RPQ 3 - investigating water potential

• making serial dilutions:

  • need to make up several solutions of different, known concentrations to test the potato cylinders in
  • use a serial dilution technique
• a serial dilution is when you create a set of solutions that decrease in concentration by the same factor each time - useful technique particularly when you need to create a very weak solution, as it means you do not have to measure out very small volumes of liquid
• e.g. how you would make 5 serial dilutions of a sucrose solution starting with an initial sucrose conc. of 2 M and diluting each solution by a factor of 2:

  • line up 5 test tubes in a rack
  • add 10cm3 of initial 2M solution to first test tube and add 5cm3 distilled water to other 4
  • using a pipette, draw 5 cm3 of the solution from 1st test tube and add to distilled water in 2nd and mix thoroughly - 10cm3 solution half as conc. as 1st (1M)
  • repeat 3 more times to create solutions 0.5M, 0.25M, 0.125M
• to dilute by a factor of 10 - take 1cm3 from original sample and add to 9cm3 of water
• you can make solutions of any concentration by finding the scale factor:

  • i.e. want to make 15cm3 of 0.4M sucrose solution
  • start with solution of known conc e.g. 1M
  • find scale factor by dividing conc of this solution by conc of solution want to make - 1/0.4 = 2.5
  • means that solution want to make is 2.5x weaker than one you have - to make the solution 2.5x weaker, use 2.5x less of it i.e. 15/2.5 = 6cm3 - transfer this amount to clean test tube
  • top up with distilled water to get volume wanted to make
  • 15-6=9 - 9cm3 distilled water
• measuring change in mass:

  • once you have made up a set of serial dilutions, you can use them to find the water potential of potato cells - need to measure how much mass the potato cells gain or lose in each solution
• 1. use a cork borer to cut potatoes into identically sized chips, about 1cm in diameter. divide chips into groups of three and measure the mass of each group using a mass balance
  2. place one group into each of your sucrose solutions and leave the chips in the solutions for at least 20 minutes (making sure all get same amount of time)
  3. remove the chips and gently pat dry with a paper towel - weigh each group again and record results - calculate percentage change in mass for each group
• final mass - initial mass / initial mass X 100
• the potato chips will gain water and therefore mass in solutions with a higher water potential than the chips, and lose water in solutions with a lower water potential
• producing a calibration curve:

  • next - use results to produce a calibration curve by plotting percentage change in mass (Y) against conc of sucrose solution (X) - can then use calibration curve to determine water potential
• the point at which your calibration curve crosses the X axis (when % change in mass is 0) is the point at which the water potential of the sucrose solution is the same as the water potential of the potato cells - find the concentration at this point and then look up the water potential for that conc of sucrose solution e.g. in a textbook
• point of taking samples straight away is so that so starch or glucose had time to diffuse through the visking tubing