Diffusion (Cambridge O Level Biology)

• Diffusion is the 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 Table

Site	Molecules Moving	From	To
Small intestine	Products of digested food such a glucose and amino acids	Lumen of small intestine	Blood in villi capillaries
Leaf	Carbon dioxide	Air spaces between mesophyll cells	Chloroplasts in mesophyll or palisade cells (for photosynthesis)
Leaf	Oxygen	Air spaces between mesophyll cells	Mitochondria of mesophyll cells (for respiration)
Lungs	Carbon dioxide	Blood in alveoli capillaries	Alveolar air space
Lungs	Oxygen	Alveolar air space	Blood in alveoli capillaries

Where does the energy for diffusion come from?

• All particles move randomly at all times
• The energy for diffusion comes from the kinetic energy of this random movement of molecules and ions

Random movement of particles diagram

Particles move randomly due to the kinetic energy all particles possess