An Ion Displacement Membrane Model

The usual assumption in treating the diffusion of ions in an electric field has been that the movement of each ion is independent of the movement of the others. The resulting equation for diffusion by a succession of spontaneous jumps has been well stated by Parlin and Eyring. This paper will consider one simple case in which a different assumption is reasonable. Diffusion of monovalent positive ions is considered as a series of jumps from one fixed negative site to another. The sites are assumed to be full (electrical neutrality). Interaction occurs by the displacement of one ion by another. An ion leaves a site if and only if another ion, not necessarily of the same species, attempts to occupy the same site. Flux ratios and net fluxes are given as functions of the electrical potential, concentration ratios, and number of sites encountered in crossing the membrane. Quantitative comparisons with observations of Hodgkin and Keynes are presented.     

STUDIES OF NATIVE GLYCOGEN ISOLATED FROM SYNCHRONIZED TETRAHYMENA PYRIFORMIS (HSM)

Native glycogen was isolated from Tetrahymena pyriformis (HSM) by isopycnic centrifugation in cesium chloride density gradients. A density of 1.62 to 1.65 was isopycnic for glycogen. Most of the banded glycogen existed as 35 to 40 mµ particles which had a sedimentation coefficient of 214. These particles were composed of aggregates of 2 to 3 mµ spherical particles. Extraction of glycogen with hot alkali reduced the sedimentation coefficient of native glycogen from 214 to 64.7 and the particle diameter from approximately 40 to 20 mµ and smaller. Cell division was synchronized by a repetitive 12-hour temperature cycle, and glycogen was measured at several times during the cell cycle. The temperature cycle consisted of 9.5 hours at 12°C and 2.5 hours at 27°C. Approximately 90 per cent of the cells divided during the last 1.5 hours of the warm period. The carbohydrate/protein ratio of cells at the end of the cold period was 0.27 and was reduced slightly during the warm period. Glucose was incorporated into glycogen during both periods, although the rate of incorporation was greater during the warm period. No preferential incorporation on the basis of particle size was noted. Incorporation was measured in both native glycogen and KOH-extracted glycogen. Tetrahymena glycogen is compared with rat liver glycogen previously isolated by similar procedures, and the significance of using combined rate-zonal and isopycnic centrifugation for isolating native glycogen is discussed.     

ELECTRON MICROSCOPE STUDY OF MITOSIS IN SEA URCHIN BLASTOMERES

The fine structure of cells at different stages of the mitotic cycle was studied in the blastomeres of 6-hour-old embryos of the sea urchin Strongylocentrotus purpuratus. The material was fixed in 1 per cent osmium tetroxide in sea water, buffered with veronal-acetate to pH 7.5, embedded in Araldite, and sectioned with glass knives. The aster, as it forms around the centriole, has the appearance of the endoplastic reticulum, with elements oriented radially from the centrosphere to the periphery of the cell. Anaphase structures described include the kinetochores, with bundles of fine filaments extending toward the centrioles, as well as continuous filaments passing between the chromosomes. Two cylindrical centrioles composed of parallel rods are present in each of the anaphase asters. At late anaphase, elements of the endoplasmic reticulum condense on the surface of the chromosomes to form a double membrane which already at this stage possesses pores or annuli. At telophase bundles of continuous filaments can be seen in the interzonal region. These filaments, as well as those associated with the chromosomes, have a diameter of approximately 15 mµ, and appear physically different from the astral structure.