2025 Cambridge IGCSE Biology (0610) 模擬試題連答案詳解

Anaerobic respiration in yeast produces ethanol and carbon dioxide. In human muscle cells, anaerobic respiration produces lactic acid only (no carbon dioxide is produced). In both types of cells, anaerobic respiration releases a much smaller amount of energy per glucose molecule compared to aerobic respiration because glucose is only partially broken down.

Correct answer: B. 1 mark for selecting the correct row containing the correct products for yeast (ethanol + carbon dioxide) and human muscles (lactic acid only), and the correct relative energy released (much less).

A higher percentage similarity in DNA base sequences indicates that two species are more closely related and shared a more recent common ancestor. Since the similarity between Q and S is the highest at 94%, they are the most closely related of the species compared.

Correct answer: B. 1 mark for identifying that the highest percentage of DNA sequence similarity (94% between Q and S) corresponds to the most closely related species.

Increasing the temperature up to the optimum temperature (around 37 degrees Celsius for many enzymes) increases the kinetic energy of both enzyme and substrate molecules. They move faster, resulting in more frequent collisions, which increases the rate of successful enzyme-substrate complex formation.

Correct answer: A. 1 mark for identifying that increased kinetic energy leads to more frequent successful collisions between enzyme and substrate.

A mature human red blood cell loses its nucleus during development to maximize space for hemoglobin, but it retains a cell membrane. A plant root hair cell is located underground, so it does not contain chloroplasts (which are needed for photosynthesis in the presence of light), but like all plant cells, it possesses a cell wall.

Correct answer: A. 1 mark for correctly identifying that a mature red blood cell has no nucleus but has a membrane, and a root hair cell has no chloroplasts but has a cell wall.

During the stationary phase of a sigmoid growth curve, the rate of reproduction/birth equals the rate of death, so the population size remains constant. This plateau is caused by limiting factors such as nutrient depletion, space limitations, or the accumulation of toxic metabolic waste.

Correct answer: C. 1 mark for identifying the stationary phase and its associated limiting factors (nutrient depletion or toxic waste accumulation).

Passive immunity is the temporary defense against a pathogen by antibodies acquired from another individual. Since the recipient's own lymphocytes do not produce the antibodies, no memory cells are formed. Breast-fed infants receive maternal antibodies through breast milk, providing them with passive immunity.

Correct answer: C. 1 mark for identifying the transmission of breast-milk antibodies as an example of passive immunity.

Bile does not contain digestive enzymes, so it does not perform chemical digestion. Instead, it emulsifies fats, breaking large fat globules/droplets into many smaller droplets. This is physical digestion because no chemical bonds in the lipids are broken; it increases the surface area of the lipids, allowing the enzyme lipase to digest them more rapidly.

Correct answer: C. 1 mark for identifying that bile physically emulsifies large fat droplets into smaller ones, increasing the surface area for lipase.

Xylem vessels are made of dead, hollow cells with cell walls reinforced by lignin. They transport water and dissolved mineral ions from the roots to the leaves (the transpiration stream), which is unidirectional (upwards only). Phloem, on the other hand, transports organic nutrients like sucrose and amino acids bidirectionally (both upwards and downwards).

Correct answer: B. 1 mark for identifying that xylem contains lignin, transports water and mineral ions, and operates in an upwards-only direction.

Anaerobic respiration in human muscle cells produces lactic acid (lactate) only, whereas in yeast (fungi) it produces ethanol and carbon dioxide (alcohol fermentation). Neither reaction uses oxygen, and both produce a relatively small amount of energy compared to aerobic respiration.

Award 1 mark for identifying the correct products of anaerobic respiration in both muscle cells (lactic acid) and yeast cells (ethanol and carbon dioxide).

The evolutionary relationship between species can be determined by comparing their DNA base sequences. Species that are closely related share a more recent common ancestor and have fewer differences in their DNA base sequences. Comparing the sequences: Species P and Species Q differ by only 1 base (the last base). All other comparisons have 2 or more differences. Thus, P and Q are the most closely related.

Award 1 mark for identifying that P and Q have the fewest differences (1 difference) and are therefore most closely related.

Enzymes are proteins with a specific 3D shape, including an active site that is complementary to the substrate. At high temperatures (such as 65 degrees C), the thermal energy breaks the bonds holding the enzyme's structure together. This denatures the enzyme, changing the shape of its active site so the substrate can no longer fit, resulting in a reaction rate of zero.

