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HKDSE Biology practice questions

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Question 1Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A student examines a cell under a light microscope and observes a rigid structure surrounding the cell membrane that stains red with a certain dye. Which statement best identifies this structure and its main function?

  1. It is the cell wall, which provides mechanical support and prevents bursting when water enters. Correct answer.
  2. It is the cytoplasm, which is the site of most metabolic reactions in the cell.
  3. It is the vacuole, which stores water and dissolved solutes to maintain turgidity.
  4. It is the cell membrane, which controls the movement of substances into and out of the cell.

Explanation

A rigid layer outside the cell membrane that stains with a cell-wall dye is the cell wall. Its main roles are mechanical support and resisting excessive water uptake so the cell does not burst. The other listed structures are internal or membranous and would not appear as a rigid outer layer.

Question 2Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A plant cell is placed in a solution whose water potential is higher than that of its cell sap. Which change is expected?

  1. Solute moves out of the cell until water potentials equalise.
  2. Water enters the cell and the cell becomes more turgid. Correct answer.
  3. The cell wall breaks as the cell swells.
  4. Water leaves the cell and the cell becomes plasmolysed.

Explanation

Water moves by osmosis from a region of higher water potential to one of lower water potential. Since the external solution has the higher water potential, water enters the plant cell, increasing turgor pressure. The rigid cell wall prevents bursting, so the cell becomes turgid rather than rupturing.

Question 3Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which molecule is the main energy currency used directly by enzyme-controlled reactions in cells?

  1. Glucose
  2. ATP Correct answer.
  3. Protein
  4. Starch

Explanation

ATP releases energy rapidly when its terminal phosphate bond is hydrolysed, and this energy drives many cellular reactions directly. Glucose and starch are energy stores that must first be broken down to release usable energy. Proteins serve mainly structural and catalytic roles rather than acting as an energy currency.

Question 4Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

An enzyme is added to a reaction mixture and the rate of reaction increases. Which explanation best accounts for this effect?

  1. The enzyme lowers the activation energy needed for the reaction to proceed. Correct answer.
  2. The enzyme raises the temperature of the mixture to speed up collisions.
  3. The enzyme increases the total energy released by the reaction.
  4. The enzyme changes the products of the reaction into simpler molecules.

Explanation

Enzymes are biological catalysts that provide an alternative reaction pathway with a lower activation energy, so more substrate molecules have enough energy to react. They do not alter the overall energy change of the reaction or the nature of the products. Heating the mixture is a separate physical effect, not the action of the enzyme.

Question 5Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which structure in the human respiratory system is the main site of gas exchange?

  1. Larynx
  2. Trachea
  3. Alveoli Correct answer.
  4. Bronchi

Explanation

Alveoli are tiny air sacs with a large total surface area, a thin wall and a rich blood supply, which together make them the main site of gas exchange. The bronchi, trachea and larynx are conducting airways that carry air but have thick walls unsuited to efficient diffusion. Gas exchange therefore occurs across the alveolar surface.

Question 6Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

In a cross between a heterozygous tall pea plant (Tt) and a homozygous short pea plant (tt), what proportion of the offspring is expected to be tall?

  1. All of the offspring
  2. Three quarters
  3. One half Correct answer.
  4. One quarter

Explanation

The Tt parent produces T and t gametes in equal numbers, while the tt parent produces only t gametes. Combining these gives Tt (tall) and tt (short) in a 1:1 ratio, so half the offspring are expected to be tall. The tall allele is dominant, so Tt plants show the tall phenotype.

Question 7Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which process removes carbon dioxide from the atmosphere and fixes it into organic compounds?

  1. Photosynthesis Correct answer.
  2. Combustion
  3. Decomposition
  4. Respiration

Explanation

Photosynthesis uses light energy to convert carbon dioxide and water into organic compounds such as glucose, releasing oxygen. Respiration, decomposition and combustion all release carbon dioxide into the atmosphere rather than removing it. Photosynthesis is therefore the key carbon-fixing process in the carbon cycle.

Question 8Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which feature of a sensory neurone allows it to carry nerve impulses towards the central nervous system?

