Ch 3 Movement of substances across cell membrane
Exercise
Multiple-choice questions (p. 3-26)
1
C
2
B
3
C
4
D
5
A
6
B
7
B
8
A
9
C
10
D
11
C
Short questions (p. 3-28)
12 HKDSE Biology Practice Paper 2012 IB Q11
13 HKDSE Combined Science Practice Paper 2012 B Q2
14 HKALE Biology 2006 I Q9
15 HKDSE Biology Sample Paper IB Q1
16 a
b
i
Glucose and galactose
1
ii
The rates of absorption of these two sugars were reduced in the
presence of cyanide.
1
This is because active transport needs energy.
1
Less / no energy is released if cyanide is present.
1
All the sugars named in the table can be absorbed when the gut is poisoned
by cyanide.
1
Diffusion does not need an energy supply.
1
Structured questions (p. 3-30)
17 a
i
Phospholipid
1
It is made up of a phosphate head
1
and two fatty acid tails.
1
ii The phosphate head is hydrophilic and the fatty acid tails are
hydrophobic. 1
The hydrophobic tails point inwards while the hydrophilic head points
outwards,
1
b
i
ii
facing the aqueous environment inside and outside the cell.
1
Both types of proteins are present in the fused cell.
1
The proteins are mixed together.
1
The phospholipid molecules give fluidity to the cell membrane. /
This allows the membrane to fuse. /
Proteins can move in the membrane and thus they can mix together.
(any 2)
1x2
18 HKALE Biology 2010 I Q7
19 HKCEE Biology 2003 I Q1b
20 HKCEE Biology 2009 I Q4
21 a
b
c
22 a
N
1
K
1
L
1
Water moves out of the cell by osmosis.
1
The cell has a higher water potential than the surrounding solution.
1
Water moves down the water potential gradient, leaving the cell.
1
Small, non-polar substances diffuse through the phospholipid bilayer.
1
Polar substances enter a cell through channel proteins
1
or carrier proteins
1
using energy.
1
Large substances enter a cell by phagocytosis.
1
Molecules have kinetic energy. /
They are moving randomly. /
The net movement of the molecules is down the concentration gradient.
(any 2)
1x2
b
At the beginning there is a concentration gradient. /
As concentration of substance A outside the membrane decreases, its
concentration inside the membrane increases. /
Substance A moves from the outside to the inside of the membrane. /
As concentration gradient becomes smaller, the rate of change decreases. /
When the concentrations of substance A are the same on both sides, there is
no change in the concentrations of substance A on both sides. (any 3)
c
i
1x3
Active transport requires energy while diffusion does not. /
Molecules can move against a concentration gradient in active
transport, while molecules can only move down the concentration
gradient in diffusion. /
Active transport involves membrane proteins while diffusion may not.
(any 2)
1x2
ii
The rate of change would be almost constant. /
The rate of change would be higher. /
The movement of molecules would not stop after the concentrations on
the two sides become the same. (any 2)
1x2
Essay (p. 3-32)
23 Structure of cell surface membrane:
The cell membrane is made up of a phospholipid bilayer.
1
Polar / hydrophilic phosphate heads point outwards.
1
Non-polar / hydrophobic hydrocarbon tails point inwards.
1
Proteins are interspersed in the phospholipid bilayer.
1
Glycoproteins are also present in the phospholipid bilayer.
1
Movement of the phospholipids with a mix of proteins has given rise to the term
‘fluid mosaic’ model.
1
Movement of molecules across the membrane:
Lipid-soluble / non-polar molecules can diffuse through the phospholipid bilayer.
/
Polar molecules (ions, etc) cannot move through the phospholipid bilayer. /
They are transported by channel proteins or carrier proteins. /
Carrier proteins are specific for a particular type of molecule (or ion) / have
specific regions to which the transported molecule / ion attaches. /
Active transport involves movement against a concentration gradient. /
Active transport involves the use of energy. /
The carrier proteins change their three-dimensional shape, transporting the
molecules across the membrane in the process. (any 6)
Communication
1×6
3
Reading to learn (p. 3-33)
1
Water is hypotonic to the spring onion cells.
1
Water enters the spring onion cells by osmosis and the cells increase in size.
1
As the waxy layer covering the outer side of the spring onion prevents the cells
in the outer layer from stretching, the cells in the inner layer expand more than
those in the outer layer.
1
2
The cells are turgid.
1
3
The cut tips will curl inwards.
1
Concentrated sucrose solution is hypertonic to the spring onion cells.
1
Water will leave the spring onion cells by osmosis. The cells will lose turgidity
and the cut tips will curl inwards.
1