LifeLine

A process of movement of molecules under a concentration gradient. Net movement of particles. Helps in the movement of substances in and out of the cells.  The particles have random movement and motion from a high to low concentration.

High to low concentration

• Move about randomly – distribute evenly
• Needs to reach a state of equilibrium
• Diffusion take place in the partially permeable cell membranes
  • Allow only some materials to pass through
    • Only molecules that are small in size
  • Large molecules
    • Facilitated diffusion – Carrier proteins
• Passive process
  • Down the concentration gradient from high > low
  • No ATP used

Facilitated diffusion

• The passive movement of molecules across a cell membrane via the aid of a membrane protein (carrier / channel protein)
  • Carrier proteins have a enzyme – substrate model 
  • Channel protein is like a passage 
• Involves large / charged molecules (e.g. ions, glucose, etc.) 

• Movement is down the concentration gradient but special carrier proteins are required to enable movement. 
• As glucose cant pass through the membrane, it will attach to the carrier proteins. Their carrier proteins will turn and throw the glucose inside.

Factors affecting diffusion

1. Concentration gradient
   • Large concentration gradient – high rate of diffusion
   • Diffusion takes place as long as there is a difference between the concentrations of a substance across a barrier
2. Temperature
   • More kinetic energy as temperature is high
   • High temperature – high rate of diffusion
3. Membrane Permeability
   • Increase in membrane permeability – high rate of diffusion
   • If the membrane is selectively permeable, permeable, impermeable or semi-permeable
4. Distance and thickness
   • The shorter the distance the substances have to move – high rate of diffusion
   • The thicker the surface the longer it will take for molecules to cross it
5. Surface area
   • More space for molecules to diffuse across the membrane
   • Larger the surface area – high rate of diffusion