2.4.5 Osmosis

Key Features of Osmosis:

1. Water Potential (Ψ):

• Water potential is the measure of the tendency of water molecules to move and is measured in kilopascals (kPa).
• Pure water has the highest water potential of 0 kPa. The addition of solutes lowers water potential, making it negative.
• Higher solute concentration = Lower (more negative) water potential.

2. Partially Permeable Membrane:

• A membrane that allows the passage of water molecules but restricts the movement of larger molecules like solutes.

3. Direction of Movement:

• Water molecules move from a region of higher water potential (less negative, fewer solutes) to a region of lower water potential (more negative, more solutes).

Osmosis in Cells:

1. Animal Cells:

• If the surrounding solution has higher water potential (hypotonic), water enters the cell, which may cause it to burst (lysis).
• If the surrounding solution has lower water potential (hypertonic), water leaves the cell, causing it to shrink (crenation).

2. Plant Cells:

• When water enters the cell in a hypotonic solution, the cell becomes turgid (swollen) due to the pressure exerted by the cell wall.
• In a hypertonic solution, water leaves the cell, causing the cell membrane to pull away from the cell wall, a process called plasmolysis.

Comparison of Osmosis and Active Transport:

Feature	Osmosis	Active Transport
Energy requirement	None (passive process).	Requires energy in the form of ATP.
Movement direction	From high water potential to low water potential.	From low concentration to high concentration.
Molecules involved	Water only.	All types of molecules (e.g., ions, nutrients).
Membrane proteins	No specific transport proteins needed.	Requires carrier proteins.

Factors Affecting Osmosis:

1. Water Potential Gradient:

• A steeper gradient increases the rate of osmosis.

2. Thickness of the Membrane:

• Thinner membranes reduce the distance water molecules need to travel.

3. Surface Area:

• A larger surface area (e.g., root hair cells) allows faster osmosis.