Award 1 mark for explaining that high temperature causes the denaturation of the enzyme and alteration of its active site shape.

Bacterial cells are prokaryotic; they possess a cell wall (made of peptidoglycan) but lack membrane-bound organelles such as mitochondria and chloroplasts. Animal cells are eukaryotic, possessing a nucleus and mitochondria, but they never have a cell wall. Therefore, the statement in option A is entirely correct.

Award 1 mark for identifying the correct combination of structures present or absent in bacterial and animal cells.

During the exponential (log) phase of a sigmoid growth curve, resources (like nutrients and space) are abundant, and waste products are low. This allows the organisms to reproduce at their maximum rate, meaning the birth rate is significantly higher than the death rate, leading to rapid population growth.

Award 1 mark for identifying that the birth rate is much higher than the death rate during the exponential/log phase.

Active immunity is the defense against a pathogen by antibody production in the body. It is acquired after vaccination or infection, where the person's own lymphocytes are stimulated to produce antibodies and memory cells. Passive immunity involves receiving antibodies from another source (e.g., across the placenta, via breast milk, or through antibody injection), which does not produce memory cells.

Award 1 mark for identifying that active immunity involves the production of antibodies by the person's own lymphocytes, stimulated by vaccination.

Physical digestion is the breakdown of food into smaller pieces without making any chemical change to the molecules themselves. Bile emulsifies large fat globules into tiny droplets. This increases the surface area of the fat for the chemical digestion by lipase, but the fat molecules themselves are not chemically broken down during emulsification, making it a physical process.

Award 1 mark for identifying the emulsification of fats by bile as physical digestion.

Xylem vessels are made of dead cells joined end-to-end with no end walls, forming a continuous tube. Their walls are thickened with lignin for support, and they transport water and minerals upwards from roots to leaves. Phloem tubes are made of living cells and transport sucrose and amino acids (translocation) throughout the plant in both directions. Therefore, Tissue X is xylem and Tissue Y is phloem.

Award 1 mark for correctly identifying Tissue X as xylem and Tissue Y as phloem based on their characteristics.

Anaerobic respiration in human muscle cells produces lactic acid only, whereas in yeast cells (alcohol fermentation) it produces alcohol (ethanol) and carbon dioxide. In both cases, because glucose is only partially broken down, the energy released per glucose molecule is much less than in aerobic respiration (which fully oxidizes glucose to carbon dioxide and water).

Award 1 mark for identifying Row A as the correct row describing anaerobic respiration in muscles and yeast and the relative energy released.

A higher percentage similarity in DNA base sequences indicates a more recent common ancestor and closer evolutionary relationship. Species W and Y share 94% similarity, whereas Species W and X share 85%. Therefore, W and Y are more closely related and shared a more recent common ancestor. Species Z is more closely related to Y (65%) than to X (59%), making choice B incorrect. The highest similarity of all is between W and Y (94%), not X and Y, making C incorrect. A low percentage similarity indicates a more distant relationship, not closer, making D incorrect.

Award 1 mark for selecting the correct statement based on high percentage DNA similarity representing a closer evolutionary relationship.

As temperature rises past the optimum (approx. 40 °C), thermal energy disrupts the hydrogen bonds holding the active site of the enzyme in its specific shape. This is called denaturation. At 50 °C, some enzyme molecules have lost their shape, reducing the reaction rate to 4.0. By 60 °C, all enzyme molecules are denatured and can no longer bind to the substrate, reducing the rate to 0.0.

Award 1 mark for identifying denaturation of enzyme molecules as the cause of the drop and cessation of reaction rate at higher temperatures.

To synthesize a protein (enzyme), many ribosomes are needed. The rough endoplasmic reticulum (which is covered in ribosomes) is responsible for transport and folding of these proteins. Vesicles are required to transport and secrete the enzymes out of the cell. Mitochondria provide the energy (ATP) required for these active metabolic processes.

Award 1 mark for identifying the correct combination of organelles involved in protein synthesis, modification, transport, and energy provision.

In the stationary phase, the population size remains constant because the rate of cell division equals the rate of cell death. This occurs due to limiting factors in a closed system, primarily the depletion of nutrients and the build-up of toxic metabolic waste products.

