Mark Scheme

Section A — Structured Questions

Question 1

(a) • A: epidermis / upper epidermis [1] • C: xylem (vessel/tissue) [1]

Accept: epidermis (tissue), epidermal (tissue) for A Reject: skin

[2 marks]

(b) Any three from: • cells are cylindrical / column-shaped / elongated [1] • cells are tightly packed (with few air spaces) [1] • positioned at top / upper surface of leaf [1] • contain many / large numbers of chloroplasts [1] • chloroplasts positioned near upper surface to absorb/receive maximum light [1]

Award maximum [3 marks]

(c) (i) • transpiration [1]

Accept: evaporation Reject: respiration, perspiration

(ii) Any three from: • guard cells take up water by osmosis [1] • guard cells become turgid [1] • (inner) cell walls are thicker / less elastic than outer cell walls / uneven thickening [1] • this causes guard cells to curve / change shape [1] • opening a pore / stoma / gap [1] • loss of water makes guard cells flaccid / less turgid [1] • stoma closes [1]

Award maximum [3 marks]

(d) (i) • 175 + 325 = 500 (total) [1] • (175 ÷ 500) × 100 = 35(%) [1]

Accept: 35.0

[2 marks]

(ii) Any two from: • (lower surface of) water lily leaf is in contact with water / submerged / underwater [1] • stomata on lower surface would be in water / blocked by water / not in air [1] • cannot function / cannot exchange gases / cannot transpire / would cause waterlogging [1] • upper surface is exposed to air / in contact with air [1]

Award maximum [2 marks]

[Total: 13 marks]

Question 2

(a) Any two from: • biological catalyst [1] • protein (molecule) [1] • speeds up / increases rate of (metabolic / biochemical / chemical) reaction [1] • without being used up / changed [1]

Award maximum [2 marks]

Accept: produced by living organisms / cells

(b) (i) • 40°C [1]

(ii) Any two from: • (more kinetic) energy [1] • enzyme and substrate molecules move faster / more (molecular) movement [1] • more frequent collisions / successful collisions / collisions between enzyme and substrate [1] • more enzyme-substrate complexes formed [1]

Award maximum [2 marks]

(iii) Any three from: • enzyme is denatured [1] • (high temperature causes) bonds in enzyme / tertiary structure to break [1] • active site changes shape / structure [1] • substrate no longer fits active site / cannot bind to active site [1] • fewer enzyme-substrate complexes formed [1]

Award maximum [3 marks]

Accept: loses its shape for changes shape

(c) (i) • pH 7 [1]

Accept: neutral pH

(ii) Any two from: • enzyme denatured / active site changed shape [1] • (very acidic/low pH causes) bonds to break / tertiary structure altered [1] • substrate cannot fit / bind to active site [1] • enzyme-substrate complexes cannot form [1]

Award maximum [2 marks]

(iii) Any one from: • volume of starch solution [1] • volume of amylase solution [1] • temperature [1] • concentration of starch solution [1] • concentration of enzyme solution [1]

Award [1 mark]

[Total: 12 marks]

Question 3

(a) • protein → amino acids [1] • starch → glucose / maltose / (reducing) sugar [1] • lipid → fatty acids and glycerol [1]

Accept: fats/oils for lipids Reject: protein → peptides only (not end product)

[3 marks]

(b) (i) • X is lacteal [1] • absorbs / transports fatty acids and glycerol / lipids / fats [1]

Accept: products of lipid/fat digestion Reject: absorbs lipids (without reference to fatty acids and glycerol)

[2 marks]

(ii) Any three from: • large network of capillaries / many capillaries [1] • large surface area (for absorption) [1] • thin wall / one cell thick / short diffusion distance [1] • good blood supply / blood flow maintains concentration gradient [1] • (products absorbed) by diffusion / active transport [1]

Award maximum [3 marks]

(c) (i) • (synthesis of) protein / enzymes / antibodies / hormones [1]

Accept: growth / repair of cells/tissues, making muscle

[1 mark]

(ii) • removal of amino / NH₂ / nitrogen-containing group (from amino acid) [1] • (amino group) converted to ammonia / urea [1]

