Osmosis

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.

  • Partially permeable means that it only allows certain types of substances to pass through.

In the diagram above, there is a flow of water particles across a semipermeable membrane.

  • On side A, there is a low concentration of water molecules and a high concentration of solute molecules. So this is a concentrated solution.
  • On side B, there is a high concentration of water molecules and a low concentration of solute molecules. So this is a dilute solution.

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.

Effect of Osmosis on Cells

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.

Effect of Osmosis on Plant Cells

Water can move in and out of plant cells by osmosis and the cell wall prevents the cell from bursting.

  • A hypotonic solution has a lower concentration of solute than the cell does – Resulting in a net movement of water into the cell
  • An isotonic solution has an equal amount of solute as the cell does – Does not result in any net movement of water in or out of the cell
  • A hypertonic solution has a greater concentration of solute than the cell does – Resulting in a net movement of water out of the cell
  • Plants tend to grow best in a hypotonic environment

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.

Effect of osmosis on animal cells

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:

  • If a red blood cell is in a hypertonic solution, then water will move out of the cell by osmosis. This can cause the cell to shrink and die.
  • If a red blood cell is in a hypotonic solution, water moves into the cell. This can cause the cell to expand and burst.

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