Cambridge International Catalogue 2024 - Final - Flipbook - Page 79
CAMBRIDGE INTERNATIONAL AS
& A LEVEL SCIENCES
Study and Revision Guides Third editions
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Sample pages
AS LEVEL
Passive transport through cell membranes
Molecules and ions are in constant motion. In gases and liquids they move freely.
As a result of their random motion, each type of molecule or ion tends to spread
out evenly within the space available. This is diffusion. Diffusion results in the
net movement of ions and molecules from a high concentration to a low
concentration.
Diffusion across a cell membrane
» Some molecules and ions are able to pass through cell membranes by simple
diffusion. The membrane is permeable to these substances.
» Some substances cannot pass through cell membranes, so the membranes are said
to be partially permeable.
» An example of simple diffusion is the movement of oxygen into cells:
– Oxygen is often at a higher concentration outside a cell than inside, because
the oxygen inside the cell is being used up in respiration.
– The random motion of oxygen molecules inside and outside the cell means
that some of them ‘hit’ the cell surface membrane.
– More of them hit the membrane on the outside than on the inside, because
there are more of them outside.
– Oxygen molecules are small and do not carry an electrical charge, so they are
able to pass freely through the phospholipid bilayer.
– Oxygen therefore diffuses from outside the cell, through the membrane, to
the inside of the cell, down its concentration gradient.
– This is passive transport, because the cell does not do anything to cause the
oxygen to move across the cell membrane.
Look inside
View sample pages from our
study and revision guides.
This practice paper comprises structured questions
similar to those you will meet in the exam.
KEY TERMS
Diffusion is the net
movement of molecules
or ions down a
concentration gradient,
as a result of the
random movement of
particles.
SKILLS FOCUS
Investigating the effect of surface area on diffusion rate
In general, large objects have a smaller surface
» For example, to calculate the surface area-toarea-to-volume ratio than small objects. You can
use blocks of agar jelly to investigate how the rate of
diffusion is affected by surface area-to-volume ratio.
volume ratio for an agar cube with side length of
2 cm, we do the following:
Method
» Make up some agar jelly using a little universal
indicator solution. If the water used to dissolve the
agar is slightly acidic, then the jelly will be red.
» Decide on the shapes and sizes of jelly to cut,
thinking about keeping the volume of each piece
constant, but varying the surface area.
» Immerse your pieces of jelly in a dilute alkali (e.g.
sodium hydrogencarbonate solution).
» As the alkali diffuses into the jelly, the indicator
will change colour. You can time how long it takes
for the whole piece of jelly to change colour.
surface area = 6 × l2
Calculating surface area-to-volume ratios
» You need to be able to calculate surface area-tovolume ratios for simple shapes such as cuboids
and cylinders.
46
Step 1: calculate the surface area of the cube:
where l is the length of a side.
surface area = 6 × 22 = 24 cm2
Step 2: calculate the volume of the cube:
volume = l3 = 23 = 8
Step 3: calculate the ratio of surface area to
volume:
surface area
ratio =
volume
24
=
=3
8
Therefore, the surface area-to-volume ratio for this
cube is 3:1.
Check your answers at www.hoddereducation.com/cambridgeextras
Sample pages
Exam-style questions and
answers
4.2 Movement of substances into and out of cells
The cell surface membrane controls what enters and leaves cells. Substances can be
transported across membranes both passively and actively.
You have 1 hour and 15 minutes to do the paper. There
are 60 marks on the paper, so you can spend just over
1 minute per mark. If you find you are spending too
long on one question, then move onto another that
you can answer more quickly. If you have time at the
end, then come back to the difficult one.
Exemplar paper
QUESTION 1
a The photograph shows a small part of a cell,
as seen using an electron microscope.
A
B
C
D
Some of the questions require you to recall information
that you have learned. Be guided by the number of
marks awarded to suggest how much detail you should
give in your answer. The more marks there are, the
more information you need to give.
Some of the questions require you to use your
knowledge and understanding in new situations. Do
not be surprised to find something completely new in
a question – something you have not seen before. Just
think carefully about it, and find something that you
do know that will help you to answer it.
Do think carefully before you begin to write. The best
answers are short and relevant – if you target your answer
well, you can get a lot of marks for a small amount of
writing. Do not say the same thing several times over,
or wander off into answers that have nothing to do with
the question. As a general rule there will be twice as
many answer lines as marks. So you should try to answer
a 3-mark question in no more than six lines of writing.
If you are writing much more than that, you almost
certainly have not focused your answer tightly enough.
Look carefully at exactly what each question wants
you to do. For example, if it asks you to ‘Explain’, then
you need to say how or why something happens, not
just describe what happens. Many students lose large
numbers of marks by not reading the question carefully.
Following each question in this practice paper, there
is an answer that might get a C or D grade, followed by
expert comments (shown by the icon
). Then there is
an answer that might get an A or B grade, again followed
by expert comments. You might like to try answering the
questions yourself first, before looking at these.
Notice that there are sometimes more ticks on the
answers than the number of marks awarded. This could be
because you need two correct responses for 1 mark (e.g.
Q1 a i) or because there are more potential marking points
than the total number of marks available (e.g. Q1 a ii).
Even if you get four or five ticks for a 3-mark question,
you cannot get more than the maximum 3 marks.
i Name the parts labelled A to D.
[2]
ii Describe how part B is involved in the
formation of extracellular enzymes.
[3]
b Give two reasons, other than the presence of
part B, why the cell in the diagram cannot
be a prokaryotic cell.
[2]
Total: 7
ANSWER A
a i
A = plasma membrane ✓, B = Golgi ✓,
C = nucleus ✗, D = phagocyte ✗
C is the nuclear envelope (or membrane),
not the nucleus itself. A phagocyte is a
cell – perhaps the student is thinking of a
phagocytic vesicle. Mark: 1/2
ii First, the enzymes are made by protein
synthesis on the ribosomes. Then they go
into the endoplasmic reticulum. Then they
are taken to the Golgi ✓, where they are
packaged. Then they go in vesicles ✓ to the
cell membrane where they are sent out by
exocytosis.
Cambridge International AS & A Level Biology Study and Revision Guide
119
This series is endorsed for the Cambridge Pathway to support the syllabuses
for examination from 2022. The Revision Guides have not been through the
Cambridge International Education endorsement process.
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