Biological membranes

• Biological membranes


  All cells and organelles are surrounded by a partially permeable membrane composed of a sea of phospholipids with protein molecules between the phospholipid molecules
• Biological membranes


  • The main function is controlling the movement of substances in and out of the cell/organelle
  • They also contain receptors for other molecules such as hormones
  • They enable adjacent cells to stick together
  • The fluidity of the membrane and the mosaic arrangement of the protein give the structure of the membrane its name – fluid mosaic model
• Movement of molecules through cell membrane


  1. Diffusion

  2. Facilitated diffusion

  3. Osmosis

  4. Active transport

  5. Exocytosis

  6. Endocytosis
• Diffusion


  Passive movement of small, non-polar lipid soluble molecules such as carbon dioxide and oxygen from an area of high concentration to an area of low concentration, directly through the phospholipid bilayer
• Facilitated diffusion

  Requires a channel protein in the cell membrane to transport polar molecules, charged and water soluble molecules across the membrane
• Osmosis


  Diffusion of water molecules from an area of low solute concentration to an area of high solute concentration through a partially permeable membrane
• Osmosis


  Diffusion of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane
• Active transport


  Can transport all types of molecules through carrier proteins from an area of low concentration to an area of high concentration, requires energy in the form of ATP
• Exocytosis and endocytosis

  Transport large particles and bulk transport, particles are enclosed in vesicles made from the cell surface membrane and transported into the cell in endocytosis, in exocytosis, vesicles containing large particles are fused with the cell surface membrane
• The rate of gas exchange by diffusion


  Becomes more rapid as surface area of the surface increases, diffusion distance decreases, diffusion gradient becomes more steep