  1. It has a long axon that conducts impulses from the cell body to the effector.
  2. It has a long dendron that conducts impulses from the receptor to the cell body. Correct answer.
  3. It has no myelin sheath so impulses travel faster.
  4. It has many short dendrites that release neurotransmitters at a synapse.

Explanation

A sensory neurone typically has a long dendron carrying impulses from a receptor towards the cell body and then a short axon leading into the central nervous system. The long axon conducting impulses to an effector is characteristic of a motor neurone. Myelin actually speeds up conduction rather than slowing it.

Question 9Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A person's blood glucose concentration rises after a carbohydrate-rich meal. Which response helps to lower it back towards normal?

  1. The liver converts glucose into glycogen and releases it into the blood.
  2. The pancreas secretes glucagon, which stimulates the liver to release glucose.
  3. The kidneys excrete all the excess glucose immediately.
  4. The pancreas secretes insulin, which stimulates cells to take up glucose. Correct answer.

Explanation

A rise in blood glucose is detected by the pancreas, which secretes insulin. Insulin stimulates body cells, especially liver and muscle cells, to take up glucose and store it as glycogen, lowering blood glucose. Glucagon has the opposite effect and would raise blood glucose further.

Question 10Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A cell is observed to have a cell wall, a large central vacuole and chloroplasts. Which conclusion is best supported?

  1. It is a plant cell. Correct answer.
  2. It is a fungal cell.
  3. It is a bacterial cell.
  4. It is an animal cell.

Explanation

The combination of a cellulose cell wall, a large permanent central vacuole and chloroplasts is characteristic of a plant cell. Animal cells lack a cell wall and chloroplasts, and typically have no large central vacuole. Bacterial and fungal cells have walls but no chloroplasts and no large central vacuole of this kind.

Question 11Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which statement correctly describes the role of the selectively permeable membrane in a cell?

  1. It prevents any substance from entering or leaving the cell.
  2. It allows all substances to pass freely so that the cell reaches equilibrium with its surroundings.
  3. It actively pumps every substance across regardless of concentration gradients.
  4. It allows some substances to pass while restricting others, helping control the cell's internal environment. Correct answer.

Explanation

A selectively permeable membrane permits certain molecules, such as water and small solutes, to cross while restricting larger or charged molecules. This selective control helps maintain a stable internal environment. It is neither fully permeable nor completely impermeable, and not every substance is actively pumped.

Question 12Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

In an experiment, pieces of potato tissue are placed in sucrose solutions of different concentrations. The mass of each piece is measured before and after. Which result would indicate that the potato tissue has lost water?

  1. The mass of the potato piece stays the same.
  2. The mass of the potato piece increases.
  3. The mass of the potato piece decreases. Correct answer.
  4. The potato piece becomes firmer and more rigid.

Explanation

If the surrounding sucrose solution has a lower water potential than the potato cells, water leaves the cells by osmosis and the tissue loses mass. An increase in mass indicates water uptake, while no change indicates the water potentials are equal. Increased firmness reflects water uptake, not loss.

Question 13Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which combination of factors would increase the rate of diffusion of a gas across a respiratory surface?

  1. A steep concentration gradient and a large surface area Correct answer.
  2. A small surface area and a long diffusion distance
  3. A steep concentration gradient and a thick surface
  4. A shallow concentration gradient and a thick surface

Explanation

The rate of diffusion increases with a steeper concentration gradient, a larger surface area and a shorter diffusion distance. A steep gradient and a large surface area both favour rapid gas exchange. A thick surface or a small surface area would slow diffusion down.

Question 14Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A student investigates the effect of pH on the activity of an enzyme and finds that activity is highest at pH 7 and falls sharply at pH 3 and pH 11. Which explanation best fits these results?

  1. Extremes of pH alter the shape of the enzyme's active site so substrate molecules no longer fit. Correct answer.
  2. Substrate concentration becomes limiting only at extreme pH values.
  3. Extremes of pH increase the kinetic energy of substrate molecules so they react faster.
  4. The enzyme is used up more quickly at extreme pH values.

