2021 HKDSE Biology Practice Paper | DSE Mock

The graph below shows the changes in the rate of transpiration and the rate of water uptake of a leafy shoot when it is transferred from still air to windy conditions at t = 10 minutes. Between t = 10 and t = 20 minutes, the rate of transpiration is significantly higher than the rate of water uptake. Which of the following statements is/are correct during this period (between 10 and 20 minutes)? (1) The water content of the leaf cells decreases. (2) The tension in the xylem vessels increases. (3) The rate of water loss from stomata is lower than the rate of water absorption by roots.

An organic effluent is continuously discharged into a river. The levels of dissolved oxygen (DO), aerobic bacteria population, and algae population at different downstream distances are as follows: At 0 m (discharge point): DO = 8.0 mg/L, bacteria = 10 units, algae = 5 units. At 100 m: DO = 2.5 mg/L, bacteria = 95 units, algae = 2 units. At 500 m: DO = 1.2 mg/L, bacteria = 60 units, algae = 15 units. At 1000 m: DO = 5.5 mg/L, bacteria = 20 units, algae = 80 units. Which of the following is the most plausible explanation for why the dissolved oxygen concentration reaches its lowest level at 500 m downstream, even though the population of aerobic bacteria has started to decrease?

Blood vessels associated with a villus in the human small intestine are described below: Vessel X is the arteriole entering the villus. Vessel Y is the venule leaving the villus (draining into the hepatic portal vein). Two hours after a person has consumed a balanced meal rich in proteins, carbohydrates, and lipids, which of the following combinations correctly compares the concentration of amino acids, oxygen, and carbon dioxide in Vessel Y with Vessel X?

A double-stranded DNA molecule contains 30% adenine (A). This DNA molecule, which contains only non-radioactive phosphorus (31P), undergoes two rounds of replication in a medium containing only nucleotides labeled with radioactive phosphorus (32P). Which of the following statements about the resulting DNA molecules is/are correct? (1) The ratio of adenine to guanine in the newly synthesized strands is 3:2. (2) 50% of the DNA molecules in the final pool contain one non-radioactive strand and one radioactive strand. (3) All of the DNA molecules in the final pool contain at least some radioactive 32P.

An active suspension of chloroplasts is kept under constant light in a chamber with a continuous supply of carbon dioxide. The light source is suddenly switched off. Which of the following combinations correctly predicts the immediate changes in the concentrations of glycerate 3-phosphate (GP), ribulose bisphosphate (RuBP), and ATP in the chloroplasts?

The concentration of a non-biodegradable persistent organic pollutant (POP) in the tissues of four different organisms in a coastal marine ecosystem are as follows: Organism P: 0.05 ppm, Organism Q: 25.40 ppm, Organism R: 1.80 ppm, Organism S: 0.45 ppm. Based on this, which of the following represents the most likely food chain among these organisms, and what is the key reason for the difference in POP concentrations?

The graph below represents the changes in air pressure within the alveoli relative to atmospheric pressure during a normal breathing cycle of a healthy human: - From t = 0 to t = 2 seconds, the alveolar pressure drops below atmospheric pressure and then returns to baseline. - From t = 2 to t = 4 seconds, the alveolar pressure rises above atmospheric pressure and then returns to baseline. Which of the following events occurs during the period from t = 0 to t = 2 seconds?

A segment of a DNA template strand has the sequence: 3'- T A C G G T C A T T C A A C T -5'. Using the codon table provided (GUA codes for Valine, GAA codes for Glutamic acid, CCA codes for Proline, AGU codes for Serine), if a mutation occurs in the DNA where the 8th base from the 3' end (adenine) is replaced by thymine, what is the effect of this mutation on the resulting polypeptide?

A person is given two injections of Antigen X: the primary injection at day 0 and a booster injection at day 28. Which of the following statements correctly explains the differences between the primary and secondary immune responses observed? (1) The secondary response is faster because memory B cells already exist to rapidly proliferate and differentiate into antibody-secreting plasma cells. (2) The secondary response is stronger and longer-lasting, maintaining high antibody levels for a more extended period. (3) Killer T cells are the primary cells responsible for secreting the antibodies in both responses.

Which of the following rows correctly compares the anaerobic respiration occurring in human skeletal muscle cells and in yeast cells?

An investigation was carried out to study the effect of different environmental factors on the rate of water uptake of a leafy shoot. Which of the following combinations of environmental conditions would lead to the highest rate of transpiration?

(1) High light intensity
(2) High relative humidity
(3) Increased air movement
(4) Elevated ambient temperature

In a photosynthesis experiment, a plant was supplied with carbon dioxide containing heavy oxygen isotope \(^{18}\text{O}\) and water containing normal oxygen \(^{16}\text{O}\). Which of the following substances produced by the plant during photosynthesis will contain \(^{18}\text{O}\)?

