membranes and membrane transport

• Lipid bilayers


  The basis of cell membranes
• Phospholipids


  • Amphipathic
  • Hydrophobic phosphate head
  • Hydrophilic fatty acid tails
• Phospholipid bilayer formation


  1. Hydrophilic phosphate heads face water

  2. Hydrophobic fatty acid tails in the middle
• All membranes in cells are composed of a phospholipid bilayer
• Kinetic theory


  Particles are in constant motion
• Simple diffusion


  Passive transport of particles from a region of high concentration to a region of low concentration
• Small uncharged particles and fat soluble molecules can diffuse across plasma membranes, but hydrophilic, charged particles cannot pass directly through cell membranes
• Integral proteins


  Permanently attached to the plasma membrane, penetrate into the centre of the phospholipid bilayer
• Peripheral proteins


  Temporarily attached to one side of the membrane, not penetrating the phospholipid bilayer
• Osmosis


  Passive transport of water molecules from a region of low solute concentration to a region of high solute concentration through a semipermeable membrane
• Aquaporins


  Integral channel proteins that selectively transport water rapidly through membranes
• Facilitated diffusion


  Passive transport of molecules from a region of high concentration to a region of low concentration through channel proteins
• Active transport


  Movement of particles from a region of low concentration to a region of high concentration using protein pumps and ATP energy
• Facilitated diffusion and active transport allow selective permeability in membranes, while simple diffusion is not selective
• Glycoproteins


  Membrane proteins with a carbohydrate chain attached
• Glycolipids


  Phospholipids with a carbohydrate chain attached
• Functions of glycoproteins and glycolipids

  • Receptors
  • Cell to cell communication
  • Immune response
  • Cell to cell adhesion
• The carbohydrate chains of glycoproteins and glycolipids are on the outside surface of the cell