Award 1 mark for identifying the stationary phase and its corresponding limiting factors.

Passive immunity is the temporary defense against a pathogen. It is acquired by transferring antibodies from another source (e.g., across the placenta or through breast milk, or via antibody injections). Because the body's own lymphocytes do not produce these antibodies, no memory cells are made. Protection is immediate, but because antibodies are eventually broken down and not replaced, the protection is only short-term.

Award 1 mark for identifying Row A as the correct set of features of passive immunity.

Bile is produced by the liver and stored in the gallbladder. It does not contain digestive enzymes. Instead, it physical digests fat by emulsification, breaking large fat globules into tiny droplets. This increases the total surface area available for the enzyme lipase to chemically digest the fats into fatty acids and glycerol.

Award 1 mark for identifying that bile physically emulsifies fat to increase the surface area for lipase action.

All four features listed are correct. Xylem vessels are made of dead cells with lignified walls that transport water and mineral ions in one direction (upwards from roots to leaves). Phloem sieve tubes consist of living cells (lacking nuclei and companion cells assist them) that transport sucrose and amino acids bidirectionally (upwards and downwards between sources and sinks).

Award 1 mark for correctly identifying that all four listed features of xylem and phloem are accurate.

Anaerobic respiration in yeast (often called fermentation) breaks down glucose to produce alcohol (ethanol) and carbon dioxide. Therefore, statements 1 and 2 are correct. Lactic acid (statement 3) is produced during anaerobic respiration in mammalian muscles, not yeast. Anaerobic respiration releases a much smaller amount of energy per glucose molecule compared to aerobic respiration because glucose is only partially broken down, making statement 4 incorrect. Thus, only statements 1 and 2 apply.

Award 1 mark for the correct option (B).
- Reject other options because they incorrectly include statement 3 or 4, or exclude statement 1 or 2.

Organisms that are closely related share a more recent common ancestor and have more similar DNA base sequences. By comparing the base sequences:
- Species 1 and Species 2 differ at only 1 position (the 5th base: G vs C).
- Species 2 and Species 3 differ at 2 positions (the 2nd and 9th bases).
- Species 3 and Species 4 differ at 3 positions (the 1st, 2nd, and 7th bases).
- Species 1 and Species 4 differ at 4 positions.
Since Species 1 and 2 have the fewest differences in their DNA sequences, they are the most closely related.

Award 1 mark for the correct option (A).
- Reject B, C, and D as these pairs have greater genetic differences.

Amylase is an enzyme (protein). Boiling at 100 °C causes the enzyme molecules to vibrate violently, breaking the weak bonds that maintain their three-dimensional structure. This causes the active site to permanently change shape (denaturation). Once denatured, the substrate (starch) can no longer fit into the active site, even if the temperature is lowered back to the optimum (37 °C). Therefore, starch is not digested.

Award 1 mark for the correct option (C).
- Reject A and B because boiling permanently denatures the enzyme so no reaction can occur.
- Reject D because starch (the substrate) is not boiled or denatured; it is the enzyme that has lost its functional shape.

Both plant cells (such as root hair cells) and bacterial cells have a cell wall (though made of different materials: cellulose in plants, peptidoglycan in bacteria), a cell membrane, cytoplasm, and ribosomes (for protein synthesis). Mitochondria and a nucleus are membrane-bound organelles found in eukaryotic plant cells but are absent in prokaryotic bacterial cells.

Award 1 mark for the correct option (C).
- Reject A because ribosomes are also present in both cells.
- Reject B because bacteria do not have a nucleus.
- Reject D because bacteria do not have mitochondria.

In a closed system (like a flask), resources are limited. During the exponential (log) phase, the population grows rapidly, leading to the rapid consumption of available nutrients and the production of toxic metabolic waste products. As nutrients deplete and toxins accumulate, the growth rate slows down and the death rate increases until the death rate equals the reproduction rate, leading to the stationary phase.

Award 1 mark for the correct option (C).
- Reject A because increased resources would prolong the exponential phase.
- Reject B because a decreasing death rate would not lead to the stationary phase.
- Reject D because temperature shock is not a standard physiological factor explaining the stationary phase transition under normal culture conditions.

