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Journal of Experimental Biology 57,55-76 (1972)
Published by Company of Biologists 1972

The Osmoregulatory System of the Amoeba, Acanthamoeba Castellanii

R. A. PAL ¹

¹ School of Biological Sciences, University of East Anglia Norwich NOR 88C and Department of Zoology, University of Sind, Famshoro (Sind), West Pakistan

1. It is estimated that Acanthamoeba castellanii eliminates a volume of water equal to its body volume in about 15-30 min. About 7% of the vacuolar discharge enters the body by means other than osmosis through the surface membrane. Food vacuoles fusing with the contractile vacuole do not significantly affect the rate of output.

2. Vacuolar output declines with the age of culture so that during the stationary phase of growth it is about half of that during early log phase of growth.

3. The rate of output of the contractile vacuole decreases with an increase of concentration of a non-penetrating solute in the external medium and shows a rectilinear relationship up to 0.07 M concentration. A low residual output after 0.07 M may be due to food vacuoles and pinocytic vacuoles.

4. On the basis of vacuolar output the excess internal osmotic pressure and permeability constant of water has been estimated as 0.07 M non-electrolyte and 0.04µm min^-1 atm.^-1 respectively.

5. On the basis of vacuolar behaviour it is concluded that the relative permeabilities of the plasma membrane to different solutes follows this order: methyl alcohol > ethylene glycol > urea > glycerol. On certain assumptions the permeability of the plasma membrane to ethylene glycol has been estimated provisionally as 0.107 x 10^-16 mol/sec/µm²/mol/l.

6. Vacuolar behaviour suggests that sodium chloride, sodium nitrate, sodium sulphate and potassium chloride, but not magnesium chloride and calcium chloride, pass into the cell freely.

7. Growth of populations of A. castellanii is almost normal in polyethylene glycol 600 up to 0.07 M concentration but in higher concentrations it is low. There are some indications of an increase in volume of A . castellanii in cultures of polyethylene glycol 600 up to 0.07 M concentration, but not in higher concentrations. For amoebae cultured in media containing polyethylene glycol 600 the rate of output of the contractile vacuole declines sharply with an increase of polyethylene glycol 600 up to 0.07 M concentration and then more gradually.

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