(1) Oxygen gas released
(2) Triose phosphate

Which of the following are the immediate consequences of discharging domestic sewage rich in organic matter into a natural river?

(1) Rapid multiplication of aerobic decomposers
(2) Increase in dissolved oxygen (DO) levels
(3) Decrease in biological oxygen demand (BOD)

A synthetic pesticide, which is non-biodegradable and fat-soluble, was washed into a lake. Over time, the pesticide accumulated in the lake's food chain: Phytoplankton -> Herbivorous fish -> Carnivorous fish -> Fish-eating birds. Which of the following statements is/are correct?

(1) The concentration of pesticide is highest in the fish-eating birds due to biomagnification.
(2) The pesticide accumulates primarily in the fatty tissues of the organisms.
(3) The total biomass of the fish-eating birds in the lake is greater than that of the phytoplankton.

If a patient has a major part of their ileum surgically removed due to a disease, which of the following functions of the digestive system would be most severely impaired?

(1) Secretion of digestive enzymes
(2) Absorption of digested nutrients
(3) Reabsorption of water

A healthy person had a meal rich in proteins and carbohydrates. Two hours later, blood samples were collected from the hepatic portal vein and the hepatic vein. Which of the following comparisons between the two blood vessels is/are correct?

(1) The glucose concentration in the hepatic portal vein is higher than that in the hepatic vein.
(2) The amino acid concentration in the hepatic portal vein is higher than that in the hepatic vein.
(3) The urea concentration in the hepatic vein is higher than that in the hepatic portal vein.

A section of double-stranded DNA has the following sequence for the template strand:

`3'- T A C G G C T T A -5'`

What is the base sequence of the mRNA transcribed from this template strand?

A single nucleotide substitution occurred in a gene coding for a functional polypeptide. This mutation resulted in a polypeptide that was significantly shorter than the original one. Which of the following codon changes in the mRNA is most likely responsible for this mutation?

Which of the following statements about the transport of organic substances in the phloem of a plant is/are correct?

(1) Sucrose is actively loaded into the sieve tubes at the source.
(2) Water enters the sieve tubes from the xylem by osmosis near the source, increasing the hydrostatic pressure.
(3) The transport of organic substances in the phloem can only occur in a downward direction.

Which of the following human activities contribute to the increase of greenhouse gases in the atmosphere?

(1) Clearing forests for agricultural land
(2) Extensive use of nitrogen-based chemical fertilizers
(3) Combustion of coal in power stations

Under which of the following conditions would the rate of water absorption of a leafy shoot be significantly lower than its rate of transpiration?

The following table shows the rate of carbon dioxide uptake of a plant under different environmental conditions:\n\n| Condition | Light intensity (units) | Temperature (\(^{\circ}\text{C}\)) | \(\text{CO}_2\) concentration (%) | Rate of \(\text{CO}_2\) uptake (arbitrary units) |\n|---|---|---|---|---|\n| 1 | 100 | 20 | 0.03 | 15 |\n| 2 | 100 | 20 | 0.12 | 15 |\n| 3 | 500 | 20 | 0.03 | 30 |\n| 4 | 500 | 20 | 0.12 | 45 |\n| 5 | 500 | 30 | 0.12 | 65 |\n\nWhich of the following statements is correct?

Organic sewage is discharged into a river. Soon, a large population of aerobic decomposers (bacteria and fungi) develops downstream. Which of the following changes will occur downstream as a direct or indirect result of this decomposer activity?\n(1) The dissolved oxygen level in the river decreases.\n(2) The concentration of inorganic nutrients (such as nitrate) increases.\n(3) The population of algae increases further downstream.

In Hong Kong, trawling has been banned in marine waters since 2012. Which of the following are the primary conservation benefits of this ban?\n(1) It prevents the physical destruction of benthic (seabed) habitats.\n(2) It reduces the non-selective capture of non-target marine species.\n(3) It directly reduces the input of plastic pollutants into the marine environment.

Which of the following descriptions about the digestion and absorption of lipids in the human small intestine is correct?

During a normal inhalation in humans, which of the following changes occur?\n(1) The external intercostal muscles and diaphragm muscles contract.\n(2) The volume of the thoracic cavity increases, which increases the pressure inside the lungs.\n(3) Air flows into the lungs because the atmospheric pressure is higher than the intra-alveolar pressure.

Which of the following combinations correctly describes the state of the heart valves when the pressure in the left ventricle is higher than the pressure in the left atrium but lower than the pressure in the aorta?

A double-stranded DNA molecule contains 3000 base pairs. If \(22\%\) of the total nitrogenous bases in this DNA molecule are adenine (A), what is the total number of guanine (G) bases in this DNA molecule?