Explanation

Each enzyme has an optimum pH at which its active site has the correct shape to bind its substrate. At extreme pH, changes in hydrogen ion concentration alter the bonds holding the enzyme's shape, so the active site no longer fits the substrate and activity falls. The enzyme is not used up in the reaction, and substrate availability does not explain a sharp pH effect.

Question 15Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which statement correctly describes the role of bile in digestion?

  1. It provides the enzyme that breaks down starch into maltose.
  2. It emulsifies fats into smaller droplets, increasing the surface area for lipase action. Correct answer.
  3. It digests fats into fatty acids and glycerol directly.
  4. It neutralises stomach acid and digests proteins into amino acids.

Explanation

Bile is produced by the liver and stored in the gall bladder. It contains bile salts that emulsify fats into small droplets, greatly increasing the surface area on which lipase can act. Bile does not itself contain digestive enzymes, so it does not break down fats, proteins or starch chemically.

Question 16Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which feature of the small intestine is most important for efficient absorption of digested nutrients?

  1. Its inner surface has many finger-like villi that greatly increase the surface area. Correct answer.
  2. It stores food for a long time before absorption begins.
  3. It secretes a strong acid to break down food chemically.
  4. Its wall is thick and muscular to push food along quickly.

Explanation

The small intestine's lining is folded into numerous villi, each with a thin wall and a rich blood supply. These features give a very large surface area and a short diffusion distance, both of which maximise absorption. A thick wall, strong acid secretion or long storage would not aid absorption.

Question 17Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A man who is heterozygous for a recessive genetic disorder marries a woman who is homozygous normal. What is the probability that their child will be affected by the disorder?

  1. All children will be carriers but none will be affected.
  2. One quarter, because both parents must pass on a recessive allele.
  3. Zero, because the child would inherit at least one normal dominant allele. Correct answer.
  4. One half, because the father passes on the recessive allele half the time.

Explanation

A recessive disorder is expressed only when an individual has two copies of the recessive allele. The mother is homozygous normal and can pass on only a normal dominant allele, so every child receives at least one dominant allele and none can be affected. Some children may be carriers if they inherit the recessive allele from the father.

Question 18Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which statement best explains why a person with a severe deficiency of red blood cells feels tired easily?

  1. Fewer red blood cells reduce the clotting ability of the blood.
  2. Fewer red blood cells lower the blood glucose concentration.
  3. Fewer red blood cells carry less carbon dioxide to the lungs for excretion.
  4. Fewer red blood cells carry less oxygen to muscles, so less energy is released by respiration. Correct answer.

Explanation

Red blood cells contain haemoglobin, which transports oxygen from the lungs to respiring tissues. A shortage of red blood cells reduces oxygen delivery, so aerobic respiration and energy release are reduced, causing tiredness. Red blood cells are not primarily responsible for clotting or glucose regulation.

Question 19Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which statement correctly describes the role of decomposers in an ecosystem?

  1. They break down dead organic matter and release nutrients back into the environment. Correct answer.
  2. They transfer energy directly from producers to primary consumers.
  3. They convert light energy into chemical energy stored in organic compounds.
  4. They fix atmospheric nitrogen into nitrates for plants to absorb.

Explanation

Decomposers, such as bacteria and fungi, break down dead organisms and waste materials. This releases mineral nutrients into the soil and water so they can be reused by producers. Converting light energy is the role of producers, and nitrogen fixation is carried out by specific nitrogen-fixing bacteria.

Question 20Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

In a food chain, only about ten per cent of the energy at one trophic level is transferred to the next. Which explanation best accounts for this loss?

  1. Energy is destroyed completely when organisms are eaten.
  2. Much energy is lost as heat through respiration and in undigested material. Correct answer.
  3. Energy is converted into matter that cannot be used by any organism.
  4. Energy is transferred to the next level with almost no loss.

Explanation

At each trophic level, organisms use energy for respiration, releasing much of it as heat, and some material is not digested or is excreted. These losses mean only a small fraction of the energy is stored as new biomass available to the next level. Energy is not destroyed but transferred and dissipated, mainly as heat.

