CB1h Core practical – Osmosis in potato slices
Specification reference: B1.16
Aim
Investigate osmosis in potatoes.
Most land plants die if flooded with sea water. Usually the concentration of
mineral salts in the soil is less than inside the roots, so water moves into
the root from the soil by osmosis. If the soil contains a high concentration
of salts then osmosis occurs out of the root, into the soil. Without sufficient
water, plants die. Some plants, called halophytes, are adapted to live in salty
areas. Their roots take in large amounts of salts from the soil, which helps
osmosis to continue from the soil into the plant. Halophytes get rid of the
extra salt they absorb in various ways.
Your task
A The tiny white crystals on this cord
grass are salt released from glands on
the surface of the leaves.
You are going to measure osmosis in plant tissue, by comparing the mass
of the tissue before and after soaking in sucrose solution. Sucrose is used
because the molecules are too large to diffuse through cell membranes.
The change in mass of the tissue shows how much water is absorbed or
lost. You can work out the solute concentration of the plant tissue from
repeating the experiment with solutions of different concentrations.
When comparing different pieces of tissue, remember to calculate the
percentage change in mass of each piece.
Method
A Label a separate boiling tube for each solution you will test. Place all
the tubes in a rack.
B Cut similar sized pieces of potato, enough for one per tube. (Make sure
they fit in a tube.)
C Blot each potato piece dry, measure and record its mass, and put it in
an empty tube.
D Fill each tube with the solution of the appropriate concentration.
Ensure you cover the potato with the solution.
E After at least 15 minutes, remove each potato piece and blot it dry.
Measure and record its mass again.
2
Core practical – Osmosis in potato slices
Exam-style questions
1 Define the term ‘osmosis’.
(1 mark)
2 Use your answer to question 1 to explain why most plants in salty
soil would have problems absorbing water through their roots. (2 marks)
3 Explain how halophyte roots are adapted to help them absorb
water from salty soil.
(2 marks)
4 Explain why sucrose, and not salt, is used in the experiment with
potato.
(2 marks)
5 Write an equipment list for the method shown on the previous page.
(2 marks)
6 Suggest, with a reason, how you would work out a suitable range of
sucrose concentrations for the solutions in the potato experiment.
(2 marks)
7 Give a reason why you should calculate percentage change in
mass when comparing results.
(1 mark)
8 Suggest one way to improve the method for the potato
experiment so that you could be more certain of the results. (1 mark)
9 Table B shows results from an experiment like the one described
in the method.
Tube
Sucrose
concentration
(%)
Mass of
Mass of
potato at start potato at end
(g)
(g)
A
0 (distilled water)
4.81
4.90
B
10
5.22
4.96
C
30
4.94
4.39
D
50
4.86
3.69
B
a For each solution, calculate the gain or loss in mass of the
potato piece.
(2 marks)
c Give a reason for the result from tube A.
d Explain the results from tubes B–D.
(1 mark)
(2 marks)
e Use the results to suggest the solute concentration of potato
tissue, giving a reason for your answer.
(1 mark)
f Describe how the method could be adapted to give a more
accurate answer to part e.
(2 marks)
10 Graph C shows the results of an experiment comparing osmosis in
tissue from a halophyte plant and a potato in the same solution.
overmatter
a Identify, with a reason, which tissue lost water fastest over the
first five minutes.
(2 marks)
b Explain why it lost water faster than the other tissue.
3
(2 marks)
c Calculate the rate of change in mass over the first four
minutes for the potato.
(1 mark)
0
Percentage change in mass (%)
b For each solution, calculate the percentage change in mass of
the potato.
(2 marks)
2
Time (mins)
4
6
−2
−4
−6
−8
−10
−12
C
0
Potato
Halophyte
8
10