The following is a section of a template strand of DNA:\n3'- T A C G G C T T A A T C -5'\nWhich of the following tRNA anticodons (written in the 5' to 3' direction) would bind to the mRNA codon transcribed from the third triplet of this template strand?

In a grassland ecosystem, the pyramid of numbers is upright. However, in a forest ecosystem, the pyramid of numbers can sometimes be inverted. Which of the following is the most likely reason for this difference?

An experiment was conducted using four identical leafy shoots to study the effect of environmental factors on the rate of transpiration. The four setups were placed under different conditions:
Setup P: Still air, \(25^\circ\text{C}\)
Setup Q: Still air, \(35^\circ\text{C}\)
Setup R: Moving air, \(25^\circ\text{C}\)
Setup S: Enclosed in a plastic bag, \(25^\circ\text{C}\)

Which of the following comparisons of transpiration rate is/are correct?
(1) P > S because the relative humidity of the air around the leaves in S increases.
(2) Q > P because the kinetic energy of water molecules is higher in Q.
(3) R > P because moving air removes the water vapor boundary layer around the leaves.

The diagram below shows a food web in a coastal marine ecosystem:
Phytoplankton -> Zooplankton -> Small fish -> Sea birds
Phytoplankton -> Benthic invertebrates -> Large fish -> Sea birds

A persistent organic pollutant (chemical X) that cannot be metabolized or excreted is released into this ecosystem. Which of the following statements is correct?

The list below describes some major blood vessels connected to the liver and kidneys in the human body:
- Vessel P connects the small intestine to the liver
- Vessel Q connects the liver to the vena cava
- Vessel R connects the aorta to the kidney
- Vessel S connects the kidney to the vena cava

Which of the following statements is/are correct?
(1) Two hours after a protein-rich meal, the concentration of amino acids in P is higher than that in Q.
(2) The concentration of urea in Q is higher than that in S.
(3) The concentration of oxygen in S is higher than that in R.

The table below shows some mRNA codons and the corresponding amino acids they code for:
Codon | Amino Acid
GAC | Aspartic acid (Asp)
CAG | Glutamine (Gln)
GUC | Valine (Val)
CUG | Leucine (Leu)

A segment of a template strand of DNA has the sequence: `3'-CAG-GAC-GTC-CTG-5'`.
Which of the following is the correct sequence of the polypeptide synthesized from this template DNA?

To study the transport of substances in plants, radioactive carbon dioxide (\(^{14}\text{CO}_2\)) was supplied to a single leaf in the middle of a herbaceous plant stem. After several hours, the distribution of radioactivity in the plant was analyzed. Which of the following statements is/are correct?
(1) Radioactive organic substances can be detected in both the roots and the young growing shoot tips.
(2) If a ring of bark (containing phloem) is removed from the stem just below the supplied leaf, no radioactivity will be detected in the roots.
(3) The translocation of these radioactive organic substances requires metabolic energy (ATP).

Which of the following human activities contributes to global warming by both increasing the source of greenhouse gases and reducing their sink?

A student sets up a flask potometer to study the rate of water uptake by a leafy shoot.

(a) Explain why a layer of oil is added on top of the water in the flask. (2 marks)

(b) The student compares the rate of water uptake under still air and moving air (using a fan). Describe and explain the difference in the rate of water uptake. (3 marks)

(c) Suggest why the rate of water uptake measured by the potometer might not exactly equal the rate of transpiration. (2 marks)

An agricultural runoff containing synthetic fertilizers entered a nearby river.

(a) Describe the immediate change in algal population in the river, and explain why this occurs. (2 marks)

(b) Explain how the rapid growth of algae eventually leads to a depletion of dissolved oxygen in the water. (3 marks)

(c) Apart from the death of aquatic organisms, suggest one negative ecological consequence of this phenomenon. (2 marks)

The human digestive system is adapted for efficient lipid digestion and absorption.

(a) Contrast the physical state of lipids before and after emulsification by bile salts, and explain how this aids chemical digestion. (3 marks)

(b) Describe how the products of lipid digestion are absorbed across the epithelial cells of the villi and transported away. (3 marks)

(c) State one structural feature of the lacteal that facilitates this transport. (2 marks)

The Polymerase Chain Reaction (PCR) is widely used in biotechnology to amplify specific DNA sequences.

(a) State the role of primers in PCR and explain why two different primers are required. (3 marks)

(b) Why is a thermostable DNA polymerase (like Taq polymerase) used instead of human DNA polymerase? (2 marks)

(c) After 3 cycles of PCR starting with a single double-stranded DNA molecule, how many copies of the target double-stranded DNA sequence are theoretically produced? Show your calculation. (2 marks)

Photosynthesis consists of the light-dependent reactions and the Calvin cycle.

