Membrane transport

• Simple Diffusion

  • occurs due the constant random motion of particles in liquids and gases
  • movement of molecules from high concentration to low concentration (down the concentration gradient)
  • passive transport (no input of energy required)
  • Move through membrane without assistanceE.g. oxygen diffusion in capillaries, urea excretion from cells
• Facilitated Diffusion


  Movement of particles that cannot easily pass through the membrane directly, from high concentration to low concentration through a protein
• Facilitated Diffusion


  • Channels proteins for inorganic charged particles
  • Carrier proteins for large insoluble lipid molecules
• Passive transport


  Eg. K+ ions in nerve cells, Ca2+ ions for muscle contractions
• Proteins tend to be specific to the substrate that it helps move across the membrane
• Facilitated diffusion can be limited by the number of proteins available and present and hence, after a certain point, the rate will not increase despite change in aforementioned factors
• Osmosis

  • diffusion of water molecules from a region of lower solute concentration to higher solute concentration
  • across a partially permeable membrane
  • Osmotic potential: tendency of water to move into a solution - greater solute concentration, greater the osmotic potential
  • occurs due to small size of water molecules that can pass through membrane
• Recall that the main function of the plasma membrane is transport. Generally, transport is defined as passive or active.
• Active transport, like (passive) facilitated diffusion requires proteins. However, these proteins use energy in form of ATP to pump molecules against their concentration gradient. There are two types of active transport: Primary: direct use of metabolic energy for transport of molecules against concentration gradient. Secondary: coupling the movement of one molecule against the concentration gradient with the movement of another along the concentration gradient of the second molecule, often created by primary active transport.
• Exocytosis: Transport of molecules in secretory vesicles that fuse with the plasma membrane upon contact to release the contents outside of the cell
• Endocytosis transport of molecules into the cell through invagination of the plasma membrane and formation of the phospholipid vesicle containing the molecule.