Question 21Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A population of insects is exposed to an insecticide, and a few individuals survive and reproduce. After several generations, most of the population is resistant. Which statement best explains this outcome?

  1. The insecticide became less effective because it broke down in the environment.
  2. The survivors learned to tolerate the insecticide and passed this tolerance to their offspring.
  3. The insecticide caused resistant mutations in every individual exposed to it.
  4. The survivors already carried resistant alleles, which became more common through natural selection. Correct answer.

Explanation

Variation already exists in the population, and some individuals carry alleles giving resistance. When the insecticide is applied, these individuals survive and reproduce, passing the resistant alleles to their offspring. Over generations the proportion of resistant individuals increases, which is natural selection in action.

Question 22Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which statement correctly describes the function of the placenta in a mammal?

  1. It stores the fetus until birth and prevents any exchange with the mother.
  2. It produces milk for the developing fetus during pregnancy.
  3. It allows the exchange of nutrients, oxygen and waste between maternal and fetal blood without mixing them. Correct answer.
  4. It mixes maternal and fetal blood so that both share the same circulation.

Explanation

The placenta brings the fetal and maternal blood supplies close together so that substances can diffuse across. Nutrients and oxygen pass to the fetus, while carbon dioxide and urea pass to the mother's blood. The two blood supplies do not normally mix, and milk production occurs in the mammary glands after birth.

Question 23Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which combination of features is characteristic of an enzyme that catalyses a reaction in the human small intestine?

  1. It has an optimum pH near neutral and is denatured by strong acid. Correct answer.
  2. It has an optimum temperature of about 70 degrees Celsius.
  3. It works best at very low temperatures because molecules move slowly.
  4. It has an optimum pH near 2 and is denatured by neutral conditions.

Explanation

Enzymes in the small intestine, such as pancreatic amylase, work best around neutral pH and body temperature. Strong acid would alter the shape of the active site and denature the enzyme. An optimum pH of about 2 describes a stomach enzyme such as pepsin, not an intestinal one.

Question 24Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

In a cross between two heterozygous individuals for a trait showing incomplete dominance, what phenotypic ratio is expected in the offspring?

  1. 1 intermediate : 2 intermediate : 1 intermediate is incorrect; the ratio is 1:2:1 of the two parental phenotypes and the intermediate. Correct answer.
  2. 3 dominant : 1 recessive
  3. 1 dominant : 1 recessive
  4. All offspring show the same intermediate phenotype.

Explanation

With incomplete dominance, the heterozygote shows a phenotype intermediate between the two homozygotes. A cross between two heterozygotes gives a genotypic ratio of 1:2:1, which corresponds to 1 of one parental phenotype, 2 intermediate and 1 of the other parental phenotype. This differs from the 3:1 ratio seen with complete dominance.

Question 25Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A plant is kept in bright light but in an atmosphere with a very low carbon dioxide concentration. Which statement best predicts the effect on the rate of photosynthesis?

  1. The rate is unaffected because light is the only factor that controls photosynthesis.
  2. The rate falls because carbon dioxide becomes the limiting factor despite adequate light. Correct answer.
  3. The rate rises because light intensity is high and light becomes the limiting factor.
  4. The rate falls only because the temperature is too low.

Explanation

Photosynthesis requires carbon dioxide as a raw material, so a very low concentration limits the rate even when light is plentiful. Under these conditions carbon dioxide, not light, is the limiting factor. Temperature is not mentioned as changed, so it cannot be the explanation for the fall.

Question 26Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which structure in a human kidney is responsible for the ultrafiltration of blood?

  1. The loop of Henle
  2. The proximal convoluted tubule
  3. The collecting duct
  4. The glomerulus Correct answer.

Explanation

Ultrafiltration occurs in the glomerulus, where high hydrostatic pressure in the capillary network forces water and small molecules through the filtration membrane into the Bowman's capsule. The other listed structures carry out selective reabsorption or concentration of the filtrate rather than the initial pressure-driven filtration step.

