Osmosis

3.2 Osmosis

Osmosis is a special type of diffusion that specifically deals with the movement of water molecules across a semi-permeable membrane.

Key Concepts

• Semi-permeable membrane: A membrane that allows some substances (like water) to pass through but blocks others (like larger molecules).
• Solute: The substance that is dissolved in a solvent.
• Solvent: The substance that dissolves the solute (usually water).
• Concentration: The amount of solute present in a given volume of solvent.

The Process of Osmosis

Osmosis describes the net movement of water molecules from a region of high water concentration (low solute concentration) to a region of low water concentration (high solute concentration) through a semi-permeable membrane.

This movement continues until the water concentration is equal on both sides of the membrane.

Types of Solutions and their Effect on Cells

The surrounding solution can be classified as either hypotonic, isotonic, or hypertonic, and this will affect the cell.

Solution Type	Water Movement	Cell Appearance	Explanation
Hypotonic Solution	Water moves into the cell	Swollen, turgid	Lower solute concentration outside the cell than inside. Water moves down the concentration gradient.
Isotonic Solution	No net water movement	Normal, flaccid	Equal solute concentration inside and outside the cell.
Hypertonic Solution	Water moves out of the cell	Shrunken, crenated	Higher solute concentration outside the cell than inside. Water moves down the concentration gradient.

Importance of Osmosis

Osmosis is crucial for many biological processes, including:

• Maintaining cell turgor pressure in plants.
• Absorption of water in the intestines.
• Regulation of blood volume and pressure in animals.