RP3 - production of a dilution series of a solute to produce calibration curve, to identify water potential of a plant tissue

Question 1

How can dilution be calculated

Answer 1

• Can rearrange and use formula below
• C1 x V1 = C2 x V2
• V2 = V1 + volume of distilled water
• Calculate dilution factor -> C2/C1
• Calculate volume of stock solution (V1) -> dilution factor x desired volume (V2)
• Calculate volume of distilled water -> desired volume (V2) - volume of stock (V1)

Question 2

Method to produce calibration curve with which to identify water potential of a plant tissue

Answer 2

• Create dilution series using 1 mol dm sucrose solution (volume of solution = control)
• Use scalpel to cut identical potato cylinders (source of plant tissue and size/shape = control)
• Blot dry with paper towel and record initial mass (remove excess water before weighing)
• Immerse one cylinder in each solution -> leave for set time in water bath (temperature, time left in solution = control) - regularly shake to ensure all surfaces exposed
• Blot dry with paper towel and record final mass (remove excess water before weighing)
• Repeat 3 or more times for each concentration

Question 3

How to process data

Answer 3

• Calculate % change in mass
• Plot graph with % change in mass against concentration (with +ve and -ve regions)
• Identify concentration where line of best fit intercepts x axis -> where water potential of sucrose solution = water potential of potato cells
• Use table in textbook to find wp of the solution

Question 4

Why is % change in mass calculated

Answer 4

Enables comparison and shows proportional change -> since tissue samples had different initial masses

Question 5

Why are potatoes blotted dry before weighing

Answer 5

• Solution on surface adds to mass (only want to measure water taken up/lost)
• Amount of solution on cube varies (ensure same amount on outside)

Question 6

Explain changes in mass when placed in different concentrations of solute

Answer 6

• Increase -> water moves into cells by osmosis - water potential of solution higher than inside cells
• Decrease -> water moves out of cells by osmosis - water potential of solution lower than inside cells
• No change -> no gain/loss of water by osmosis - water potential of solution equals water potential of cells