Osmosis involves the movement of water molecules down a concentration gradient from an area of high concentration to an area of low concentration, through a semi-permeable membrane.
Cell membranes are semi-permeable, so allow small molecules such as water, oxygen, carbon dioxide and glucose to pass through, but prevent larger molecules such as sucrose and starch from passing through.
When a cell is placed into a liquid containing water, one of three possible situations will arise:
- If the cell has a lower water concentration than its environment, water will diffuse into the cell by osmosis, and the cell will swell. Although water molecules can diffuse both ways across the cell membrane, more water will enter the cell than leave it. Animal cells do not have cell walls, so they may eventually burst. In contrast, plant cells are prevented from bursting by the presence of their cell walls: the cells will become turgid (swollen and hard), and the pressure inside the cells rises until eventually no more water can enter them.
- If the cell has a higher concentration of water than its environment, more water will diffuse out of the cell than into it, and the cell will shrivel.
- If the cell has exactly the same water concentration as its environment, there will be no net movement of water across the cell membrane and the cell will stay the same size.