Osmosis (SQA National 5 Biology): Revision Note

The process of osmosis

• Osmosis is a type of diffusion that involves the movement of water molecules

• Osmosis can be defined as:

the movement of water molecules from a higher water concentration to a lower water concentration through a selectively permeable membrane.

• Note that a selectively permeable cell membrane can also be described as partially permeable

Osmosis in plant & animal cells

• Plant and animal cells are surrounded by selectively permeable cell membranes; this means that water can move in and out of cells by osmosis

• Osmosis affects animal and plant cells in different ways

Animal cells

• Animal cells can burst or shrink as follows:

  • animal cells placed in a strong sugar solution will shrink

    • the cell cytoplasm has a higher water concentration than the surroundings

    • water moves out by osmosis

    • cell volume decreases

  • animal cells placed in distilled water will burst

    • the cell cytoplasm has a lower water concentration than the surroundings

    • water enters by osmosis

    • cell volume increases

• Maintaining a stable water potential in animal tissues is essential to prevent cell damage

Plant cells

• The effect of osmosis on plant cells differs to animal cells due to the presence of plant cell walls

  • Cell walls are rigid so maintain cell shape even when the volume of the cytoplasm changes

• Plant cells can become turgid or plasmolysed as follows:

  • plant cells placed in a strong sugar solution will become plasmolysed

    • the cell cytoplasm has a higher water concentration than the surroundings

    • water moves out by osmosis

    • cytoplasm volume decreases

    • cell membrane pulls away from the cell wall

  • plant cells placed in distilled water will become turgid

    • the cell cytoplasm has a lower water concentration than the surroundings

    • water enters by osmosis

    • cytoplasm volume increases

    • cytoplasm pushes against the cell wall