Osmosis is a specific type of diffusion that takes place with water molecules. It is the movement of water molecules from a region of high concentration to a region of low concentration. This happens across a partially permeable membrane.
In the diagram above, there is a flow of water particles across a semipermeable membrane.
This is why the water molecules move from side B to side A, from a dilute solution to a concentrated solution.
Water moves across a partially permeable membrane and down a concentration gradient. Osmosis does not require energy, so it is a passive process.
If the concentration of water is the same on both sides, then the movement of water particles will be the same in both directions. However, there will be no net movement of the water molecules.
The cytoplasm of cells is a relatively concentrated solution, meaning that it has a lower concentration of water. When a cell is in water, water will move into the cell by osmosis.
Osmosis has different effects on cells, depending on their type. Let’s look at plant cells first.
Water can move in and out of plant cells by osmosis and the cell wall prevents the cell from bursting.
If a plant cell is placed in water, the cell contents push against the cell wall and the cell becomes turgid. This means the cell enlarges and swells with water.
If a plant cell is placed in a concentrated solution, water moves out of the cell by osmosis, causing the cell to shrink. This is known as the cell becoming flaccid (weaker and softer).
The process in which cells lose water in a hypertonic solution is called plasmolysis. In a very concentrated solution, a cell can become fully plasmolysed.
Animal cells do not have a cell wall, so they are more affected by changes in the concentration of the solutions around the cell. Consider the case of red blood cells as an example: