Required practical 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 a method to produce of a calibration curve with which to identify
the water potential of plant tissue (eg. potato) - Part 1: collecting data

Answer 2

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)
2. Use scalpel / cork borer to cut 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 30oC
5. Blot dry with a paper towel and measure /record final mass of each piece

Question 3

What are control variables of this experiment?

Answer 3

● Volume of solution, eg. 20 cm3
● Size, shape and surface area of plant tissue
● Source of plant tissue ie variety or age
● Blot dry to remove excess water before weighing
● Length of time in solution
● Temperature
● Regularly stir / shake to ensure all surfaces exposed
● Blot dry to remove excess water before weighing

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 solution

Question 5

Explain why % change in mass is calculated

Answer 5

● Enables comparison / shows proportional change

● As plant tissue samples had different initial masses

Question 6

Explain why the potatoes are blotted dry before weighing

Answer 6

● Solution on surface will add to mass (only want to measure water taken up or lost)

● Amount of solution on cube varies (so ensure same amount of solution on outside)

Question 7

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

Answer 7

● Water moved into cells by osmosis

● As water potential of solution higher than inside cells

Question 8

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

Answer 8

● Water moved out of cells by osmosis

● As water potential of solution lower than inside cells

Question 9

Explain the no change in plant tissue mass when placed in different
concentrations of solute

Answer 9

● No net gain/loss of water by osmosis

● As water potential of solution = water potential of cells