Required Practical 3

Question 1

Describe how to calculate dilutions.

Answer 1

You can rearrange and use the formula the formula: C1 x V1 = C2 x V2 with V2 = V1 + volume of distilled water**, or:

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 the 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.

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 a calibration curve with which to identify the water potential of plant tissue.

Answer 3

PART 1: COLLECTING DATA

1. Create a series of dilutions using 1 mol dm⁻³ sucrose solution (0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm⁻³).
   • Volumes of solutions should be kept the same.
2. Use cork borer to extract potato and scalpel to cut potato into identical cylinders (one for each concentration).
   • Size, shape and surface area of plant should remain same.
   • Source of plant (variety/age) should remain same.
3. Blot dry with paper towel and measure and record initial mass of each cylinder.
   • Blot dry to remove excess water before weighing.
4. Immerse one cylinder in each concentration and leave for a set time (20-30 mins) in a water bath at 30°C.
   • Length of time in solution and temperature should remain the same.
   • Regularly stir/shake to ensure all surfaces exposed.
5. Blot dry with paper towel and measure and record final mass of each piece.
   • Blot dry to remove excess water before weighing.
6. Repeat 3 or more times at each concentration.

PART 2: PROCESSING DATA

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

Question 4

Why calculate % change in mass?

Answer 4

Enables comparison/shows proportional change as plant tissue samples had different initial masses.

Question 5

Why blot dry before weighing?

Answer 5

• 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 6

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

Answer 6

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

• Not net gain/loss of water by osmosis
• As water potential of solution = water potential of cells