Why does a fresh water protozoan require a contractile vacuole that constantly works to pump out water?
The cell membrane is semi-permeable. The concentration of solutes (sodiium, potassium etc) is generally greater INSIDE the cell than OUTSIDE the cell. This concentration gradient will cause water to flow through (into) the cell membrane to equilibrate the osmotic pressure (make the concentration in and out the same)....this will cause the cell to swell to the point of bursting! POW!!!! No more cell..... Kinda
