As you will remember animal cells have partially permeable cell membrane. This means that if they are placed in pure water because their cytoplasm is a stronger solution than the pure water, water will pass into the cells by osmosis. The cells will therefore swell up n biology, osmosis is the movement of molecules from a zone of high water potential to a zone of lower water potential through a semi-permeable membrane. The phenomenon of osmosis occurs at different concentrations of water potential in animal cells and plant cells..