Passive immunity involves receiving ready-made antibodies from another source (e.g., via the placenta, breast milk, or an injection of immunoglobulin). Because the recipient’s own immune system is not stimulated to respond, their lymphocytes do not produce antibodies and no memory cells are created. Consequently, passive immunity is temporary because these foreign antibodies are eventually broken down by the body.

Award 1 mark for the correct option (C).
- Reject A and B because they describe characteristics of active immunity.
- Reject D because passive immunity can also be acquired artificially through immunoglobulin injections.

Physical digestion is the breakdown of large food particles into smaller ones without chemically altering the molecules. Bile contains bile salts that emulsify fats, meaning they break large fat droplets into many smaller droplets. This increases the total surface area of the lipid exposed to lipase, speeding up chemical digestion. Neutralization of acid (option A) is a chemical function, not physical digestion.

Award 1 mark for the correct option (B).
- Reject A because neutralizing stomach acid is a chemical process, not physical digestion.
- Reject C because chemical breakdown into fatty acids and glycerol is catalyzed by lipase (chemical digestion).
- Reject D because pepsin works in acidic conditions of the stomach, whereas bile operates in the alkaline duodenum.

When the ventricles contract, the pressure inside them rises rapidly. To prevent blood from flowing backward into the atria, the atrioventricular (bicuspid and tricuspid) valves must close immediately. At the same time, the high ventricular pressure forces the semilunar valves to open, allowing blood to be pumped out of the heart into the aorta and pulmonary artery.

Award 1 mark for the correct option (D).
- Reject A, B, and C as they represent incorrect valve state combinations during ventricular systole.

During vigorous exercise, muscles respire anaerobically, producing lactic acid. This lactic acid builds up in the muscles and must be transported to the liver via the bloodstream. After exercise, rapid and deep breathing continues (repaying the oxygen debt) to supply enough oxygen to the liver to break down the accumulated lactic acid.

A is correct: 1 mark for identifying lactic acid as the accumulating substance and explaining that continued breathing provides oxygen to break down lactic acid in the liver.

Organisms with more similar DNA base sequences are more closely related and share a more recent common ancestor. Comparing the DNA base sequences of the given pairs:
- Species W and Species X differ by only 1 base (at position 8: T in W, C in X).
- Species W and Species Z differ by 4 bases (at positions 1, 3, 8, and 9).
- Species X and Species Y differ by 2 bases (at positions 3 and 9).
- Species X and Species Z differ by 3 bases (at positions 1, 3, and 9).
Therefore, species W and X are the most closely related of the options provided.

A is correct: 1 mark for analyzing the differences in sequence base pairs and concluding that species W and X are most closely related due to having the fewest differences (only 1 mismatch).

At low temperatures, both enzyme and substrate molecules have low kinetic energy, which causes them to move slowly. This results in a low rate of reaction due to a low frequency of successful collisions between the substrate and the active site. Denaturation occurs at high temperatures, which involves a change in the three-dimensional shape of the active site (preventing substrate binding), not the breakdown of peptide bonds.

A is correct: 1 mark for linking low temperatures to low kinetic energy and fewer successful collisions.

During the lag phase, the bacterial population does not increase significantly because the cells are adapting to their new environment, absorbing nutrients, and synthesizing the specific enzymes and proteins needed for cell division. The birth rate does not equal the death rate until the stationary phase.

A is correct: 1 mark for identifying the correct description of the lag phase.

Active immunity is defence against a pathogen by antibody production inside the body, which leads to the creation of memory cells and long-term protection. Passive immunity is the temporary defence against a pathogen by antibodies acquired from another source (e.g., across the placenta or via breast milk), which does not lead to the production of memory cells.

A is correct: 1 mark for distinguishing that active immunity produces memory cells while passive immunity does not.

Incisors are chisel-shaped teeth located at the front of the mouth, adapted for biting and cutting food. Canines are pointed and used for tearing, while premolars and molars have broad, flat surfaces with ridges to crush and grind food.

A is correct: 1 mark for the correct combination of tooth type, function, and structural adaptation.

In early spring, new leaves and buds cannot yet photosynthesize to meet their own metabolic needs, so they act as sinks. The food energy (stored as starch) in the roots or tubers is converted into soluble sucrose and transported upwards through the phloem to support this new growth, making the storage organs in the roots the source.

A is correct: 1 mark for identifying the root storage organs as the source and the growing buds/leaves as the sink in early spring.

