THE CONTRACTILE VACUOLE AND THE ADJUSTMENT TO CHANGING CONCENTRATION IN FRESH WATER AMOEBAE

Abstract

1. Amoeba lacerata, a fresl water amoeba, is able to adjust and live in any concentration of the salts of sea water up to 125 per cent.2. Contractile vacuoles are formed in all concentrations and are separate from food vacuoles.3. Food is engulfed with little or no water and small protoplasmic vacuoles coalesce with it. When digestion is completed, the food residues are eliminated with little or no fluid.4. The major part of fluid elimination occurs by way of the contractile vacuoles.5. Contractile vacuoles grow in size by coalescence and osmotic swelling.6. The rate of fluid elimination is affected by a number of factors, but under constant optimal conditions it varies inversely with the concentration of the medium. It is suggested that the rate is proportional to the rate of catabolism.7. The osmotic pressure of the protoplasm varies in the same direction as that of the outside medium. The protoplasm of completely adjusted amoebae is very nearly equal to that of the medium, the osmotic difference being less than that of 5 per cent sea water (0.13 atmosphere).8. Contractile vacuoles have been shown to contain osmotically active substances; they shrink in hypertonic and swell in hypotonic solutions regardless of whether remain in the cell or have been removed from the cell.9. The volume of the amoeba is only temporarily dependent on the concentration of the medium. After adjustment, the volume shows no dependence upon concentration of the medium. If the amoeba is placed in a hypertonic solution the cell shrinks and then swells to its original volume within three hours time. Conversely, if placed in a hypotonic solution, it swells and then shrinks to its original volume.10. The cell membrane is either permeable to substances in the medium especially when adjusting to a change in the concentration of the medium, or is capable of modifying its internal osmotic activity in some other way.11. The contractile vacuoles are obviously excretory and also osmoregulatory organelles since they remove waste substances which would otherwise cause increases in the osmotic pressure in the cell.