Accept: amino group removed

[2 marks]

[Total: 11 marks]

Question 4

(a) (i) • F: pulmonary artery [1] • H: aorta [1]

Must be fully correct for mark

(ii) • D / left ventricle [1] • needs to pump blood further / around (whole) body / at higher pressure / to overcome greater resistance [1]

Accept: right ventricle only pumps to lungs / shorter distance

[2 marks]

(b) Any four from: • blood enters right atrium from vena cava [1] • (blood flows) through valve into right ventricle [1] • right ventricle contracts [1] • (blood flows) through pulmonary artery to lungs [1] • (blood returns from lungs via) pulmonary vein to left atrium [1] • (blood flows) through valve into left ventricle [1] • left ventricle contracts [1] • (blood flows) through aorta to body [1]

Award maximum [4 marks]

Accept logical sequence with correct chambers and vessels

(c) (i) • blood in pulmonary vein has just collected oxygen from lungs / is oxygenated [1] • this blood flows directly into left atrium then left ventricle then aorta / no oxygen used between pulmonary vein and aorta [1]

[2 marks]

(ii) • 19.0 – 4.5 = 14.5 (% decrease) [1] • (14.5 ÷ 19.0) × 100 = 76.3(%) [1]

Accept: 76, 76.32 Accept correct answer with no working for full marks

[2 marks]

[Total: 12 marks]

Section B — Extended Response

Question 5

(a) • (micro)organism that causes disease [1] • examples: bacterium, virus, fungus, protoctist [1]

Accept: one valid example for second mark

[2 marks]

(b) Mark using levels of response:

Level 3 (7–8 marks): • Detailed and coherent explanation covering all three bullet points • Accurate use of terminology (antigens, antibodies, lymphocytes, phagocytes, memory cells) • Clear understanding of sequence of immune response • Links between detection, response and long-term immunity clearly explained

Level 2 (4–6 marks): • Explanation covers at least two bullet points adequately • Some correct terminology used • Understanding of immune response shown but may lack detail or have minor errors • Some links between stages of immunity made

Level 1 (1–3 marks): • Limited explanation, may only address one bullet point • Limited use of correct terminology • Basic understanding shown • Little or no linking of ideas

0 marks: No creditworthy material

Indicative content:

Detection: • pathogens have antigens on surface • antigens are recognised as foreign / non-self • (specific) white blood cells / lymphocytes detect antigens

Response: • phagocytes / phagocytic white blood cells engulf pathogens / phagocytosis • lymphocytes produce antibodies • antibodies are complementary to / specific to antigens • antibodies bind to antigens • antibodies destroy pathogens / cause pathogens to clump together • lymphocytes can divide to produce many cells quickly

Long-term protection: • some lymphocytes become memory cells • memory cells remain in blood (for many years) • if same pathogen enters body again, memory cells recognise antigens • memory cells produce antibodies more rapidly / in greater numbers • secondary response is faster • provides immunity

[8 marks]

(c) Mark using levels of response:

Level 3 (5–6 marks): • Discussion of multiple control methods with reference to specific diseases from Table 5.1 • Evaluation of effectiveness with reasoning • Considers advantages and disadvantages / limitations • Well-structured response with clear judgements

Level 2 (3–4 marks): • Discussion of some control methods with some reference to examples • Some evaluation present but may be limited • Some structure to response

Level 1 (1–2 marks): • Basic description of control method(s) • Little or no evaluation • May not refer to specific diseases

0 marks: No creditworthy material

Indicative content:

Methods and evaluation: • improving sanitation / clean water supply controls cholera effectively (targets transmission route) • insecticides / mosquito nets / draining standing water controls malaria by reducing vector population • vaccination provides long-term immunity / prevents influenza infection before it occurs • very effective but needs regular updating due to virus mutation • personal hygiene / avoiding contact with infected surfaces prevents athlete's foot • relatively simple but relies on individual behaviour • isolation / quarantine of infected individuals prevents spread (particularly airborne diseases like influenza) • antibiotics can treat bacterial diseases like cholera but not viral/fungal/protoctist diseases • public health education increases awareness and compliance with control measures • cost and accessibility may limit effectiveness in less developed countries • vector control for malaria requires sustained effort and resources • cultural / social factors may affect implementation

