3.2 Osmosis

Definition

Osmosis is the net movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration.

Partially Permeable Membranes

A partially permeable membrane allows some molecules to pass through but blocks others. This property is crucial for osmosis. Examples include cell membranes.

Water Potential

Water moves from areas of high water potential to areas of low water potential. Water potential is a measure of the tendency of water to move from one area to another.

Osmotic Concentration

Osmotic concentration refers to the number of solute particles in a solution. A higher concentration of solutes leads to a lower water potential.

Types of Solutions and their Effect on Cells

The movement of water across a membrane depends on the concentration of solutes on either side. We can categorize solutions as follows:

• Hypotonic Solution: A solution with a lower concentration of solutes than the inside of the cell. Water will move into the cell.
• Isotonic Solution: A solution with the same concentration of solutes as the inside of the cell. There is no net movement of water.
• Hypertonic Solution: A solution with a higher concentration of solutes than the inside of the cell. Water will move out of the cell.

Osmosis in Cells

The movement of water in and out of cells is essential for their survival. The type of solution surrounding a cell determines whether water enters or leaves.

Table Summarizing Osmosis

Solution Type	Concentration of Solutes (compared to inside the cell)	Movement of Water	Effect on Cell
Hypotonic	Lower	Water moves into the cell	Cell swells (may burst)
Isotonic	Same	No net movement of water	Cell maintains its normal shape
Hypertonic	Higher	Water moves out of the cell	Cell shrinks

Key Concepts

• Water moves down its water potential gradient.
• Osmosis is a passive process, meaning it does not require energy.
• The movement of water in and out of cells is vital for their function.