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A level Biology PAG answers
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All of the answers for the extention questions for every A level Biology PAG
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Practical Endorsement GCE BiologyPAG1 – PAG 12
Answers to PAG extension questions
PAG1.1
1. What is the purpose of mitosis for a living organism?
Growth and repair of tissues, asexual reproduction.
2. What is a key distinguishing visible feature of each stage of mitosis?
Prophase – chromosomes visible, metaphase – chromosomes line up, anaphase – chromatids separate,
telophase – two nuclei visible, cell splits.
3. Once active cell division ends, the cells will enter interphase. Explain why it is incorrect to say that these
cells are “resting”.
Interphase is not resting as such activities as checking the DNA for errors, protein synthesis, respiration,
mRNA synthesis all occur during this time.
4. Why is a sample from the roots of Allium spp. a good specimen for studying mitosis?
Cells are actively growing so it is easy to find cells in various stages of mitosis.
5. State which stage of the cell cycle the majority of the cells in your specimen were in and suggest why.
Interphase. Even actively dividing cells spend most time of the cell cycle in this stage so is inevitable that
many cells in a root tip will be in interphase.
PAG1.2
1. Could you use a sample of your own blood for this practical? What precautions would you need to take if
you did?
Yes, students could use their own blood. Their proposed precautions should include measures to ensure
that the puncture wound from which they extract their own blood does not become infected (pre- and
post-swabbing with ethanol, covering with a sterile dressing) and measures to ensure that nobody else
is contaminated with their blood (students work only with their own blood, all materials disposed of in
bleach, good general hygiene e.g. handwashing).
2. Make an estimate of the concentration of erythrocytes in this sample of blood. What facts do you need
to produce this estimate? How accurately can you gauge each of these things? What do you judge to be
the uncertainty in this estimate? How would you go about making a more accurate measurement and
what additional apparatus would you need?
The way students deal with the uncertainty here is as interesting as the actual ‘right’ answer. They are
being asked to pile estimates on top of estimates to get to a number – what is the volume of the drop
they added to the slide?, how can they work out what fraction of that volume they are seeing in any one
field of view?, what is the nature of the ‘wedge’ of blood they have created by smearing (how much
thicker is the start than the end)? 5 billion (5 x 10 9) erythrocytes per cm3 is probably close to the truth. A
haemocytometer would of course be the ideal way to get an accurate measurement and this in turn
brings in the concept of serial dilution (students should realise from this practical that the concentration
is going to be too high to easily count the cells in ‘neat’ blood – so dilution or smearing is required).
3. Using your calculation of the diameter of a typical erythrocyte, and assuming these cells are spherical,
what is the volume of a typical erythrocyte? Given what you know about the shape of erythrocytes do
© OCR 2020 Page 1 v1.0 – Feb 2020
, Practical Endorsement GCE Biology
PAG1 – PAG 12
you think this is likely to be an over-estimate or an under-estimate?
Check the student’s calculation of diameter to allow ‘error carried forward’.
If the calculated diameter is 7 µm the volume of a spherical cell would be 180 µm 3 or 1.8 x 10-16 m3.
Given their biconcave shape the true volume of erythrocytes will be considerably lower.
4. What are the functions of the various different cell types you have identified in the sample?
Erythrocytes – carry and deliver oxygen (from lungs to tissues) and carbon dioxide (from tissues to
lungs)
Thrombocytes – blood clotting
Lymphocytes – specific immunity
Neutrophils – non-specific immunity (phagocytosis of pathogens)
Eosinophils – inflammation and response to parasites
Basophils – inflammatory response.
PAG1.3
1. Using a calibrated eyepiece graticule the mean diameter of an alveolus was calculated to be 170 µm. Do
you think this sounds too small, too large or about right? What is the reasoning behind your answer?
Hint: even before you have completed your own detailed measurements, how do you know roughly how
large an alveolus must be based on your knowledge and what you can see in your specimen?
This is about right. Students could make several points to support this.
