Required Practical 3

• Describe how to calculate dilutions:

  Use the formula C1 x V1 = C2 x V2

  • C1 = Concentration of stock solution
  • V1 = Volume of stock solution used to make new concentration
  • C2 = Concentration of solution you are making
  • V2 = Volume of new solution you are making

  V2 = V1 + volume of distilled water to dilute with
• Describe a method to produce a calibration curve with which to identify the water potential of plant tissue e.g. potato

  1. Create a series of dilutions using a 1 mol dm-3 solution (0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm-3)
  2. Use scalpel/cork borer to cut a potato into identical cylinders
  3. Blot dry with a paper towel and measure/record initial mass of each piece
  4. Immerse one chip in each solution and leave for a set time (20-30 mins) in a water bath at 30 degrees
  5. Blot dry with a paper towel and measure/record final mass of each piece
• List some control variables for each step of the practical

  1)Volume of solution e.g. 20cm3

  
  

  2)- Size, shape and surface area of plant tissue

  - Source of plant tissue

  
  

  3)Blot dry to remove any excess water before weighing

  
  

  4)- Length of time in solution

  - Temperature

  - Regularly stir/shake to ensure all surfaces exposed

  
  

  5)Blot dry to remove any excess water before weighing
• Describe how to process data from the results obtained during the practical

  1)Calculate % change in mass = (final - initial mass)/initial mass

  
  

  2)Plot graph with concentration on x axis and percentage change in mass on y axis (calibration curve)

  • Must show positive and negative regions

  3)Identify concentration where line of best fit intercepts x axis (0% change)

  • Water potential of sucrose solution = water potential of potato cells

  4)Use a table in a textbook to find water potential of that solution
• Why calculate % change in mass?

  • Enables comparison/shows proportional change
  • As plant tissue samples had different initial masses
• Why blot dry before weighing?

  • Solution on surface will add to mass
  • Amount of solution on cube varies
• Explain why there is an increase in mass when plant tissue is placed in different concentrations of solute?

  • Water moved into cells via osmosis
  • As water potential of solution higher than inside cells
• Explain why there is a decrease in mass when plant tissue is placed in different concentrations of solute?

  • Water moved out of cells via osmosis
  • As water potential of solution lower than inside cells
• Explain why there is no change in mass when plant tissue is placed in different concentrations of solute?

  • No net gain/loss of water via osmosis
  • As water potential of solution = water potential of cells