A hypotonic solution is one in which there is a higher concentration of water molecules outside the plant cell. When a plant cell is placed in a hypotonic solution, water molecules will flow into the cell through the semipermeable membrane. The plant cell swells until the water concentration inside the cell matches the water concentration outside the cell. This swelling causes the plant cell to become turgid, meaning hard and rigid. Turgidity is how plants are able to remain stiff and upright.
A hypertonic solution is one in which the concentration of water molecules is lower outside the plant cell than the concentration inside the cell. When a plant cell is submerged in a hypertonic medium, water moves from inside the cell to the outside. This movement causes the cell to shrivel and the cytoplasm to pull away from the cell wall, a process known as plasmolysis. When plant cells become shriveled from a lack of water, the plant will wilt.
An isotonic solution is one in which the water concentration outside the cell equals the water concentration inside the cell. No movement of water molecules occurs when a plant cell is placed in an isotonic solution. This lack of osmosis is known as incipient plasmolysis, meaning the cell is neither turgid nor shriveled. Incipient plasmolysis causes the green leaves of a plant to droop since they lack the sturdy structure of turgid plant cells.
Reverse osmosis is the process of solvent molecules moving from a low-concentration solution to a high-concentration solution. This movement is the opposite of traditional osmosis and occurs as a result of a great amount of external pressure. Reverse osmosis does not happen naturally and is most widely used as a process to purify water. Therefore, it is important to note that reverse osmosis does not occur in plants and has no effect on the structure of plant cells.