2021 HKDSE Integrated Science Paper 1 Qs. 2, 6 & 8 and Paper 2 Q.2

Paper 1 Q.2

Satisfactory performance. While a lot of candidates realised that chemical X would cause

vasodilation in arterioles, some candidates wrongly stated that vasodilation takes place in

the blood capillaries. Quite a number of candidates had mistaken that a faster blood flow

caused the facial flushing. In fact, facial flushing caused by vasodilation results in an

increased blood flow to the capillaries rather than a faster blood flow there.

✓

✓

Satisfactory performance. Candidates generally knew that allele D had undergone point

(gene) mutation or single base substitution to give rise to allele D*. Some of them gave

wrong answers such as DNA replication or just stated substitution instead of single base

substitution.

✓

✓

Fair. Many candidates seemingly had overlooked the fact that the DNA nucleotide

sequences given were coding (non-template) strands of the alleles and therefore wrongly

gave mRNA nucleotide sequences complementary to these coding (non-template) strands.

These candidates failed to realise that the mRNA is made according to the complementary

base pairing rule using the ‘non-coding (template) strand’. Thus, the mRNA made has the

same nucleotide sequence as the coding (non-template) strand of the DNA, but containing

the base uracil instead of thymine.

✓

The performance was very poor. Some candidates were able to state the principle of ‘lock and key hypothesis’, but they were unable to use this principle to explain why the ALDH coded by allele D* was non-functional. These candidates failed to state that a different amino acid might have been coded by the mRNA transcribed from allele D*, and hence the shape or 3D structure of the active site of the enzyme is altered in such a way as to make the enzyme no longer be the same as the functional ALDH. They only gave vague answers such as the functional and non-functional ALDHs were different in structures or shapes. Quite a number of candidates did not show they understood the concept of ‘lock and key hypothesis’ well enough. They wrongly related the ‘lock and key hypothesis’ to the process of transcription or translation, giving wrong answers such as ‘the non-functional ALDH became unable to bind with the nucleotides to synthesise mRNA’ or ‘the non-functional ALDH became unable to bind with the amino acid to form polypeptide’.

✓

?

The performance was very poor. Very few candidates knew that PCR is a means for

amplifying DNA segments in a sample. Some candidates gave wrong answers such as

‘cell division’, ‘DNA replication’, or ‘gel electrophoresis’.

Satisfactory performance.

The performance was poor. Only a few candidates were able to identify the correct

individuals and give the appropriate reason with respect to the information shown in the

photograph. Most of the candidates were unable to interpret the results of gel electrophoresis

or explain that the extra DNA band is due to the action of the restriction enzyme on allele D.

✓

Satisfactory performance. Many candidates realised that population P is the least likely to

over-consume alcoholic beverages, but they failed to relate the highest gene frequency of

allele D* with the occurrence of unpleasant effects brought on by acetaldehyde

accumulation.

✓

✓

Paper 1 Q.7

✓

✓

Satisfactory performance. Candidates were generally familiar with the transfer of DDT

along the food chain from phytoplankton to penguins, but very few of them were able to

explain the bioaccumulation of DDT. They failed to point out the fate of DDT in living

organisms, i.e. it can neither be metabolised nor excreted.

✓

✓

✓

The performance was poor. Very few candidates recognised that the process shown in the

question was denitrification, which involves the actions of denitrifying bacteria under an

anaerobic condition. Candidates in general showed a poor understanding of the nitrogen

cycle: e.g. misidentified nitrifying bacteria as the microorganism involved

✓ ✓

➢ stating the wrong or irrelevant soil conditions for denitrification process, e.g. soil being

acidic (low pH), dry or moist; soil rich in humus, fertilisers, nitrates or decomposers etc.

✓

✓

✓

Fair. Many candidates mentioned that N2O accelerates the greenhouse effect but were

unable to relate explicitly how this enhances global warming. Moreover, quite a number of

candidates just stated that the glaciers would melt. They were unable to pinpoint the fact

that glaciers would melt more quickly with global warming.

✓

✓

✓

Very good performance.

✓

✓

✓

Paper 1 Q.8

Fair. Candidates were generally able to state that the allele for Huntington’s disease is

dominant. However, not many of them demonstrated a proper deduction to reach this answer.

