2.4.6 Osmosis in Plant Cells

Osmosis and Plant Cell Structure:

1. Key Components:

• Cell wall: Provides structural support and prevents the cell from bursting.
• Cell membrane: Partially permeable, allowing the movement of water in and out of the cell.
• Vacuole: Stores water and solutes, contributing to the water potential within the cell.

2. Water Potential (Ψ):

• A measure of the tendency of water to move.
• Total water potential in plant cells is determined by:
• Solute potential (Ψs): The effect of solute concentration (always negative).
• Pressure potential (Ψp): The pressure exerted by the cell wall (positive value).

Effects of Osmosis on Plant Cells:

3. Turgid Cell (Hypotonic Solution):

• Water enters the cell as the external solution has a higher water potential than the cytoplasm.
• The vacuole swells, pushing the cytoplasm against the cell wall.
• The cell becomes turgid, which helps maintain the plant's structure and upright position.
• Pressure potential (Ψp) increases, opposing further water intake.

4. Flaccid Cell (Isotonic Solution):

• The external solution has a similar water potential to the cytoplasm.
• No net movement of water occurs.
• The cell becomes flaccid (limp), and the plant may wilt.

5. Plasmolysed Cell (Hypertonic Solution):

• Water leaves the cell as the external solution has a lower water potential than the cytoplasm.
• The vacuole shrinks, and the cytoplasm pulls away from the cell wall.
• The cell is plasmolysed, which can cause damage if prolonged.

Practical Investigation:

• Aim: To observe the effects of osmosis on plant cells.
• Method:
  1. Place thin slices of plant tissue (e.g., onion or potato) in solutions with varying water potentials.
  2. Observe cells under a microscope or measure changes in tissue mass.
• Results:
  • Cells in distilled water become turgid.
  • Cells in concentrated salt/sugar solutions become plasmolysed.

Importance of Osmosis in Plants:

1. Maintains Turgidity:

• Turgid cells provide structural support, helping leaves and stems stay upright for optimal photosynthesis.

2. Water Transport:

• Drives water uptake from the soil via root hair cells and its movement through the plant.

3. Stomatal Function:

• Osmosis regulates guard cell turgor, controlling the opening and closing of stomata for gas exchange.