(a) Explain how light energy absorbed by chlorophyll is converted into chemical energy in the form of ATP and NADPH. (3 marks)

(b) Describe how the products of the light-dependent stage are utilized in the Calvin cycle to form triose phosphate (3C sugar). (3 marks)

(c) If a plant is kept in darkness, why does the concentration of ribulose bisphosphate (RuBP, 5C) rapidly decrease? (2 marks)

Microplastics are tiny plastic particles that pollute marine environments.

(a) Define "bioaccumulation" and explain how microplastics accumulate in higher trophic levels of a marine food web. (3 marks)

(b) Distinguish between bioaccumulation and biomagnification. (2 marks)

(c) State two measures humans can take to reduce the impact of plastic pollution in the oceans. (2 marks)

The human respiratory and circulatory systems are structurally adapted for efficient oxygen transport.

(a) Explain how the structure of an alveolus is adapted to maximize the rate of oxygen diffusion into the blood. (4 marks)

(b) Red blood cells (RBCs) lack a nucleus and mitochondria. Explain how each of these structural features is an adaptation for oxygen transport. (4 marks)

A point mutation occurs in a protein-coding gene, changing a codon from UGG (coding for Tryptophan) to UGA (a stop codon).

(a) Name this type of mutation and describe its direct effect on translation. (3 marks)

(b) Explain how this mutation affects the final protein's structure and function. (3 marks)

(c) Suggest why some mutations do not change the amino acid sequence of a protein. (1 mark)

Stomata regulate gas exchange and water loss in plants.

(a) Describe the mechanism of stomatal opening in response to light, in terms of solute concentration and water potential of guard cells. (4 marks)

(b) Why do stomata close during a severe drought, even if there is bright sunlight? Explain the adaptive significance of this response. (3 marks)

A new highway is constructed through a dense, pristine forest ecosystem, splitting it into two isolated parts.

(a) Explain how habitat fragmentation caused by the highway can lead to a decrease in genetic diversity of a small mammal species. (3 marks)

(b) Suggest two mitigation measures that can be implemented to reduce the negative impact of the highway on wildlife. (2 marks)

(c) Describe how conservationists could monitor the effectiveness of these mitigation measures over time. (3 marks)

Describe and compare the mechanisms of water transport in the xylem and translocation of organic nutrients in the phloem. Discuss how prolonged drought with high temperatures affects both transport processes.

Section A: Human Physiology: Regulation and Control\n\nAn athlete participates in a 20-kilometer run on a hot (33 degrees Celsius) and humid (85% relative humidity) day.\n\n(a) (i) Explain why the high ambient temperature and high relative humidity pose a severe challenge to the athlete's thermoregulation during the run. (4 marks)\n(ii) Describe how the cardiovascular system responds to help lower body temperature under such conditions, and explain how this response affects the oxygen supply to the skeletal muscles. (3 marks)\n\n(b) During the run, the athlete loses a significant amount of water through sweating, leading to dehydration.\n(i) Explain how the body detects this water loss and regulates the reabsorption of water in the kidneys to conserve water. State the specific hormone and target cells involved. (6 marks)\n(ii) Describe the expected trend of the solute concentration of the athlete's urine over the course of the 2-hour run. Explain your answer. (3 marks)\n\n(c) After completing the run, the athlete rapidly consumes a large volume (3 litres) of pure water in a short period. Explain why this action might lead to cell swelling (especially in the brain), and how the kidneys respond to restore normal blood osmotic pressure. (4 marks)

Section D: Biotechnology\n\nRecombinant DNA technology is widely used to produce human therapeutic proteins, such as erythropoietin (EPO) which is used to treat anemia.\n\n(a) (i) Explain why mRNA, rather than genomic DNA, is isolated from human cells to synthesize the EPO gene using reverse transcriptase. (3 marks)\n(ii) Describe how the Polymerase Chain Reaction (PCR) is used to amplify the synthesized cDNA, specifying the roles of primers and DNA polymerase. (4 marks)\n\n(b) The amplified cDNA is then inserted into a plasmid vector.\n(i) Explain the importance of using the same restriction endonuclease to cut both the cDNA and the plasmid vector. (3 marks)\n(ii) Besides the target gene, the recombinant plasmid contains a promoter sequence and a selectable marker gene (e.g., ampicillin resistance gene). Explain the function of each of these two components in the production process. (4 marks)\n\n(c) The recombinant plasmid is introduced into host cells for protein expression.\n(i) If Escherichia coli (bacterium) is used as the host, explain why the expressed EPO protein may not be biologically active. (2 marks)\n(ii) Suggest one alternative eukaryotic host system that can be used to overcome the limitation mentioned in (c)(i). State two biosafety or ethical concerns associated with using genetically modified organisms for pharmaceutical production. (4 marks)