When the left ventricle contracts, its pressure rises. As soon as the ventricular pressure exceeds the pressure in the left atrium, the bicuspid (atrioventricular) valve is pushed closed to prevent the backflow of blood into the atrium. However, because this pressure is still lower than the pressure in the aorta, the aortic semilunar valve is not yet pushed open and remains closed. Therefore, both valves are closed.

D is correct: 1 mark for determining that both the bicuspid valve and the aortic semilunar valve are closed.

(a) Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. (b) Similarity: Both processes break down glucose without oxygen and release a relatively small amount of energy compared to aerobic respiration. Differences: In yeast, anaerobic respiration produces ethanol and carbon dioxide (alcohol fermentation). In human muscle cells, anaerobic respiration produces lactic acid only (lactic acid fermentation) and no carbon dioxide. (c)(i) Boiling the glucose solution expels any dissolved oxygen, ensuring anaerobic conditions are maintained. It also sterilizes the solution, killing any contaminating microorganisms. Cooling is essential to prevent denaturing the enzymes in the yeast cells or killing the yeast cells when they are added. (c)(ii) As temperature increases from \(20\ ^\circ\text{C}\) to the optimum temperature (around \(35\text{--}40\ ^\circ\text{C}\)), the kinetic energy of yeast enzymes and substrate molecules increases, leading to more frequent successful collisions and a higher rate of carbon dioxide production. Above the optimum temperature up to \(70\ ^\circ\text{C}\), high kinetic energy disrupts the chemical bonds holding the enzyme's tertiary structure, denaturing the enzymes. The active site changes shape, so substrate molecules can no longer bind, causing the rate of carbon dioxide production to decrease rapidly to zero.

(a) Max 2 marks: chemical reactions in cells that break down nutrient molecules to release energy [1]; without using oxygen [1]. (b) Max 4 marks: yeast produces ethanol [1]; yeast produces carbon dioxide [1]; muscle cells produce lactic acid only / no carbon dioxide [1]; both release relatively small amounts of energy / both do not use oxygen [1]. (c)(i) Max 3 marks: boiling expels/removes dissolved oxygen [1]; boiling sterilizes/kills other microbes [1]; cooling prevents denaturation of yeast enzymes / killing yeast [1]. (c)(ii) Max 4 marks: increasing temperature to optimum increases kinetic energy of molecules [1]; leads to more frequent successful collisions [1]; above optimum / up to \(70\ ^\circ\text{C}\), enzymes denature [1]; active site shape changes and substrate can no longer bind / rate drops to zero [1].

(a)(i) Monocotyledonous plants have leaves with parallel venation and floral parts in multiples of three. Dicotyledonous plants have leaves with net-like/reticulate venation and floral parts in multiples of four or five. (a)(ii) Vertebrates possess a backbone or vertebral column, which invertebrates lack. (b) DNA base sequences represent the direct genetic code of organisms. Species with more closely related evolutionary backgrounds share a more recent common ancestor and have more similar DNA base sequences. Relying solely on morphology can be misleading because unrelated species may evolve similar physical characteristics due to adapting to similar environments (convergent evolution). (c)(i) Specimen A is an Insect (Insecta). Specimen B is an Arachnid (Arachnida). Specimen C is a Myriapod (Myriapoda). Specimen D is a Crustacean (Crustacea). (c)(ii) Two features shared by all arthropods are jointed legs (appendages) and a hard exoskeleton made of chitin.

(a)(i) Max 2 marks: Monocots have parallel veins AND dicots have net-like/reticulate veins [1]; monocots have floral parts in multiples of 3 AND dicots have floral parts in multiples of 4 or 5 [1]. (a)(ii) 1 mark: presence of backbone / vertebral column / spine [1]. (b) Max 4 marks: DNA base sequences show direct genetic relationships [1]; closely related species have more similar base sequences [1]; indicates a more recent common ancestor [1]; physical features can be misleading due to convergent evolution / environmental adaptation [1]. (c)(i) 4 marks: Specimen A: Insect / Insecta [1]; Specimen B: Arachnid / Arachnida [1]; Specimen C: Myriapod / Myriapoda [1]; Specimen D: Crustacean / Crustacea [1]. (c)(ii) Max 2 marks: jointed legs / jointed limbs [1]; chitinous exoskeleton [1]; segmented body [1].

