Chapter 8: Movement through cell membranes

Chapter 8: Movement through cell membranes

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); e.g. oxygen and carbon dioxide diffuse across the alveolus membranes from regions of relatively high concentration to regions of relatively low concentration.

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; e.g. water is continuously moving across the membrane of an Amoeba by osmosis from the freshwater (high water concentration) to the interior of the cell (lower water concentration)

Application of Osmosis
Osmosis is used by food industry for preservation of food; e.g. tinned fruit/vegetables are placed in a concentrated sugar (syrup) or salt solution – this causes any bacteria/fungi present to die due to water leaving the cells (bacterial and fungal cells have a high water concentration relative to the syrup/salt solution).

Turgor and Plasmolysis

  • Turgor is the pressure exerted by the cells contents on the cell wall of a plant cell
  • Plasmolysis is the loss of water from a plant cell such that the cell membrane detaches from the cell wall.

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.