The movement of water in between the plant cells happens according to the solution concentration that exists in the surrounding. The osmotic pressure in the cytoplasm of the plant cell is equilibrated by the solution external to the cell. Now, the external solution is termed as isotonic. If the concentration of the external solution is lower than that of the cytoplasm then it is termed as hypotonic. If the external solution concentration is higher than that of the cytoplasm then it is termed as hypertonic. The plant cells bulge by water intake from the hypotonic surroundings and shrinks by the expulsion of water to the hypertonic surroundings.
When water moves out of the plant cell then the process is called as plasmolysis. The cell membrane will get separated from the cell when the cell shrinks from the cell wall during the plasmolysis. The shrinking of the plant cell occurs when it is placed in the hypertonic solution which has higher concentration of solute than that is present in the protoplasm of the cell. The movement of water occurs from the cytoplasm and then from the vacuole. The water that is taken away from the cell by diffusion into the external fluid makes the protoplast to shrink away from the cell wall and the cell state is called as ‘plasmolysed’. The water movement occurs usually from the region of higher water potential (inside cell) to the region of lower water potential (outside cell). When the movement of water does not take place from the cell into an isotonic solution then the two regions are said to be in equilibrium. The cells are called as flaccid.
When the plant cell is kept in hypotonic solution, the water moves from the solution into the cell creating a pressure inside the cell on the cell wall. This pressure is called as turgor pressure. The pressure that is created by the protoplasts on the cell wall due to the water movement into the cell is called as pressure potential ᵠp. The cell does not break up as the cell wall is very much rigid. The growth and extention of the cells happen due to the turgor pressure that is created by the entry of water.