 

 volume. The cell consists of a thin peripheral layer of protoplasm, several 

 microns in thickness, surrounding a very large central vacuole, several milli- 



