Membrane Structure & Transport

• Phospholipid bilayer


  The basic structure of the cell membrane, formed by two layers of phospholipids with hydrophilic heads facing outwards and hydrophobic tails facing inwards

  
• Phospholipids


  • Amphipathic - have a hydrophilic phosphate head and hydrophobic hydrocarbon tails
  • Form a phospholipid monolayer when in water, with heads facing out and tails facing in
  • Form a phospholipid bilayer when at sufficient concentration, with two layers of phospholipids

  
• Micelle


  A spherical structure formed when phospholipids are mixed with water, with the hydrophilic heads facing outwards and the hydrophobic tails facing inwards

  
• Phospholipid bilayer


  • Composed of two layers of phospholipids with hydrophobic tails facing inwards and hydrophilic heads facing outwards
  • Allows for some fluidity due to weak hydrophobic interactions between the hydrocarbon tails
  • Acts as a barrier to most water-soluble substances due to the non-polar fatty acid tails

  
• Cholesterol


  A lipid that is a component of animal cell membranes, positioned between phospholipids with its hydrophobic region interacting with the phospholipid tails and its hydrophilic hydroxyl group interacting with the phosphate heads

  
• Role of cholesterol in mammalian membranes

  • Stabilizes the membrane at higher temperatures by preventing the membrane from becoming too fluid
  • Increases fluidity of the membrane at colder temperatures to prevent it from becoming too rigid
  • Reduces the permeability of the membrane to hydrophilic ions like sodium and hydrogen
  • Increases the mechanical strength and stability of the membrane
• Integral membrane proteins


  Partially hydrophobic proteins embedded in the phospholipid bilayer, either in one or both layers

  
• Peripheral membrane proteins


  Hydrophilic proteins temporarily attached to the surface of integral proteins or connected to the membrane via a hydrocarbon chain
• Functions of membrane proteins


  • Transport
  • Receptors
  • Cell adhesion
  • Cell-to-cell recognition
  • Immobilized enzymes
• History of fluid mosaic model

  1. Gorter and Grendel model (1920s) - phospholipids arranged in a bilayer

  2. Davson and Danielli model (1930s) - proteins arranged in layers above and below the phospholipid bilayer

  3. Singer and Nicolson fluid mosaic model (1970s) - proteins are both peripheral and integral, dispersed throughout the fluid membrane
• Simple diffusion


  The net movement of particles from a region of higher concentration to a region of lower concentration, driven by the random motion of the particles
• Simple diffusion


  A type of transportation that involves particles passing between phospholipids in the plasma membrane
• Simple diffusion


  The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration
• Molecules or ions move down a concentration gradient during simple diffusion
• As a result of diffusion, molecules or ions tend to reach an equilibrium (given sufficient time), where they are evenly spread within a given volume of space
• Factors affecting the rate of diffusion across a membrane

  • Steepness of the concentration gradient
  • Temperature
  • Surface area
  • Properties of the molecules or ions
• Large molecules


  Diffuse more slowly as they require more energy to move
• Uncharged molecules (e.g. oxygen)


  Diffuse faster as they move directly across the phospholipid bilayer
• Non-polar molecules


  Diffuse more quickly as they are soluble in the non-polar phospholipid bilayer
• Polar molecules


  Smaller polar molecules (e.g. urea) can diffuse at low rates, although larger polar molecules cannot easily pass through the hydrophobic part of the membrane
• Facilitated diffusion


  The movement of substances that cannot diffuse through the phospholipid bilayer of cell membranes, with the help of certain proteins
• Substances that require facilitated diffusion

  • Large polar molecules such as glucose and amino acids
  • Ions such as sodium ions (Na+) and chloride ions (Cl-)
• Channel proteins


  Water-filled pores that allow charged substances (e.g. ions) to diffuse through the cell membrane
• Carrier proteins


  Proteins that can switch between two shapes to allow the movement of molecules or ions across the membrane
• During facilitated diffusion, the net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
• Osmosis


  The diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane
• Osmolarity


  The solute concentration of a solution; a solution with high osmolarity has a high solute concentration and a solution with low osmolarity has a low solute concentration
• Water will move from a solution of low osmolarity to a solution of high osmolarity across a partially permeable membrane