Diffusion (CIE IGCSE Biology: Co-ordinated Sciences (Double Award))

What is Diffusion in Biology?

• Diffusion is the net movement of molecules from a region of its higher concentration to a region of its lower concentration
• Molecules move down a concentration gradient, as a result of their random movement

Diffusion across the cell membrane

• For living cells, the principle of the movement down a concentration gradient is the same, but the cell is surrounded by a cell membrane which can restrict the free movement of the molecules
• The cell membrane is a partially permeable membrane - this means it allows some molecules to cross easily, but others with difficulty or not at all
• The simplest sort of selection is based on the size of the molecules
• Diffusion helps living organisms to:
  • obtain many of their requirements
  • get rid of many of their waste products
  • carry out gas exchange for respiration

Examples of diffusion in living organisms 

• You will need to learn examples of substances that organisms obtain by diffusion
• Don’t forget that plants require oxygen for respiration at all times, as well as carbon dioxide for photosynthesis when conditions for photosynthesis are right (e.g. enough light and a suitable temperature)

Examples of Diffusion Table

Where does the energy for diffusion come from?

• All particles move randomly at all times
• This is known as Brownian motion
• The energy for diffusion comes from the kinetic energy of this random movement of molecules and ions

Brownian motion diagram

Brownian motion being demonstrated by particles in a beaker