Page 22 - Q & A STPM 2022 Biology

Biology STPM Chapter 3 Membrane Structure and Transport
Question 14
The diagram below shows two adjacent plant cells, P and Q. The values of their
pressure potential (ψ ) and solute potential (ψ ) are given in kPa.
p s
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= –2 000 = –1800
s s
= 1 000 = 400
p p
Semester
1
(a) State the direction in which water would move betwen the two cells.
Explain your answer. (2 marks)
(b) Complete the table below to show the values of the pressure potential, ψ p
and the water potential, ψ of each cell when equilibrum has been reached.
w
Assume that changes in the value ψ are negligible.
s
Potential Cell P Cell Q
Pressure potential, ψ
p
Water potential, ψ
w
(4 marks)
(c) Then, the cells are transferred into distilled water and allowed to reach
equilibrum. Assume the changes in the values below if ψ are negligible.
s
(i) Give the new value of water potential, ψ of the two cells. (1 mark)
(ii) Give the new value of the pressure potential, ψ of cell P. (1 mark)
P
Answer :
(a) Cell P Cell Q
ψ = ψ + ψ P ψ = ψ + ψ P
w
s
s
w
= –2000 kPa + 1000 kPa = –1800 kPa + 400 kPa
= –1000 kPa = –1400 kPa
Water flows from cell P to cell Q. This is because water flows from a
region of higher water potential to a region of lower water potential.
(b)
Potential Cell P Cell Q
Pressure potential, ψ –1200 + 2000 = 800 –1200 + 1800 = 600
P
Water potential, ψ 1200 1200
w
(c) ψ = 0 kPa (d) 0 = ψ + ψ P
S
w
= –2000 + ψ P
ψ = 2000 kPa
P
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Chapter 3.indd 62 11/2/21 9:21 AM