Question 27Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which molecule carries the amino acid sequence from the nucleus to the ribosome during protein synthesis?

  1. Ribosomal RNA
  2. Deoxyribonucleic acid
  3. Messenger RNA Correct answer.
  4. Transfer RNA

Explanation

Messenger RNA is transcribed from DNA in the nucleus and carries the base sequence, read as codons, to the ribosome. Transfer RNA delivers amino acids and ribosomal RNA forms part of the ribosome, but neither carries the sequence information from the nucleus.

Question 28Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which blood vessel carries oxygenated blood away from the heart to the body tissues?

  1. The hepatic portal vein
  2. The vena cava
  3. The pulmonary artery
  4. The aorta Correct answer.

Explanation

The aorta is the major artery leaving the left ventricle and distributes oxygenated blood to the systemic circulation. The pulmonary artery and vena cava carry deoxygenated blood, and the hepatic portal vein carries nutrient-rich blood between the gut and liver.

Question 29Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which part of a flowering plant develops into the fruit after fertilisation?

  1. The ovule
  2. The ovary Correct answer.
  3. The stigma
  4. The anther

Explanation

After fertilisation, the ovary wall develops into the fruit and the ovules inside become seeds. The anther and stigma are parts of the flower involved in producing and receiving pollen rather than forming the fruit.

Question 30Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which component of a balanced ecosystem converts atmospheric nitrogen into nitrogen compounds usable by plants?

  1. Denitrifying bacteria
  2. Nitrogen-fixing bacteria Correct answer.
  3. Decomposing fungi
  4. Nitrifying bacteria

Explanation

Nitrogen-fixing bacteria, often living in root nodules of legumes, reduce atmospheric nitrogen to ammonia, which is then converted into usable nitrogen compounds. The other organisms act at different stages of the nitrogen cycle.

Question 31Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which type of immunity is acquired when antibodies pass from a mother to her baby through breast milk?

  1. Natural passive immunity Correct answer.
  2. Artificial passive immunity
  3. Natural active immunity
  4. Artificial active immunity

Explanation

Passive immunity arises when ready-made antibodies are supplied rather than produced by the recipient's own immune system. Because the transfer occurs naturally through breast milk rather than by medical intervention, it is natural passive immunity.

Question 32Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which process in the carbon cycle releases carbon dioxide into the atmosphere?

  1. Respiration Correct answer.
  2. Photosynthesis
  3. Transpiration
  4. Nitrogen fixation

Explanation

Respiration breaks down organic molecules such as glucose and releases carbon dioxide as a waste product, adding it to the atmosphere. Photosynthesis removes carbon dioxide, while nitrogen fixation and transpiration involve other elements and processes.

Question 33Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which characteristic is shared by all members of the kingdom Fungi?

  1. Their cell walls contain chitin Correct answer.
  2. They reproduce only by binary fission
  3. Their cells contain chloroplasts
  4. They are always multicellular

Explanation

Fungal cell walls are made of chitin, which distinguishes them from plants whose walls contain cellulose. Fungi are heterotrophic and therefore lack chloroplasts, some such as yeasts are unicellular, and they typically reproduce by spores rather than binary fission.

Question 34Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A student places a piece of red blood cell suspension in distilled water. Which observation is expected?

  1. The cells shrink and become crenated
  2. The cells remain unchanged in size
  3. The cells swell and burst by osmosis Correct answer.
  4. The cells lose water and become rigid

Explanation

Distilled water is hypotonic to the cytoplasm of red blood cells, so water enters the cells by osmosis across their partially permeable membranes. With no cell wall to resist the pressure, the cells swell and eventually burst, a process called haemolysis.

Question 35Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

An enzyme is added to a substrate solution and the rate of reaction is measured. Which change is most likely to increase the initial rate when substrate concentration is not limiting?

  1. Removing the enzyme from the mixture
  2. Adding a heavy metal ion that denatures the enzyme
  3. Raising the temperature towards the enzyme's optimum Correct answer.
  4. Lowering the temperature well below the optimum

Explanation

When substrate is plentiful, the rate is limited by the number of successful enzyme-substrate collisions, which increase as temperature rises towards the optimum. Beyond the optimum the enzyme denatures, so a moderate increase towards the optimum raises the rate.