(a) The enzyme acts as the lock and the substrate acts as the key. The enzyme has a specific 3D shape containing an active site. The substrate has a shape that is complementary to the active site. The substrate binds to the active site, forming an temporary enzyme-substrate complex where the reaction occurs. Products are formed and released, leaving the enzyme unchanged and ready to bind with another substrate. (b)(i) Buffer solutions maintain a constant pH. This is important because changes in pH can alter the charge and 3D shape of the enzyme's active site, potentially denaturing it. (b)(ii) Samples of the reaction mixture are taken at regular intervals and added to iodine solution. Starch turns iodine from yellow-brown to blue-black. Digestion is complete when iodine remains yellow-brown. (c) At low temperatures, enzymes and substrate molecules have low kinetic energy and move slowly. This results in fewer successful collisions per unit time, forming fewer enzyme-substrate complexes and slowing down the rate of reaction. (d) A catalyst is a substance that increases the rate of a chemical reaction and is not changed or used up in the reaction. Enzymes are biological catalysts because they are proteins made by living organisms that speed up metabolic reactions.

(a) Max 4 marks: enzyme active site has a specific shape [1]; substrate has a complementary shape to the active site [1]; substrate binds to active site to form enzyme-substrate complex [1]; product is released leaving enzyme unchanged [1]. (b)(i) Max 2 marks: buffers maintain/control pH [1]; prevent changes in pH which could denature the enzyme / affect active site [1]. (b)(ii) Max 2 marks: add iodine solution [1]; blue-black color disappears / iodine remains orange/yellow-brown when starch is gone [1]. (c) Max 3 marks: low kinetic energy of molecules [1]; slower movement leading to fewer collisions [1]; fewer enzyme-substrate complexes formed per unit time [1] (Reject: 'enzymes are denatured' at low temperature). (d) Max 2 marks: catalyst speeds up reaction without being consumed/changed [1]; enzymes are proteins produced by living organisms to speed up metabolic reactions [1].

(a)(i) Mitochondria are the site of aerobic respiration, which releases energy (ATP) for cellular processes. (a)(ii) Ribosomes are the site of protein synthesis. (b)(i) The permanent vacuole contains cell sap (a concentrated solution of solutes), which draws water into the cell by osmosis. This causes the vacuole and cytoplasm to expand and push against the rigid cell wall, creating turgor pressure. The inelastic cell wall resists this expansion, maintaining cell turgidity which keeps the plant tissues firm and upright. (b)(ii) Root hair cells are located underground where there is no light. Since chloroplasts contain chlorophyll to absorb light for photosynthesis, they are not needed in cells that receive no light. (c) The sperm cell is adapted by having: a flagellum (tail) that beats to enable movement/swimming toward the egg; many mitochondria in the middle piece to release energy for swimming; an acrosome in the head containing digestive enzymes to penetrate the jelly coat of the egg; a haploid nucleus containing \(23\) chromosomes to restore the diploid number upon fertilization; and a streamlined shape to reduce drag.

(a)(i) Max 2 marks: site of aerobic respiration [1]; releases energy / ATP [1] (Reject: 'produces/creates energy'). (a)(ii) 1 mark: site of protein synthesis [1]. (b)(i) Max 3 marks: cell sap in vacuole draws in water by osmosis [1]; turgor pressure generated as cytoplasm pushes against cell wall [1]; cell wall is rigid/inelastic to resist bursting / maintain turgor [1]; keeps plant firm/upright [1]. (b)(ii) Max 2 marks: root hair cells are underground/in dark [1]; no light available for photosynthesis [1]; chloroplasts/chlorophyll would be a waste of energy to produce [1]. (c) Max 5 marks: flagellum/tail for swimming [1]; mitochondria in mid-section release energy [1]; acrosome contains enzymes to digest egg jelly coat [1]; haploid nucleus carries half the chromosomes [1]; streamlined shape reduces drag/resistance [1].

(a) A population is a group of organisms of one species, living in the same area at the same time. (b)(i) The four phases are: 1. Lag phase, 2. Exponential (log) phase, 3. Stationary phase, 4. Death (decline) phase. (b)(ii) In the stationary phase, the birth rate equals the death rate. This is because resources such as nutrients and oxygen become limited due to consumption. Furthermore, toxic waste products accumulate to levels that inhibit growth or kill some bacteria, and physical space becomes limited. (c) Over the past two centuries, improvements in medical care (such as vaccines and antibiotics) have dramatically reduced infant mortality and increased life expectancy. Better sanitation and clean water supplies have minimized the transmission of waterborne pathogens. Advanced agriculture has increased food production and security. These advancements decreased death rates while birth rates remained relatively high, preventing the population from reaching a stationary phase and causing an exponential phase of human population growth.

