Membrane Permeability

• Explain why the beetroot must be washed thoroughly before you start your investigation.

  The beetroot cylinders are washed and dried to remove excess surface pigment from the cut cells at the edge. This excess pigment would distort the transmission readings, giving inaccurate results.
• How does temperature affect the permeability of beetroot membrane?

  Heating the membrane can cause gaps to form between the phospholipid molecules and the membrane will become more permeable. The protein in the membrane can be denatured by heat. Beetroot cells contain betalain, a bright red, water soluble pigment, in the cell vacuoles.
• How does pH affect the permeability of the beetroot cell membrane?

  The ideal pH range for beetroot cell membrane permeability is between pH 6 and 8. At this range, the cell membranes are able to maintain their structural integrity and function effectively. Deviations from this range can result in increased permeability and potential damage to the cell.
• Cell-surface membranes are made up of a phospholipid bilayer which makes them

  selectively permeable. This permeability can be changed by different variables, such

  as temperature and concentration of solvents, like ethanol.
• The permeability of a membrane can be measured by using beetroot cells, which

  contain a purple pigment called betalain. When the cell-surface membrane has a

  higher permeability, more pigment leaks out of cells. The permeability can therefore be

  measured by the amount of pigment leaked from beetroot cells into an aqueous

  solution using a colorimeter.
• 1. Cut beetroot into 6-10 identical cubes using a scalpel.
  2. Wipe/rinse to clean off any pigment released as a result.
  3. If investigating temperature: place each of the cubes of beetroot in an equal volume of distilled water (5-15ml).
  4. Place each test tube in a water bath at a range of temperatures (30-80°C).
• 5. If investigating concentration of solvents: create a dilution series of ethanol

  using distilled water. Ethanol concentrations should range from 0-100% ethanol.

  6. Leave the samples for 20 minutes - the pigment will leak out of the beetroot.

  7. Set the colorimeter to a blue filter and zero using a cuvette with distilled water.

  8. Filter each sample into a cuvette using filter paper.

  9. Measure the absorbance for each solution. A higher absorbance indicates

  higher pigment concentration, and hence a more permeable membrane.
• Hazard: Scalpel

  Risk: Cuts from sharp object

  Mitigation: Cut away from fingers; use forceps to hold sample whilst cutting, keep scalpel away from the edge of the desk
• Hazard: Broken glass

  Risk: Cuts from sharp object

  Mitigation: Take care when handling glassware; keep away from edge of desk
• Hazard: Ethanol

  Risk: Irritant/flammable

  Mitigation: Wear eye protection; keep away from naked flames
• Hazard: Hot liquids

  Risk: Scalding

  Mitigation: Handle with care; use tongs to remove boiling tubes from water bath; wear eye protection, keep away from the edge of the desk
• The graph plotted from this experiment will be absorbance against ethanol concentration/temperature.
• Temperature Conclusion:

  • As the temperature increases, the permeability of the cell-surface membrane also increases. This is because the proteins in the membrane denature as the heat damages the bonds in their tertiary structure. This creates gaps in the membrane, so it is easier for molecules to pass through it.
  • At low temperatures, phospholipids have little energy and are packed closely together to make the membrane rigid. This causes a decrease in permeability and restricts molecules from crossing the membrane.
• Concentration of Ethanol Conclusion:

  • Ethanol causes the cell-surface membrane to rupture, releasing the betalain pigment from the cell. Higher concentrations of ethanol will cause more disruption to the membrane and more gaps will form. Thus, as concentration of ethanol increases, the permeability of the cell-surface membrane also increases.