membrane permeability

• IV

  • the temperature of the water in the water bath
• DV

  • The percentage transmission of light through the resulting solution
• control variables

  • Volume of distilled water – 5cm³ of distilled water should be used to fill the boiling tubes each time
  • Time left in water – leave each boiling tube containing beetroot for 30 minutes
  • Size of beetroot piece – use a ruler and knife to cut cylindrical beetroot pieces of 1cm in length
  • Colorimeter used – same colorimeter should be used on the same blue/green setting each time, measuring percentage absorbance. Calibration with distilled water should be carried out each time
  • Volume of beetroot solution – 2cm³ of beetroot solution should be added to a cuvette each time
• why beetroot?


  Beetroot cells contain pigment called betalains in their vacuoles. We can observe the effect of temperature on cell membranes in beetroot by observing the leakage of this pigment, indicating the weakening of the cell membrane. 
• control 


  Keeping a beetroot piece in 10cm³ of distilled water at room temperature can provide control results.
• method

  1. Use a cork borer and knife to cut 8 x 1cm lengthed beetroot cylinders.
  2. Fill 8 boiling tubes each with 5cm³ of distilled water and place them into 8 separate water baths of different temperatures 0-70 degrees
  3. add a piece of beetroot to each boiling tube and leave for 30 minutes.
• method continued

  1. Remove the beetroot pieces with a pair of forceps and then shake the tubes to disperse the dye.
  2. Set a colorimeter to percentage absorbance. Calibrate by filling a cuvette with distilled water first then add 2cm³ of beetroot solution from the first temperature to a new cuvette.
  3. Place this cuvette into the colorimeter to read the percentage absorbance. Repeat this for all other pieces.
• results/calculations


  Percentage transmission = 100 – percentage absorbance
• conclusions

  • As temperature increased, the percentage transmission slightly increased to a point at which it greatly increased.
  • Greater kinetic energy created more gaps in the phospholipid bilayer for the betalains to leak through. At a certain point, broken hydrogen bonds caused proteins to change shape and denature – leaving larger holes in the cell membrane
• evaluation points

  • Some beetroot may have skin on affecting surface area (random error)
  • Difficulty in maintaining temperature (random error) – set water baths for higher temperatures and set refrigerators for lower temperatures
  • Accurate reading of the colorimeter (systematic error)