Question 36Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

In a breeding experiment, a black guinea pig is crossed with a white guinea pig and all the offspring are black. Which conclusion is best supported?

  1. The offspring are heterozygous with a white phenotype
  2. Black is dominant and the black parent is homozygous Correct answer.
  3. Both parents are heterozygous
  4. White is dominant and the black parent is homozygous

Explanation

When all offspring show one parental phenotype, that phenotype is dominant and the parent showing it must be homozygous, otherwise some recessive offspring would appear. The black parent is therefore homozygous dominant and all the offspring are heterozygous black.

Question 37Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which adaptation of a xerophytic plant reduces water loss by transpiration?

  1. Broad, flat leaves with many stomata
  2. Sunken stomata in pits on the leaf surface Correct answer.
  3. Large air spaces in the spongy mesophyll
  4. A thin cuticle covering the leaf epidermis

Explanation

Sunken stomata create a humid microclimate in the pits that reduces the water potential gradient between the leaf interior and the air, slowing transpiration. A thin cuticle, broad leaves and large air spaces would all tend to increase water loss.

Question 38Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

During strenuous exercise, a person's breathing rate and depth increase. Which change in the blood most directly stimulates this response?

  1. A fall in blood glucose concentration
  2. A rise in carbon dioxide concentration Correct answer.
  3. A rise in oxygen concentration
  4. A rise in blood pH

Explanation

Increased respiration during exercise raises blood carbon dioxide, which is detected by chemoreceptors and stimulates the respiratory centre to increase ventilation. Oxygen changes are a weaker stimulus, and a rise in carbon dioxide actually lowers pH rather than raising it.

Question 39Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which statement correctly describes the role of the loop of Henle in the mammalian kidney?

  1. It actively secretes glucose into the urine
  2. It creates an osmotic gradient in the medulla that allows water to be reabsorbed from the collecting duct Correct answer.
  3. It filters large proteins from the blood into the Bowman's capsule
  4. It stores urine before release from the kidney

Explanation

The loop of Henle uses active transport of ions and the countercurrent arrangement to build a high solute concentration in the medulla. This gradient draws water out of the collecting duct by osmosis, concentrating the urine.

Question 40Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which feature of an insect-pollinated flower is most likely to attract pollinators?

  1. Long filaments hanging outside the flower
  2. Feathery stigmas exposed to the wind
  3. Small, dull petals without scent
  4. Large, brightly coloured petals Correct answer.

Explanation

Insect-pollinated flowers typically have large, colourful petals and scent or nectar to attract insects that transfer pollen. Feathery stigmas and hanging anthers are characteristic of wind-pollinated flowers, which do not need to attract animals.

Question 41Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A woman with blood group AB marries a man with blood group O. Which blood groups are possible among their children?

  1. Groups A and B only Correct answer.
  2. Group AB only
  3. Groups AB and O only
  4. Groups A, B, AB and O

Explanation

The AB parent carries alleles IA and IB, while the O parent carries two i alleles. Each child receives either IA or IB together with i, giving genotype IAi (group A) or IBi (group B), so only groups A and B are possible.

Question 42Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which statement best explains why a food chain rarely has more than five trophic levels?

  1. Decomposers remove all the energy before the next level
  2. Consumers at higher levels cannot digest plant material
  3. Energy is lost at each level, so too little remains to support further consumers Correct answer.
  4. The number of species decreases at each trophic level

Explanation

Only a small fraction of the energy at each trophic level is incorporated into the next, with most lost as heat through respiration. After several transfers the remaining energy is insufficient to support a viable population of further consumers.

Question 43Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which statement correctly describes the role of the pancreas in digestion?

  1. It secretes enzymes and hydrogencarbonate into the small intestine Correct answer.
  2. It absorbs most of the digested nutrients
  3. It secretes bile to emulsify fats
  4. It stores glycogen and releases it as glucose into the gut

Explanation

The pancreas releases pancreatic juice containing amylase, protease and lipase together with hydrogencarbonate, which neutralises acidic chyme. Bile is produced by the liver, absorption occurs mainly in the small intestine, and glycogen is not released into the gut.

