Osmosis in Plant Cells (Cambridge International A-Level Biology)

Osmosis: Plant Cells

Spec point — explain the movement of water between cells and solutions in terms of water potential and explain the different effects of the movement of water on plant cells and animal cells

• Osmosis is the net movement of water molecules from a region of higher water potential(dilute solution) to a region of lower water potential (concentrated solution), through a selectively permeable membrane

• If a plant cell is placed in pure water or a dilute solution, water will enter the plant cell through its selectively permeable cell surface membrane by osmosis

  • This is because the pure water or dilute solution has a higher water potential than the plant cell

• As water enters the vacuole of the plant cell, the volume of the plant cell increases

• The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell

  • The inelastic cell wall prevents the cell from bursting
  • The pressure created by the cell wall also stops too much water entering and this also helps to prevent the cell from bursting

• When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid

• This turgidity is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant

  • This allows the plant to stand upright with its leaves held out to absorb sunlight

• If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts

Osmosis of water into a plant cell

• If a plant cell is placed in a solution with a lower water potential than the plant cell (such as a concentrated sucrose solution), water will leave the plant cell through its selectively permeable cell surface membrane by osmosis

• As water leaves the vacuole of the plant cell, the volume of the plant cell decreases

• The protoplast gradually shrinks and no longer exerts pressure on the cell wall

• As the protoplast continues to shrink, it begins to pull away from the cell wall

• This process is known as plasmolysis

  • The plant cell is plasmolysed

Plasmolysis of a plant cell that has been placed in a solution with a lower water potential than the cell itself<

Exam Hint Remember—plant cell membranes are composed of a phospholipid bilayer and are selectively permeable (only certain molecules can cross), whereas plant cell walls are made of cellulose and are freely permeable.