Many will, very sensibly, relate it to their own measurements. It’s very unlikely their own mean will be
exactly 170 µm so we are looking for students realising, and explaining, that their results lend credence
to the 170 µm figure if they are reasonably close. The concept of ‘order of magnitude’ is useful here. In
explaining why a result of their own (e.g. 250 µm) supports the 170 µm figure students could comment
on: naturally occurring variability; the nature of sampling (an answer further supported by reference to
the standard deviation of the mean they have calculated, or even simply the range within their sample
would be worth additional credit); possible differences between their sample and the way it was
prepared and the sample used to derive the 170 µm figure including: different species, different age,
healthy or diseased tissue, different fixation causing different degree of shrinking, inflated or deflated at
the point of fixation.
Alternatively or additionally students could make several valid points about ‘sanity checking’ the 170 µm
figure by a combination of knowledge and observation. For example: knowing that erythrocytes are
approximately 7 µm in diameter allows us to get a visual impression of scale either by finding an
erythrocyte for direct comparison or by realising that the diameter of the lumen of the capillaries will be
about the same; bearing in mind that alveolar diameter will change significantly during the breathing
cycle adds to the uncertainty of any estimate; knowing that the diffusion distance from air to blood is
very short (of the order of just 1 µm) is another way of getting a sanity check on the scale of what is
being viewed.
2. Why is it difficult to see blood capillaries? The capillaries have a small diameter and very thin walls (the
endothelial cells are flattened (and hence unstained)) meaning there is very little material that will take
up stain. The lumen is likely to be empty and is itself very narrow. The capillaries therefore are small,
low contrast and unobtrusive in these specimens.
© OCR 2020 Page 2 v1.0 – Feb 2020
, Practical Endorsement GCE Biology
PAG1 – PAG 12
3. Why are bronchioles so numerous? Why are capillaries so numerous? Numerous bronchioles are
necessary to achieve effective ventilation – allowing air to get to and from all the alveoli. Numerous
capillaries are necessary to achieve effective perfusion – allowing blood to flow adjacent to every one of
the alveoli. Matching ventilation and perfusion makes an efficient gas exchange system.
4. Why are lung arterioles relatively thin-walled? The blood pressure on the arterial side of the pulmonary
circulation is lower than on the arterial side of the systemic circulation. Therefore thinner arteriole walls
are adequate to withstand and maintain this pressure. In addition there is no need in pulmonary
arterioles’ walls for smooth muscle to allow vasoconstriction since all areas of both lungs will always
remain fully perfused.
5. Explain how the features of lung structure seen on these slides account for efficient gas exchange in the
lungs. Concepts to include: thin walls of alveoli and capillaries (short diffusion distance); large number of
alveoli and capillaries (large total surface area) (matching ventilation and perfusion); large numbers of
arterioles and bronchioles (maintaining a steep concentration gradient through effective perfusion and
ventilation respectively).
PAG 2.1
1. Blood leaves the kidney via the renal vein and eventually returns to the heart. This blood will pass
through a number of blood vessels, organs and chambers of the heart before it returns to the systemic
circulation via the aorta. Name the blood vessels, organs and chambers of the heart the blood will pass
through, in the correct order.
Renal vein vena cava right atrium right ventricle pulmonary artery lungs pulmonary
vein left atrium left ventricle aorta.
2. (a) Calculate how many times thicker the right ventricle is compared to the right atrium.
(b) Calculate how many times thicker the left ventricle is compared to the right ventricle.
(c) Explain your answers to (a) and (b).
(a) Student’s own measurements – ventricle should be thicker than atrium. Correct and appropriate
units should be used.
(b) Student’s own measurements – left ventricle should be thicker than the right. Correct and
appropriate units should be used.
(c) Atria are thinner walled as only generate enough of a force to pump blood into the ventricles below.
Left ventricle needs to be thicker than the right to generate the pressure required for blood to circulate
the entire systemic circulation whereas the right ventricle only needs to circulate blood to the
pulmonary circulation.
3. What is the function of the atrioventricular valves? To separate the atria from the ventricles and prevent
blood from flowing back into the atria when ventricular systole occurs.
4. (a) Describe the role of the coronary arteries.
(b) What are the possible consequences of a blockage in a coronary artery?
(a) To deliver oxygen and glucose to the cardiac muscle for respiration.
(b) Blockage will cause these not to be delivered, respiration could be prevented, leading to death of
cardiac muscle and possibly causing a heart attack.
© OCR 2020 Page 3 v1.0 – Feb 2020
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