(a) Max 2 marks: group of organisms of one species [1]; living in same area at same time [1]. (b)(i) 4 marks: Lag phase [1]; Exponential / log phase [1]; Stationary phase [1]; Death / decline phase [1]. (b)(ii) Max 3 marks: birth/reproduction rate equals death/mortality rate [1]; nutrients / food / oxygen become limiting [1]; accumulation of toxic metabolic wastes [1]; space becomes limited [1]. (c) Max 4 marks: medical care reduced death rates / increased lifespan [1]; sanitation/clean water reduced pathogen spread [1]; food production prevented starvation / increased population limit [1]; resulted in exponential/rapid growth stage [1].

(a) A pathogen is a disease-causing organism. (b)(i) Skin acts as a continuous physical barrier of dead cells that prevents entry. Hairs in the nose trap dust particles and pathogens in inhaled air. (b)(ii) Hydrochloric acid in the stomach destroys pathogens ingested with food or drink. Mucus traps pathogens and contains enzymes like lysozyme, which is also found in tears and saliva to destroy bacterial cell walls. (c)(i) A vaccine contains a harmless, weakened, or dead form of a pathogen or its antigens. Once injected, lymphocytes recognize these foreign antigens and are activated. They undergo mitosis to produce a clone of identical cells, some of which secrete specific antibodies to destroy the antigen. Importantly, some lymphocytes differentiate into memory cells that persist in the blood for a long time. If the actual pathogen enters the body in the future, memory cells recognize it immediately and rapidly produce large quantities of specific antibodies, neutralizing the pathogen before illness occurs. (c)(ii) Passive immunity involves receiving pre-made antibodies from an external source (maternal breast milk). Because the infant's own lymphocytes are not activated by an antigen, no memory cells are produced. Over time, these maternal antibodies are naturally broken down and cleared from the body, leaving the infant without long-term protection.

(a) 1 mark: disease-causing organism [1]. (b)(i) Max 2 marks: skin is physical barrier [1]; nose hairs trap dust/microbes [1]. (b)(ii) Max 2 marks: stomach acid kills ingested bacteria [1]; lysozyme / enzymes in tears/saliva/mucus destroy bacterial walls [1]. (c)(i) Max 5 marks: vaccine contains harmless/dead/weakened pathogen or antigen [1]; antigens trigger response by lymphocytes [1]; lymphocytes divide/multiply [1]; antibodies are produced [1]; memory cells are produced and persist [1]; secondary response is rapid and produces more antibodies [1]. (c)(ii) Max 3.33 marks: passive immunity is transfer of pre-made antibodies [1]; no antigen exposure occurs, so host lymphocytes are not activated [1]; no memory cells are produced by the baby [1]; antibodies are foreign proteins and are broken down over time [1].

(a) (i) The independent variable is the factor changed by the experimenter, which is the glucose concentration (%). The dependent variable is the factor measured, which is the volume of gas collected (cm\(^3\)).
(a) (ii) Liquid paraffin is less dense than water, so it floats on the surface, creating an airtight barrier that prevents oxygen diffusion into the yeast mixture, maintaining anaerobic conditions.
(a) (iii) To find the mean: sum the three values and divide by 3. \(24.5 + 26.0 + 23.0 = 73.5\). \(73.5 / 3 = 24.5\) cm\(^3\).
(b) A control is used to prove that the yeast respiration is causing the gas production. By using boiled yeast, enzymes are denatured and no respiration occurs. Alternatively, removing the glucose (carbon source) prevents respiration.
(c) Respiration is temperature-dependent (enzyme-controlled), so temperature must be kept constant using a water bath. pH also affects enzymes and must be stabilized using buffer solutions.
(d) Leakage is a common error in gas collection apparatus. Sealing joints with petroleum jelly is a standard solution in practical work.

