Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Answers to end-of-chapter questions
(in this case) by osmosis;
through partially permeable cell surface
membrane of cell;
as cell fills with water, cell / protoplast
expands and pressure (potential) increases;
until water potential of cell = zero / water
potential of pure water;
cell wall rigid / will not stretch (far), and
prevents entry of more water;
cell is turgid; [max. 5]
[Total: 8]
10a the greater the concentration difference, the
greater the rate of transport;[1]
b (net) diffusion and facilitated diffusion only
occur if there is a concentration, difference /
gradient, across the membrane
or
at equilibrium / if no concentration
difference, there is no, net exchange /
transport across membrane / rate of
transport, is same in both directions; AW
active transport can occur even if no
concentration difference;
because molecules / ions are being pumped;
AW[3]
c i active transport;[1]
ii active transport depends on a supply of
ATP;
provided by respiration;[2]
d graph for diffusion is linear / straight line
(with no maximum rate);
purely physical process / not dependent
on transport proteins / channel or carrier
proteins;
graph for facilitated diffusion is a curve with a
maximum rate; AW
facilitated diffusion depends on presence
of, transport proteins / channel or carrier
proteins;
as concentration increases, the receptor
sites of these proteins become more and
more saturated / the more saturated these
become, the less the effect of increasing
concentration;
rate reaches a maximum when all, transport
/ channel or carrier proteins, are working at
full capacity / when all receptor sites are, full
/ saturated;
NB: This is similar to the effect of substrate
concentration on rate of enzyme activity.
[max. 5]
[Total: 12]
11a the living contents of a plant cell;[1]
b i at 90% = 22 kPa (accept 21 or 23 kPa), at
95% = 100 kPa, at 100% = 350 kPa;[1]
ii change 90–95 % = 78 kPa (accept 77 or
79 kPa);
change 95–100% = 250 kPa;[2]
iii as water enters the cell, the cell wall is
stretched / protoplast pushes against cell
wall;
cell wall is (relatively) rigid;
water cannot be compressed;
therefore pressure builds up more and
more rapidly (for given volume of water)
/ small increase in amount of water has
large effect on pressure; AW [max. 2]
(This could be compared with pumping up
a bicycle tyre – pressure increases much
more rapidly for a given amount of air
towards the end due to the elastic limit of
the tyre being reached.)
iv 350 kPa;[1]
c i zero (kPa);[1]
ii 86%;[1]
iii incipient plasmolysis;[1]
iv water potential = solute potential;[1]
v ψ = ψ s + ψ p ;[1]
vi the cell continues to lose water /
protoplast continues to shrink;
protoplast pulls away from cell wall =
plasmolysis;
shrinks until equilibrium is reached;
when water potential of cell = water
potential of outside solution;
solute potential gets lower / more
negative;
because cell contents becoming more
concentrated; [max. 5]
d only a small amount of water is needed to
bring about a large change in pressure;
because the cell wall is (relatively) rigid;
Cambridge International AS and A Level Biology © Cambridge University Press 2014