cpac 5

• Cell-surface membranes


  Made up of a phospholipid bilayer which makes them selectively permeable
• Permeability


  Can be changed by different variables, such as temperature and concentration of solvents like ethanol
• Beetroot cells


  Contain a purple pigment called betalain
• Permeability measurement


  Amount of pigment leaked from beetroot cells into an aqueous solution using a colorimeter
• Equipment


  • Water baths
  • Thermometer
  • Distilled water
  • Syringe
  • Beetroot
  • Cork borer
  • White tile
  • Knife
  • Pipette
  • Test tubes
  • Colorimeter
  • Cuvettes
  • Forceps
• Method


  1. Cut beetroot into identical cylinders

  2. Place each cylinder in distilled water in water bath at range of temperatures 0-70°C

  3. Leave for 15 minutes

  4. Remove beetroot, decant liquid into clean test tubes

  5. Set colorimeter to blue filter, zero with distilled water, filter samples into cuvettes

  6. Measure absorbance
• Higher absorbance indicates higher pigment concentration, and hence a more permeable membrane
• Hazards


  • Scalpel
  • Broken glass
  • Hot liquids
  • Ethanol
• Safety precautions


  • Cut away from fingers, use forceps, keep away from edge of desk
  • Take care when handling glassware, keep away from edge of desk
  • Handle with care, use tongs, wear eye protection, keep away from edge of desk
  • Wear eye protection, keep away from naked flames
• In emergency


  • Elevate cuts, apply pressure, seek medical assistance
  • Wash eyes and skin with cold water
• As temperature increases


  Permeability of cell-surface membrane increases
• Reason for increased permeability


  Proteins in membrane denature as heat damages bonds in tertiary structure, creating gaps in membrane
• At low temperatures


  Phospholipids have little energy and are packed closely together, making membrane rigid, decreasing permeability
• At very low temperatures, ice crystals can form which pierce the cell membrane and increase the permeability