RPQ 4 - investigating cell membrane permeability

• you can investigate how different variables (e.g. temp, solvent concentration) affect the cell membrane permeability by doing experiments using beetroots.

  Beetroot cells contain a coloured pigment that leaks out - the higher the permeability the more pigment leaks out of the cell
• 1. use a scalpel to carefully cut five equal sized pieces of beetroot (on a cutting board). rinse the pieces to remove any pigment released during cutting
• 2. add the five pieces to five different test tubes, each containing 5cm3 of water - use a measuring cylinder or pipette to measure the water
• 3. place each test tube in a water bath at a different temperatures for the same length of time (measured using a stopwatch)
• 4. remove the beetroot pieces from the tubes, leaving just the coloured liquid
• 5. use a colorimeter - machine that passes light of a specific wavelength through a liquid and measures how much of that light is absorbed. Many colorimeters use filters to make sure the light passing through the liquid is at the desired wavelength
• 6. firstly, switch the colorimeter on and allow five minutes for it to stabilise, then set the colorimeter so you're using a blue filtre (wavelength about 470nm)
• 7. add distilled water to a cuvette so it is three quarters full (cuvette is a small container that fits inside a colorimeter). Put the cuvette into the colorimeter. two of the cuvette's sides may be ridged or frosted - make sure put the cuvette into the colorimeter the correct way, so that light will be passing through the clear slides. calibrate the machine to 0
• 8. next use a pipette to transfer a sample of the liquid from the first test tube to a clean cuvette - again should be about 3/4 full
• 9. put the cuvette in the colorimeter and read and record the absorbance of the solution
• 10. repeat steps 8-9 for the liquids remaining in the 4 test tubes using a clean pipette and cuvette each time
• 11. ready to analyse results - higher the absorbance the more pigment released so the higher the permeability of the membrane
• depending on resources available - could connect the colorimeter to a computer and use software to collect the data and draw a graph of the results
• could do a similar experiment with beetroot to investigate the effect of solvents on the permeability of cell membranes i.e. by placing the beetroot cubes in different concentrations of a particular solvent (such as alcohol or acetone). surrounding cells in an increasing concentration of a solvent increases membrane permeability because the solvent dissolves the lipids in the cell membrane, causing it to lose its structure