42 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



Other being kept in the dark for a period and then illuminated. Samples were 

 removed at intervals and analyzed for sodium and potassium; representative 

 data are presented in figures i and 2. After a latency of approximately 30 hours 

 a progressive loss of potassium occurs, amounting to 20% of the original 

 potassium content of the cell in 82 hours. Upon illumination there occurs an 

 abrupt accumulation of potassium which, for a short time, remains consider- 

 ably above the level of the illuminated controls, then returns to this concen- 

 tration level (fig. i). 



The sodium content of the cells gradually increases over the course of the 

 experiment, 72 hours, in the dark. On illumination the sodium content is re- 



10 20 30 40 50 60 



HOURS 

 Fig. 2. Influence of illumination and darkness on the sodium content of Ulva lactuca. 



duced in a period of 20 hours to the level of the controls, while that of the 

 illuminated controls remains essentially constant (fig. 2). 



The data indicate a marked influence of photosynthesis on the maintenance 

 of the normal distribution of potassium and sodium in the cells of Viva. The 

 condition of darkness is sufficient to cause a loss of potassium and gain of 

 sodium, presumably because the cell is using up its carbohydrate reserves or 

 intermediates. The reexcretion of sodium and reaccumulation of potassium 

 when light is admitted to fronds previously maintained in the dark lend further 

 support to this interpretation, for under conditions of illumination the im- 

 portant glycolytic intermediate, phosphoglyceric acid, is made available to 

 cellular metabolism by photosynthesis (3, 13). The influence of light on the 

 uptake of electrolytes in algae has been previously observed by Hoagland and 

 Davis (20) and Jacques and Osterhout (15). The latter authors interpret the 

 uptake of potassium on more intense illumination in terms of an increase in 



