practical 4

• Cell-surface membranes


  Made up of a phospholipid bilayer which makes them selectively permeable
• Permeability of cell-surface membranes

  • Can be changed by different variables, such as temperature and concentration of solvents, like ethanol
• Measuring permeability of cell-surface membranes

  1. Use beetroot cells which contain a purple pigment called betalain

  2. When cell-surface membrane has higher permeability, more pigment leaks out

  3. Permeability measured by amount of pigment leaked into aqueous solution

  4. Using a colorimeter
• Equipment


  • Beetroot
  • Scalpel
  • Forceps
  • Cutting board
  • Ruler
  • Tongs
  • Distilled water
  • Boiling tubes
  • Boiling tube rack
  • Colorimeter
  • Cuvettes
  • Filter paper
  • Timer
  • Water bath
  • Thermometer
  • Ethanol
• Method


  1. Cut beetroot into identical cubes

  2. Clean off any pigment released

  3. If investigating temperature: place cubes in equal volume of distilled water, place in water bath at range of temperatures

  4. If investigating concentration of solvents: create dilution series of ethanol using distilled water, concentrations 0-100%

  5. Leave samples for 20 minutes

  6. Set colorimeter to blue filter and zero using distilled water

  7. Filter each sample into cuvette using filter paper

  8. Measure absorbance of each solution
• Higher absorbance indicates higher pigment concentration, and hence a more permeable membrane
• Hazards


  • Scalpel (cuts)
  • Broken glass (cuts)
  • Ethanol (irritant/flammable)
  • Hot liquids (scalding)
• Safety precautions


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


  • Elevate cuts, apply pressure, seek medical assistance
  • Elevate cuts, apply pressure, do not remove glass from wound, seek medical assistance
  • Wash eyes and skin with cold water
  • Run burn under cold water, seek medical assistance
• As temperature increases


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

  Proteins in membrane denature as heat damages bonds in tertiary structure, creating gaps in membrane
• Reason for decreased permeability at low temperatures

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

  Permeability of cell-surface membrane increases
• Reason for increased permeability with higher ethanol concentrations

  Ethanol causes the cell-surface membrane to rupture, releasing the betalain pigment from the cell, and more gaps will form