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Answers:
LEVEL: AS
BIOLOGY – Exchange and transport
3. Water potential (12 minutes)
(a) The diagram represents a turgid plant cell. Label the parts indicated using the appropriate
letter.
A . The part which is fully permeable
B The part which is partially/selectively permeable
C The part which contains cell sap
(3 marks)
(b) In an experiment several fully turgid plant cells were placed in either concentrated glucose
solution or distilled water. Complete the table to show what happens to the cells placed in
concentrated glucose solution.
(2 marks)
External solution
Overall direction of water
movement
Appearance of part
containing cell sap
out of cell
vacuole shrinks
nil
no change
Concentrated glucose
Distilled water
(i) Define the term ‘water potential’.
(1 mark)
The tendency for water molecules to (enter or) leave a system.
(ii) What is the water potential of pure water?
(1 mark)
Zero.
Copyright © Pearson Education Limited 2001
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Answers:
LEVEL: AS
BIOLOGY – Exchange and transport
(d) The graph shows the change in cell volume and water potential values of a flaccid plant cell
placed in distilled water.
Using the graph
(i) Describe the change in water potential of the cell.
(1 mark)
Water potential becomes less negative/becomes closer to zero.
(ii) Explain your answer.
(1 mark)
As water enters vacuole because cell sap is hypertonic to the distilled water.
(e) The solute potential of the cell is never zero. Explain.
(1 mark)
The sap always contains a very dilute solution of salts.
(Total marks 10)
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