Cell membranes and transport

• Cell membranes


  Made up of proteins and phospholipids
• Fluid mosaic model

  Model of the structure of biological membranes, where proteins are studded through a phospholipid bilayer like a mosaic
• Fluid mosaic model

  • Movement of molecules within a layer of the bilayer is its fluidity
• Phospholipids

  One sheet of phospholipids opposite each other, with hydrophilic phosphate heads pointing out and interacting with water, and hydrophobic tails pointing towards each other
• Proteins

  • Scattered throughout phospholipid bilayer
• Extrinsic proteins

  Found on either outer surfaces of bilayer, providing structural support and forming recognition sites
• Intrinsic proteins

  Extend across both layers of the phospholipid bilayer, some are carriers transporting water soluble substances and others allow active transport of ions across by forming channels
• Cell membranes


  • Selectively permeable
  • Only allow certain molecules through
• Lipid soluble molecules

  Oxygen and carbon dioxide can dissolve in the phospholipid bilayer and diffuse across the membrane
• Lipid soluble molecules dissolving and diffusing

  1. Dissolve in the phospholipid

  2. Diffuse across the membrane
• Water soluble substances

  Glucose, polar molecules and ions cannot readily diffuse through bilayer and must pass through intrinsic protein molecules
• Water soluble substances passing through

  1. Can't readily diffuse through phospholipids

  2. Must pass through intrinsic protein molecules
• Permeability increase

  1. Temperature increases vibrations of phospholipids, moving them further apart

  2. Organic solvents dissolve phospholipids
• Simple diffusion

  The movement of molecules from an area of high concentration to lower concentration, down a concentration gradient, until they are equally distributed
• Simple diffusion

  • Passive process
  • Doesn't require energy from ATP
  • Through phospholipid bilayer
• Diffusion rate increases by…

  • Higher concentration gradient
  • Shorter diffusion distance
  • Larger surface area
  • Smaller molecules
  • Being non-polar or fat soluble
  • Increased temperature
• Facilitated diffusion


  • Allows diffusion for polar molecules that can't pass directly through phospholipid bilayer
  • Protein channels or carriers are used
  • Happens down a concentration gradient
  • Channel and carrier protein availability limits the rate of facilitated diffusion
• Channel proteins


  • Hydrophilic so ions can pass through
• Carrier proteins


  • Allow diffusion of larger molecules such as sugars and amino acids
• Active transport


  • Molecules move against a concentration gradient
  • Process requires energy in the form of ATP from respiration
  • Process occurs through intrinsic carrier proteins
  • Rate is limited by availability of carrier proteins
• Co-transport


  • A type of facilitated diffusion where two different substances use the same carrier protein at the same time
  • Example: Sodium-glucose co-transport
• Osmosis


  The movement of water from an area of high water potential to low water potential across a selectively permeable membrane
• Water potential


  The tendency of water molecules to move
• Solute potential


  • The osmotic strength of the solution
  • Water potential of pure water is 0 and becomes more negative as the concentration of the solution increases
  • Higher concentration - more negative solute potential
• Pressure potential


  • Water entering a plant cell by osmosis expands the vacuole and pushes the cytoplasm against the cell wall
  • Cell wall can only expand a little so pressure outward builds up, restricting the entry of more water, making the cell turgid
  • The balance of the pressure potential and solute potential determines if water moves in or out
• Osmosis and plant cells


  Water potential = pressure potential + solute potential
• Turgor and plasmolysis


  • If the water potential of the external solution is higher than inside the cell, it is hypotonic and water moves into the cell
  • If the water potential of the external solution is lower than the solution inside the cell, it is hypertonic and water moves out of the cell
  • If the cell and external solution have the same water potential, it is isotonic and there is no net water movement
• Incipient plasmolysis


  • Pressure potential is at 0
  • Cytoplasm begins to be pulled away from cell wall
• Plasmolysed


  • Cytoplasm completely pulled away from cell wall
  • Cells are flaccid
• Osmosis and animal cells

  • Pressure potential doesn't need to be considered as animal cells don't have cell walls
  • No pressure potential as no cell wall
  • Animal cells are in an isotonic solution
  • Cells can burst in hypotonic solutions
  • Cells can shrink in hypertonic solutions
• Endocytosis


  • Phagocytosis - Plasma membrane engulfs material
  • Pinocytosis - Uptake of liquids, vesicles produced are smaller
• Exocytosis


  • Substances leave the cell, having been transported through the cytoplasm in a vesicle, which fuses with the cell membrane
  • ATP required