Diffusion

• Simple diffusion is the movement of substances from an area of high concentration to an area of low concentration, down a concentration gradient across a partially permeable membrane directly through the phospholipid bilayer. 
• Simple diffusion:

  • There is a concentration gradient 
  • Molecules have kinetic energy 
  • No additional energy from ATP is required- it is a passive process. 
  • No specific proteins are required.  
• Molecules that can diffuse through the phospholipid bilayer:

  • Lipid soluble e.g. steroid hormones 
  • Small and non-polar e.g. CO2, O2 
  • Very small & polar e.g. water (slowly)
• Factors affecting the rate of diffusion:

  • Temperature 
  • Concentration gradient 
  • Thickness of exchange surface 
  • The surface area 
• Facilitated diffusion is the movement of substances from an area of high concentration to an area of low concentration, down a concentration gradient across the membrane via carrier or channel proteins.
• Unlike simple diffusion, the rate of facilitated diffusion is limited by the number of carrier/channel proteins in the membrane.
• Facilitated diffusion:

  • There is a concentration gradient 
  • Protein channels/carriers are required. 
  • Molecules have kinetic energy 
  • No additional energy from ATP is required  
• Channel Proteins

  • These act as pores in the membrane.
  • Some only allow specific ions through e.g. Na ions (due to specific shape).
  • They can be gated so they only open/close in specific circumstances.  
• Carrier proteins

  • These allow specific large molecules to pass through.
  • When the molecule (e.g. glucose) binds to a specific site on the carrier, the protein changes shape and transfers the molecule to the other side of the membrane. 
• Different membranes around different cells can have different channels and carrier proteins in their bilayer and so can control the entry and exit of different substances into and out of the cell. 
• Factors affecting the rate of facilitated diffusion:

  • Concentration gradient
  • Temperature
  • The number of channel/ carrier proteins