Osmosis and Diffusion

Selective Permeability

Selectively permeability refers to the cell membrane controlling what substances enter and leave the cell.
Selective permeability is controlled by proteins embedded in the cell membrane.


  • Diffusion is the movement of particles from a region of high concentration to a region of low concentration (i.e. down a concentration gradient)

An example is in the movement of oxygen and carbon dioxide across the alveolar membranes from regions of relatively high concentration to regions of relatively low concentration. See Chapter 35.


  • Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration across a semi-permeable membrane

An example is in Amoeba  where water is continuously moving into the cell by osmosis from the freshwater (high water concentration) to the interior of the cell (lower water concentration). See Chapter 22.

Application of Osmosis

Osmosis is used by food industry for preservation of food. Any microorganisms present will die due to water leaving the cells.

  • Tinned fruits are placed in a concentrated sugar (syrup) solution
  • Meats can be salted

Turgor, Plasmolysis and Crenation

  • Turgor is the pressure exerted by the cells contents on the cell wall of a plant cell. The cell wall prevent it from bursting.
  • Plasmolysis is the loss of water from a plant cell such that the cell membrane detaches from the cell wall.
  • Crenation is the loss of water from an animal cell. It shrivels up.
Cretinised red blood cell (TOP) and normal red blood cell (BOTTOM)
A wilted plant

Mandatory Practical Activity: to demonstrate osmosis


  • Visking tubing
  • Water
  • Syringe
  • Sucrose
  • Beakers


  • Set up the equipment as shown in the diagram and leave for at least 30 minutes or overnight.


  • The mass of the control tubing remained the same.
  • The mass of the test tubing increased.


  • The mass of the test tubing increased due to osmosis – movement of water across the Visking tubing membrane.
  • Sucrose is a molecule that cannot pass through the Visking tubing membrane.