RP03: Investigating Water Potential

Question 1

What is purpose of calibration curve

Answer 1

They’re used to determine conc of an unknown sample by comparing it to a set of standard values with known concs

Question 2

How is calibration curve used to find conc of plant tissue

Answer 2

Plot calibration curve of percentage change in mass against conc

Find x-intercept where plant tissue is isotonic to sucrose solution

Question 3

What occurs when plant tissue is placed in hypotonic solution

Answer 3

Water moves into plant tissue by osmosis, plant tissue increases in mass

Question 4

What occurs when plant tissue is placed in a hypertonic solution

Answer 4

Water moves out of plant tissue by osmosis, plant decreases in mass

Question 5

Why are potato discs left in solution for 20 mins

Answer 5

To allow time for osmosis until plant tissue reaches equilibrium with its surrounding solution

Question 6

What is water potential determined by

Answer 6

Conc of solutes

Higher the solute conc then the lower the water potential

Question 7

Outline procedure of investigating osmosis using potato tissue

Answer 7

1. Make a simple dilution of 1M sucrose to produce 5 concentrations. Add 5 cm^3 to 5 different test tubes.
2. Cut a potato into equal sized chips and weigh.
3. Place a chip in each test tube and leave for 20 minutes.
4. Take out, dab the excess water and weigh them again.
5. Calculate the percentage change in mass.

Question 8

What is percentage change used rather than actual change in mass

Answer 8

Potato chips may not all have same starting mass

% change allows comparison

Question 9

What is indicated by x-intercept of calibration curve

Answer 9

Conc that is isotonic to solution tested

Question 10

Explain change in mass in potato chips

Answer 10

The potato chips with concentration lower than the sucrose solution (higher water potential) lose mass as there is a net movement of water out of the cells.

The potato chips with concentration higher than the sucrose solution (lower water potential) gain mass as there is a net movement of water into the cells.

Question 11

Why are potato chips dabbed dry after removing from sucrose solution

Answer 11

To remove any excess water clinging to its surface

Question 12

What are controlled variables of this practical

Answer 12

Vol of sucrose solution

Size of potato chips

Length of time left in solution

Dab each potato disc with paper towels