Total: 13.33 Marks
- (a)(i) [2 marks] 1 mark for identifying glucose concentration as the independent variable, and 1 mark for identifying volume of gas (or carbon dioxide) collected as the dependent variable.
- (a)(ii) [2 marks] 1 mark for stating that it prevents oxygen entering, 1 mark for stating this ensures anaerobic conditions/respiration.
- (a)(iii) [2 marks] 1 mark for showing correct working (sum divided by 3), 1 mark for correct calculation of 24.5 cm\(^3\).
- (b) [2 marks] 1 mark for stating a valid modification (using boiled/dead yeast OR using water in place of glucose), 1 mark for keeping all other factors identical.
- (c) [4 marks] 1 mark for identifying temperature + 1 mark for using a water bath; 1 mark for identifying pH + 1 mark for using a buffer solution.
- (d) [1.33 marks] 0.67 marks for identifying a valid error (e.g., gas leaks / temperature fluctuations), 0.66 marks for a corresponding, valid improvement.

(a) Potato cylinders act as the source of the enzyme. Variation in cylinder size would change the surface area (affecting the rate of enzyme release/reaction) and total enzyme quantity, making the test unfair.
(b) The trend is typical of pH-enzyme activity curves. It shows a rise to a peak (optimum) at pH 6, followed by a decline as the enzyme denatures at pH values above the optimum.
(c) The peak height of foam (48 mm) directly correlates with the highest rate of reaction, which occurs at pH 6.
(d) Temperature affects enzyme kinetic energy and must be kept constant. Hydrogen peroxide is the substrate; its concentration determines reaction rate and must be controlled.
(e) To find a more precise optimum, the interval between tested pH values must be reduced (narrower increments) within the range where the peak occurred (between pH 5 and 7).

Total: 13.33 Marks
- (a) [2 marks] 1 mark for ensuring equal surface area / volume of potato tissue, 1 mark for maintaining a constant enzyme/catalase concentration.
- (b) [3 marks] 1 mark for stating that activity increases as pH increases up to pH 6, 1 mark for identifying the peak at pH 6, 1 mark for stating that activity decreases above pH 6.
- (c) [2 marks] 1 mark for identifying pH 6 as the optimum, 1 mark for linking this choice to the maximum foam height / highest rate of oxygen release.
- (d) [4 marks] 1 mark for identifying temperature + 1 mark for control method (water bath); 1 mark for identifying hydrogen peroxide concentration/volume + 1 mark for control method (measuring with a precise syringe/using same stock).
- (e) [2.33 marks] 1.33 marks for suggesting a narrower range of pH intervals (e.g., intervals of 0.2 or 0.5 pH units), 1.00 mark for specifying testing between pH 5 and pH 7.

(a) Magnification is calculated using the formula \(M = \frac{I}{A}\). Sustituting the given values: \(112.5 \text{ mm} / 45 \text{ mm} = 2.5\). Magnification is expressed with a '\(\times\)' symbol in front, so \(\times 2.5\).
(b) Dicotyledonous leaves typically display reticulate (net-like) venation patterns, in contrast to monocotyledonous leaves which show parallel veins. They also possess broad laminas and petioles.
(c) A dichotomous key must contain binary choices at each level. Step 1 splits the organisms by leg count (separating Species S). Step 2 splits the remaining group by presence/absence of wings (separating Species R). Step 3 splits the final two species (P and Q) by antennae length.
(d) Leaf litter contains soil pathogens and potentially harmful biting or stinging organisms. Wearing protective gloves and washing hands are appropriate safety measures.

Total: 13.33 Marks
- (a) [3 marks] 1 mark for stating the correct formula, 1 mark for correct substitution (112.5 / 45), 1 mark for correct calculation of 2.5 (accept x2.5 / magnification of 2.5).
- (b) [2 marks] 1 mark for reticulate/net-like venation, 1 mark for broad leaf blade/petiole.
- (c) [6.33 marks]
- 2.00 marks for step 1: correctly separating Species S (4 pairs of legs) from others (3 pairs of legs).
- 2.00 marks for step 2: correctly separating Species R (wings absent) from P and Q (wings present).
- 2.33 marks for step 3: correctly separating Species P and Q using antennae length.
- (d) [2 marks] 1 mark for identifying a valid hazard (stinging insects, sharp twigs, pathogens), 1 mark for a corresponding safety precaution (wearing gloves, using forceps/pooter, washing hands).