The Localization of Radiophosphate in Cells

The exact mode in which ions are distributed in protoplasm is at present unknown. Using radioactive isotopes, several authors have attempted, by placing photographic film in close contact with tissues and thus obtaining a radio-autograph, to record where added ions are localized in the cell. Unfortunately this procedure has not yielded any great amount of information on cellular localization, although excellent differentiation of different tissue masses has been obtained. 1 The difficulty in obtaining clear radio-autographs of cells is that the cell diameters are of the same order of magnitude as the distance that β-rays, not normal to the film. travel before striking the film. The net result is a general fog which obscures the desired detail. To obtain pictures with more detail it is necessary therefore either to col-limate or to focus the electrons coming from the tissue to the film, or else to use cells of much larger dimensions. In this study the second alternative has been employed in that cells of Nitella or Chara sp., which have diameters of the order of 1-2 mm and lengths of about 40 mm, were used.

Experimental. Cells of Chara or Nitella were prepared for use as previously described. 2 The cells were removed from pond water, blotted to remove excess water and placed on end in a tube containing 0.001 M radioactive sodium phosphate solution at 15°C and pH 7. In some experiments only enough phosphate solution was used to cover 1-2 mm of the cell as measured along its axis. The tube was then filled with washed paraffin oil. By this arrangement it was possible to expose only a minimal part of the surface of the cell to phosphate solution and hence obtain some idea of the rapidity of diffusion and possible circulation of the phosphate due to cyclosis. To be sure there must be a thin film of water extending up from the P04 solution along the cell surface but this method probably lets ion diffusion proceed as asymmetrically as possible. After various times of immersion the cells were removed from the tubes, washed thoroughly in running distilled water, placed on a glass slide and rapidly frozen and dried in vacuo. A piece of X-ray film was then placed over the dried cell and exposed for a suitable time.