Page 20 - Q & A STPM 2022 Biology

Biology STPM Chapter 3 Membrane Structure and Transport
Answer :
(a) Osmosis is the passive movement of water molecules from a region
of high-water potential to a region of lower water potential through a
partially permeable membrane.
(b) Cell A = Cell B =
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ψ = ψ + ψ ψ = ψ + ψ P
S
P
S
= -2000 + 1000 = -1600 + 800
= -1000 kPa = -800 kPa
Semester
(c) Water flows down a water potential gradient from a region of high-water
1 potential to a region of low-water potential.
(d) At equilibrium,
Cell A =
ψ = -900 kPa
ψ = -900 + 2000
P
= 1100 kPa
Cell B =
ψ = -900 kPa
ψ = -900 + 1600
P = 700 kPa

Question 13
The diagram below shows two adjacent cells and their values for ψ and ψ
s
p
given in kPa.

ψ = -2000 kPa ψ = -1800 kPa
S S
ψ = +800 kPa ψ = +1000 kPa
P P

Cell P Cell Q
(a) (i) Give the formula to determine water potential of a plant cell. (1 mark)
(ii) Define osmosis in terms of water potential. (2 marks)
(b) State the direction of water movement between the two cells. Explain your
answer. (2 marks)
(c) Calculate the pressure potential, (ψ ) and water potential (ψ) of cell P and
p
cell Q when equilibrium is reached. Assume that changes in solute potential
are negligible. (1 mark)
(d) State two differences between osmosis and sodium-potassium pump.
(2 marks)

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Chapter 3.indd 60 11/2/21 9:21 AM