Biological Membranes

• Cell and organelle membrane

  Partially permeable membrane composed of a sea of phospholipids with protein molecules between the phospholipid molecules
• Cell/organelle membrane


  • Controls the movement of substances in and out of the cell/organelle
  • Contains receptors for other molecules such as hormones
  • Enables adjacent cells to stick together
• Fluid mosaic model


  The fluidity of the membrane and the mosaic arrangement of the protein give the structure of the membrane its name
• Movement of molecules through cell membrane


  1. Depends on the properties of the molecule

  2. Depends on the requirements of the cell
• 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 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


  • Surface area of the surface increases
  • Diffusion distance decreases
  • Diffusion gradient becomes more steep