The Process of Diffusion (WJEC GCSE Biology: Combined Science)

• Diffusion is the movement of substances down a concentration gradient from an area of higher concentration to an area of lower concentration
  • This movement continues until the concentration of substances is equal; this is known as equilibrium
• Diffusion is an entirely passive process, meaning that no energy is required
• Note that diffusion occurs as the result of random movement of molecules

Diffusion diagram

Perfume particles in air diffuse from a region of higher concentration to a region of lower concentration until they are evenly distributed

Diffusion & cell membranes

• All cells are surrounded by a cell membrane which separates the inside of the cell from its outside environment
• Substances can diffuse through the cell membrane to enter or exit the cell
  • E.g. oxygen and carbon dioxide can diffuse across cell membranes during gas exchange
• Not all substances can diffuse across cell membranes; they are said to be partially permeable

Diffusion across a cell membrane diagram

Cell membranes are partially permeable, meaning that some substances can diffuse across while others cannot

Modelling diffusion

• Visking tubing (sometimes referred to as dialysis tubing) is a non-living partially permeable material made from cellulose
• It can be used to model the process of diffusion
• Pores in the membrane are small enough to prevent the passage of large molecules (such as starch) but allow smaller molecules (such as glucose) to pass through by diffusion

Investigating diffusion with Visking tubing

1. Tie off the Visking tubing at one end and fill with a solution of starch and glucose
2. Tie off the other end of the tubing
3. Place in a beaker of distilled water and leave overnight
4. Test the contents of both the Visking tubing and the beaker for:
   • Glucose
     • Using Benedict's solution
   • Starch
     • Using iodine solution

• The results should show that:
  • Glucose is present in the beaker and in the Visking tubing
    • It has diffused through the pores of the Visking tubing
  • Starch is only present in the Visking tubing
    • Starch molecules are too large to diffuse through the Visking tubing pores

Visking tubing diffusion diagram