Osmosis is the diffusion of water through the cell membrane. The cell membrane is selectively permeable and water is able to pass through it. If there is a higher concentration of water, outside the cell membrane, relative to the inside, inward osmosis will occur. The contents of the cell will swell. This can be seen under a compound microscope using onion cells. If a wet-mount slide of onion epidermis is prepared and the cells are placed in distilled water, the cytoplasm of the cell will expand due to inward osmosis. There will be a greater concentration of water outside the cell, relative to the inside and inward osmosis will occur. In microscopic organisms living in freshwater, there will always be inward osmosis occurring and the homeostasis of these cells would be threatened because they could potentially burst. However, mechanisms like contractile vacuoles use energy to push out the excess water from these cells.