Biology
HKDSE Mock Exam XII
Answers
Paper 1 Section A
*Questions which are common items.
Question no. Answer Question no. Answer Question no. Answer
*1 A *13 A *25 D
*2 A 14 C *26 D
*3 A 15 C 27 A
*4 D *16 B 28 B
*5 B 17 D 29 C
*6 B *18 B 30 B
*7 C *19 D *31 B
*8 A *20 C *32 C
*9 B *21 D 33 B
*10 D *22 B 34 A
*11 C 23 B *35 B
*12 B 24 A 36 C
Paper 1 Section B
Question 1 is a common item.
1 E 1m
B 1m
A 1m
C 1m
Question 2 is a common item.
2 a Capillary 1m
b Cell Q has a biconcave disc shape. 1m
This provides a large surface area to volume ratio / a short distance for the diffusion of oxygen into
and out of the cell. 1m
c pancreatic vein à (posterior) vena cava à heart à pulmonary artery à lung
à pulmonary vein à heart à aorta à hepatic artery à liver 2m
Question 3 is a common item.
3 a The water potential of the cell sap of the epidermal cells is higher than that of the salt solution. 1m
Water moves out of the cell by osmosis. 1m
The vacuole shrinks in size, 1m
pulling the cytoplasm / cell membrane away from the cell wall. 1m
b The epidermal cells have different water potential. 1m
c i Correct title 1m
Correct choice of axes 1m
Axes with labels 1m
Correct plotting and joining of lines 1m
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The percentage of plasmolysed cells against
concentration of salt solution
100
90
percentage of plasmolysed cells
80
70
60
50
40
30
20
10
0
0.1 0.2 0.3 0.4 0.5 0.6
concentration of salt solution (mol/cm3)
ii 0.37 mol/cm3 1m
d Obtain more strips of epidermis from different parts of the onion /
include more pieces of epidermis in each salt solution /
count more cells in each piece of epidermis
to calculate a mean. 1m
This is to minimize the individual differences of water potential between different parts of the onion
/ the epidermal cells. 1m
(or other reasonable answers)
Question 4 is a common item.
4 a Translocation 1m
b The radioactive carbon dioxide diffuses into the air space of the leaf. It dissolves in the water film
on the surfaces of mesophyll cells and then diffuses into the cells. 1m
The mesophyll cells use the radioactive carbon dioxide to carry out photosynthesis, producing
glucose with radioactivity. 1m
The radioactive glucose is converted to a radioactive sugar (sucrose), which is then transported to
the roots via the phloem. 1m
c The radioactive sugar is actively used to synthesize other substances for growth (e.g. cellulose
for making new cell walls) in the bud, 1m
whereas the radioactive sugar (sucrose) keeps on flowing in the stem. 1m
d More carbohydrates is transported from the leaf to the nearest growing region / region of storage. 1m
The radioactivity in the root of plant A is much higher than that in the root of plant B
as the leaf treated with radioactive carbon dioxide in plant A is closer to the root. 1m
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This is an item for Biology only.
5 a Primary succession 1m
b i Each year, collect the above-ground parts of all the plants in a designated sample area of 1 m2
(or using a quadrat). 1m
Dry all the harvested plants in an oven at around 100 oC 1m
until a constant mass is obtained upon repeated weighing. 1m
ii The rate of increase in plant biomass during the first 20 years is higher than that after 100 years.
1m
It is because during the first 20 years, the area was occupied by herbaceous plants and shrubs,
while after 100 years, trees became the dominant species. 1m
Herbaceous plants and shrubs are fast-growing, while trees grow at a slower rate. 1m
This is an item for Biology only.
