Required Practical 3

Question 1

Describe how to calculate dilutions.

Answer 1

Use the formula: C1 x V1 = C2 x V2

C1 = concentration of stock solution.
V1 = volume of stock solution used to make new concentration.

C2 = concentration of solution you are making.
V2 = volume of new solution you are making.

(V2 = V1 + volume of distilled water to dilute with)

Question 2

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

Answer 2

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 3

Why calculate % change in mass?

Answer 3

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

Question 4

Why blot dry before weighing?

Answer 4

• 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 5

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

Answer 5

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