Page 21 - Q & A STPM 2022 Biology

Biology STPM Chapter 3 Membrane Structure and Transport
Answer :
(a) (i) ψ = ψ + ψ p
s
(ii) Osmosis is the movement of water molecules from a region of
higher water potential to a region of lower water potential through
a partially permeable membrane.
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(b) ψ cell P = –2000 + 800
= –1200 kPa Semester
ψ cell Q = –1800 + 1000
= –800 kPa 1
Since ψ cell Q is higher than ψ cell P , water flows from cell Q to cell P. This
is because water flows down a water potential gradient from a region
of higher water potential to a region of lower water potential.

(c) At equilibrium,
ψ cell P = ψ cell Q

= –1200 + (–800)
2
= –1000 kPa
ψ for cell P = –1000 + 2000
P
= 1000 kPa
ψ for cell Q = –1000 + 1800
P
= 800 kPa
(d) – Osmosis is a passive process while sodium-potassium pump is an
active process.
– Osmosis does not require energy while sodium-potassium pump
requires energy.
– Movement of water in osmosis is down its concentration gradient
while movement of ions in the sodium-potassium pump is against
its concentration gradient.
– Molecules pass through a semi-permeable membrane while ions are
transported through a carrier protein in sodium-potassium pump.
– Only water molecules are involved in osmosis while the sodium-
potassium pump transports ions.

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Chapter 3.indd 61 11/2/21 9:21 AM