A cell is selectively permeable, meaning that some substances can pass directly through the cell membrane while other substances cannot. However, molecules enter and exit the cell through three processes of osmosis, diffusion and active transport. Osmosis - Can be said as a movement of water from the upper semipermeable membrane to the lower one, like a cell wall, when it is smaller the molecules continue to diffuse until the area where the molecules are located reaches a state of equilibrium , such as studying the effect of sucrose solution on a mass of potatoes. If the concentration of water entering the tissue of the plant cell is higher than the concentration of water in the solution, then water is lost in the cell through osmosis. This will cause it to shrink and make the plant cell into a state of plasmolysis. Also, because the potatoes were in a distilled water solution, the cell could swell as the water moves into the cell vacuole and pushes against the cell wall, eventually the cell holds as much water as it can hold. The strong cell wall prevents the cell from breaking down and when this happens it is said to be turgid. This explains the definition of osmosis, osmosis is a particular type of diffusion involving water molecules, it occurs when two solutions are separated by semipermeable media