6 a Correct drawing of arrows 1m
b Synaptic vesicles containing neurotransmitter are present only in the endings of axons. 1m
Receptors of the neurotransmitter are present only in the endings of dendrons. 1m
c i The hand would withdraw 1m
but the person would not feel the pain. 1m
ii The hand would not withdraw 1m
and the person would not feel the pain. 1m
d When the person decided to straighten his arm, the motor areas of the cerebrum generated
nerve impulses. 1m
The nerve impulses were transmitted to the muscles on the upper arm along motor neurones. 1m
Upon stimulation by the nerve impulses, the triceps contracted 1m
while the biceps relaxed, 1m
causing the straightening of the arm.
Question 7 is a common item which is equivalent to CS (Biology) question 5.
7 a The tying of the pancreatic duct resulted in the backflow of pancreatic juice to the pancreatic tissues
that secrete the juice. 1m
Digestive enzymes (e.g. proteases) in the pancreatic juice broke down the pancreatic tissues. 1m
It can be deduced that the islets of Langerhans did not degenerate as the dog did not develop diabetes.
1m
b The pancreatic extract contained insulin. 1m
Since Banting and Best were not diabetics / their pancreas could secrete insulin normally,
the injection caused their blood insulin level to reach a higher than normal level. 1m
As insulin stimulates the conversion of more blood glucose into glycogen by the liver / uptake of
more blood glucose by body cells, the extra insulin would reduce the blood glucose to a low level. 1m
As a result of insufficient blood glucose supply to the brain, Banting and Best felt dizzy. 1m
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c
Nature of science Elaboration
Science is affected by the The development of microscope allowed 1m
technology and the types of Langerhans to discover the islets of Langerhans.
equipment available at the time.
Scientists build on the work of Banting and Best knew that the removal of pancreas 1m
other scientists. from a dog can make it diabetic.
Question 8 is a common item which is similar to CS (Biology) question 6.
8 a FAP is not a sex-linked genetic disease 1m
because the gene (APC) is not located on a sex chromosome / is located on an autosome. 1m
b Individual 7 / 10 is normal, so he / she must have received at least one normal allele from either of
his / her parents (individual 3 or 4). 1m
Individuals 3 and 4 have FAP, so each of them must carry at least one FAP-causing allele. 1m
Thus, at least one of the individuals 3 and 4 is heterozygous. 1m
In heterozygous condition, only the dominant allele is expressed. 1m
Therefore, the FAP-causing allele is dominant. 1m
c These genes would direct the cell to produce certain proteins 1m
which inhibit cell division / cause the destruction of the cells with mutated DNA. 1m
d The development of polyps into malignant tumours takes time for mutations to accumulate. 1m
Having colonoscopy every year allows the early detection and removal of polyps, ensuring that polyps
do not have the chance to accumulate mutations. 1m
This is an item for Biology only.
9 a P, Q and S 1m
b Both Q and S contacted the COVID-19 virus before 1m
but Q cannot / has not yet produced antibodies against the virus while S can form antibodies
to destroy the virus. 1m
c i No. The antigen of the COVID-19 virus is different from that of the MERS virus. 1m
The immunity / memory cell produced is specific to the antigen of the COVID-19 virus. 1m
ii Measles / tuberculosis / smallpox / poliomyelitis / influenza 1m
(Or other acceptable answers)
Question 10 is a common item which is equivalent to CS (Biology) question 7.
10 How gases are exchanged in the placenta:
l Oxygen diffuses from the maternal blood to the embryo’s blood. 1m
l Carbon dioxide diffuses from the embryo's blood to the maternal blood. 1m
Adaptations of lung and placenta to gas exchange:
l Numerous air sacs in the lungs and the large number of finger-like embryonic villi in the placenta 1m
provide a large surface area for diffusion of gases. 1m
l The epithelia making up the walls of the air sacs are only one-celled thick. The walls of the embryo’s
capillaries and the embryonic villi are also very thin. 1m
These features provide a short distance for the diffusion of gases. 1m
l There are numerous capillaries surrounding the air sacs. In the placenta, there are a lot of blood vessels.
1m
The blood in these blood vessels transports the gases away readily. A steep concentration gradient
can be maintained for efficient diffusion of gases. 1m
Communication 3m
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