RP 3 -Production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissue.

Question 1

Describe how a dilution can be calculated

Answer 1

1. Calculate dilution factor = desired concentration (C2) / stock concentration (C1)
2. Calculate volume of stock solution (V1) = dilution factor x final desired volume (V2)
3. Calculate volume of distilled water = final desired volume (V2) - volume of stock solution (V1)

Question 2

Describe how you would use a 0.5 mol dm-3 solution of sucrose (stock solution) to produce 30
cm3 of a 0.15 mol dm-3 sucrose solution. (2)

Answer 2

1.Calculate dilution factor (desired concentration / stock concentration): 0.15 / 0.5 = 0.3
2. Calculate volume of stock solution (dilution factor x final volume): 0.3 x 30 cm3 = 9 cm3
3. Calculate volume of distilled water (final volume - stock solution volume): 30 cm3
- 9 cm3 = 21 cm3

Question 3

Describe a method to produce of a calibration curve with which to identify
the water potential of plant tissue (eg. potato) and the relevant controls
Part 1: collecting data

Answer 3

1. Create a series of dilutions using a 1 mol
   dm-3 sucrose solution (0.0, 0.2, 0.4, 0.6, 0.8,
   1.0 mol dm-3 )
   ● Volume of solution, eg. 20 cm3
2. Use scalpel / cork borer to cut potato into
   identical cylinders
   ● Size, shape and surface area of plant tissue
   ● Source of plant tissue ie variety or age
3. Blot dry with a paper towel and measure /
   record initial mass of each piece
   ● Blot dry to remove excess water before weighing
4. Immerse one chip in each solution and
   leave for a set time (20-30 mins) in a
   water bath at 30oC
   ● Length of time in solution
   ● Temperature
   ● Regularly stir / shake to ensure all surfaces exposed
5. Blot dry with a paper towel and measure /
   record final mass of each piece
   ● Blot dry to remove excess water before weighing
   Repeat 3 or > times at each conc

Question 4

Describe a method to produce of a calibration curve with which to identify
the water potential of plant tissue (eg. potato)
Part 2: processing data

Answer 4

6. Calculate % change in mass = (final - initial mass)/ initial mass
7. Plot a graph with concentration on x axis and percentage change in mass
   on y axis (calibration curve)
   ○ Must show positive and negative regions
8. Identify concentration where line of best fit intercepts x axis (0% change)
   ○ Water potential of sucrose solution = water potential of potato cells
9. Use a table in a textbook to find the water potential of that solutio

Question 5

Explain why % change in
mass is calculated. (2)

Answer 5

• enables comparison
• as plant tissue samples had different initial masses

Question 6

Explain why the potatoes
are blotted dry before
weighing. (2

Answer 6

-solution on surface will add to mass (we only want to measure water taken up or lost)
- amount of solution on the cube varies (so ensure same amount of solution on outside)

Question 7

Explain the changes in plant tissue mass when placed in different
concentrations of solute

Answer 7

increase in mass
Water moved into cells by osmosis
• As water potential of solution higher than inside cells
decrease in mass
Water moved out of cells by osmosis
• As water potential of solution lower than inside cells
no change
No net gain/loss of water by osmosis
As water potential of solution = water potential of cells