Question 44Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which change would most likely increase the rate of transpiration from a leafy shoot?

  1. Applying a waterproof coating to the leaves
  2. Lowering the light intensity
  3. Increasing the humidity of the surrounding air
  4. Increasing the wind speed around the leaves Correct answer.

Explanation

Higher wind speed removes the layer of humid air around the leaf, maintaining a steep water potential gradient and increasing the rate of transpiration. High humidity, low light and a waterproof coating all reduce water loss.

Question 45Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A mutation changes a single base in a gene but the amino acid sequence of the protein is unchanged. Which explanation best accounts for this?

  1. The ribosome corrected the error during translation
  2. The mutation occurred in a non-coding region of DNA
  3. The protein was degraded before it could function
  4. The genetic code is degenerate, so different codons can specify the same amino acid Correct answer.

Explanation

Because several codons can code for the same amino acid, a substitution may produce a different codon that still specifies the same amino acid, leaving the protein unchanged. This silent mutation reflects the degeneracy of the genetic code.

Question 46Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

In a population of birds, individuals with medium-sized beaks survive best while those with very small or very large beaks survive poorly. Which type of selection does this describe?

  1. Directional selection
  2. Disruptive selection
  3. Artificial selection
  4. Stabilising selection Correct answer.

Explanation

Stabilising selection favours the intermediate phenotype and reduces variation at the extremes, keeping the population mean unchanged. Directional selection shifts the mean towards one extreme, while disruptive selection favours both extremes over the intermediate.

Question 47Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A cell is treated with a drug that prevents the formation of the spindle fibres. Which stage of mitosis would be most directly affected?

  1. Prophase
  2. Telophase
  3. Interphase
  4. Metaphase Correct answer.

Explanation

Spindle fibres attach to chromosomes and align them at the equator during metaphase, and later pull chromatids apart in anaphase. Without a functional spindle, chromosomes cannot align correctly at the metaphase plate, disrupting the orderly separation of chromatids.

Question 48Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

In an ecosystem, a primary consumer ingests plant material but only a small proportion of the energy in that material becomes available to the next trophic level. Which factor contributes most to this loss?

  1. Energy lost as heat during the consumer's respiration Correct answer.
  2. Energy retained in undigested plant cell walls
  3. Energy stored in the consumer's body tissues
  4. Energy passed on in the consumer's gametes

Explanation

Most of the energy a consumer obtains is used in respiration and released as heat, which cannot be passed on to the next trophic level. Although undigested material is also lost, respiration accounts for the largest share of energy loss between trophic levels.

Question 49Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

Which statement correctly describes the role of the placenta in the exchange of materials between mother and fetus?

  1. It prevents all substances from reaching the fetus
  2. It mixes maternal and fetal blood so that materials can diffuse freely
  3. It allows selective transfer of nutrients and oxygen while keeping the two blood supplies separate Correct answer.
  4. It produces red blood cells for the fetus throughout pregnancy

Explanation

The placenta brings maternal and fetal blood close together in villi so that nutrients, oxygen and wastes can diffuse across, but the two circulations remain separate. This arrangement permits exchange while limiting the transfer of some harmful substances.

Question 50Biology (Category A Senior Secondary elective subject; content domains not enumerated in the retrieved source)

A plant is provided with water containing a dissolved mineral ion that is present at a lower concentration in the soil solution than in the root hair cells. Which process best explains how the ion continues to enter the root?

  1. Active transport using energy from respiration Correct answer.
  2. Transpiration pull drawing the ion upward
  3. Osmosis of water carrying the ion
  4. Simple diffusion down a concentration gradient

Explanation

When the ion concentration inside the root hair cell is higher than in the soil solution, uptake against the gradient requires active transport powered by respiration. Diffusion and osmosis cannot move the ion against its concentration gradient, and transpiration pull acts on water in the xylem.