Cellular and Molecular Biology

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Osmotic Balance of Cells


All cells must be able to maintain a proper water balance and control concentrations of dissolved materials. Diffusion and osmosis govern this process. Diffusion means that dissolved substances (solutes) tend to move from areas of high concentration to areas of low concentration.

In osmosis, water moves from a region of low solute concentration to high solute concentration. Cells can be in a hypotonic environment , with a solute concentration outside the cell lower than solute inside or hypertonic, with a solute concentration outside the cell higher than inside, or isotonic with solute concentrations inside and outside being equal.

Pond-dwelling microbes such as the Paramecium must take in water from pond surroundings to keep internal concentration within acceptable limits.

Is pond water surrounding the Paramecium normally hypotonic, hypertonic, or isotonic with respect to the cell?




Hypotonic at all times.


Hypotonic only when it rains.