They are weak in making logical deductions:

◼ Instead of making deduction from the information given, many candidates simply

assumed that the disease was controlled by a dominant allele and described what would

happen under such assumption.

✓?

They are weak in making logical deductions:

◼ A wrong deduction that the normal daughter (individual 3) should have received alleles

for normal condition from ‘both parents’ (individuals 1 and 2) was made. In fact, the case

of individual 3 could only give the deduction that this individual should have received the

allele for normal condition from ‘either of her parents’.

They are weak in making logical deductions:

◼ Having asserted that the allele for normal condition should be recessive as parents with

Huntington’s disease could give birth to a normal daughter, candidates provided no

explanation for such assertion.

In addition, some candidates held a misconception that the allele for Huntington’s disease is

sex-linked.

?

✓✓

✓

Satisfactory performance. Some candidates did not realise that depression is associated

with a low level of serotonin in the synaptic clefts in the brain. Quite a number of

candidates wrongly stated that the inhibition of reuptake of serotonin would increase the

serotonin production.

✓

✓

✓

✓ ?

✓ ✓

✓

The performance was poor. Very few candidates recognised that Huntington’s disease is

due to a gene defect and thus could not be revealed by the appearance (shape) or number of

chromosomes. Moreover, many candidates seemed to have no clues on which genetic

screening test can be adopted for finding out such gene defect.

Paper 2 Q.2

Paper 2 – Q.2(a)

Fair. Candidates were generally able to outline the lytic cycle of a virus. However, some of

them just mentioned the replication process of the genetic material of the virus without

explicitly stating the production of new viruses. Quite a number of candidates did not

mention that the newly formed viruses would infect other tissue cells and damage them.

They also failed to point out that such severe damage of cells would lead to the

malfunctioning of the organ / system involved (e.g. lungs in the respiratory system), causing

symptoms to appear.

✓

✓

✓

✓

✓

?

Fair. Most of the candidates showed an understanding of the aim of the quarantine measure

is to isolate people who might have been infected to prevent them from having the chance to

spread COVID-19 in the community.

✓

✓

Showed an understanding of the aim of the quarantine

measure but gave no explanation with reference to Figure 1.

Many of them overlooked the requirement of the question and did not make reference to the

information given in Figure 1:

• Time is needed for the viral load in the infected person to reach a sufficient level to be

tested positive;

• Symptoms would have shown up within 14 days after infection.

✓

✓

Very good performance.

Very good performance.

Satisfactory performance. Many candidates were able to point out that the protective

measures help to prevent the healthcare system from being overburdened during the course

of an epidemic.

✓

✓

Some candidates, however, overlooked the clue given in the question and mis-interpreted the

graph. Many candidates erroneously thought that the total number of cases would be greater

when protective measures were not adopted. They failed to grasp the importance of having

protective measures is to flatten the curve, i.e. to lower and delay the peak of cases, so as to

allow more time for the healthcare system to better cope with patient load.

Satisfactory performance. Many candidates stated correctly that killer T cells are

differentiated from T cells, but wrongly mentioned that the T killer cells destroy the

virus. In fact, the killer T cells kill the virus-infected cells.

✓✓

✓

Paper 2 – Q2(b)

The performance was poor. Most candidates failed to distinguish between the cohort

study and the case-control study.

✓

Fair. Many candidates correctly calculated the relative risk. They also noticed the

relative risk is greater than 1 and thus reached the conclusion that type II diabetes is

regarded as a risk factor for stroke.

✓

✓

✓

Fair. Many candidates pointed out that the ‘bad’ cholesterol is the low-density

lipoprotein cholesterol, but quite a number of them had written ‘low-density cholesterol’

instead. Candidates generally realised that a high level of LDL cholesterol would cause

the blockage of blood vessels. Yet, only a few were able to clearly point out such

blockage is due to the build up of fatty deposits in the wall of blood vessels. Moreover,

some candidates failed to state that a stroke is a result of the shortage of oxygen supply,

not just blood supply, to the brain cells.

✓

✓

✓

Satisfactory performance. Although many candidates were able to give the expected

answers, some of them confused the functions of the cerebrum with the cerebellum. In

addition, quite a number of candidates were not able to state the functions of cerebrum

and cerebellum in a precise way.

✓

✓

✓✓

✓

THANK YOU!