[6 marks]

[Total: 16 marks]

Question 6

(a) • carbon dioxide + water → glucose + oxygen [1] • correct formula: CO₂ + H₂O → C₆H₁₂O₆ + O₂ is also acceptable [1]

Accept: (in presence of) light (energy) Accept: named sugar for glucose Reject: incorrect formulae

[2 marks]

(b) Mark using levels of response:

Level 3 (5–6 marks): • Clear and detailed method described • Control variables and fair testing explained • Expected results stated with justification • Logical sequence of steps

Level 2 (3–4 marks): • Method described with some detail • Some reference to fair testing or control • Expected results stated • Sequence may lack some detail

Level 1 (1–2 marks): • Basic method outlined • Limited reference to fair testing • Results may be vague or incomplete

0 marks: No creditworthy material

Indicative content:

Method: • use plant with broad leaves / use variegated leaf / geranium • cover part of leaf with aluminium foil / black paper / keep plant in dark for 24-48 hours • place plant in bright light for several hours • remove leaf and test for starch • decolourise leaf by boiling in alcohol / ethanol • add iodine solution

Fair test: • use same plant / same leaf • same length of time in light • same intensity of light • control leaf / uncovered section of leaf for comparison

Expected results: • area exposed to light turns blue-black with iodine • covered area / leaf kept in dark remains brown/orange • shows starch is present where light available • starch is product of photosynthesis • no starch where no light • proves light is necessary for photosynthesis

[6 marks]

(c) Mark using levels of response:

Level 3 (7–8 marks): • Comprehensive evaluation covering all three areas • Both beneficial and harmful effects considered • Clear links to increasing CO₂ shown • Scientific understanding demonstrated • Balanced and well-reasoned judgements made • Well-structured response

Level 2 (4–6 marks): • Evaluation covers at least two areas adequately • Some beneficial and harmful effects discussed • Some scientific understanding shown • May be imbalanced or lack detail in places • Some structure present

Level 1 (1–3 marks): • Limited evaluation, may focus on only one area • May only discuss harmful or beneficial effects • Limited scientific understanding • Lacks structure or coherence

0 marks: No creditworthy material

Indicative content:

Global ecosystems – harmful: • climate change / global warming • melting ice caps / rising sea levels • coral bleaching / damage to marine ecosystems • changes in temperature affect species distribution • habitat loss • species extinction • disruption of food chains / food webs • changes in rainfall patterns cause drought or flooding • disruption of migration patterns / breeding cycles

Global ecosystems – beneficial: • some plants may grow better in warmer conditions • expansion of growing regions in some areas • increased plant growth in some ecosystems

Agricultural food production – harmful: • extreme weather events damage crops • droughts reduce crop yields • flooding destroys crops • pests and diseases spread to new areas • changes to growing seasons • reduction in food security

Agricultural food production – beneficial: • longer growing seasons in some regions • increased crop yields in some areas (if water available) • new crops can be grown in previously unsuitable areas • CO₂ fertilisation effect may increase growth

Rate of photosynthesis – beneficial: • CO₂ is a raw material for photosynthesis • increased CO₂ concentration increases rate of photosynthesis (if other factors not limiting) • more glucose produced • faster plant growth

Rate of photosynthesis – limitations: • other limiting factors (temperature, light intensity, water availability) • may prevent increased photosynthesis • nutrients may become limiting • benefits may be offset by negative effects of climate change

[8 marks]

[Total: 16 marks]

Sample Answers with Examiner Commentary

Question 5(b) — Sample Answers

Grade A (high distinction) answer*

When a pathogen enters the body, it has antigens on its surface which are foreign molecules. White blood cells called lymphocytes detect these antigens and recognise them as non-self, meaning they don't belong to the body.

There are two main types of white blood cells involved in fighting infection. Phagocytes carry out phagocytosis, which means they engulf and digest pathogens. Lymphocytes produce antibodies which are proteins that are complementary in shape to the specific antigens on the pathogen. Each antibody only fits one type of antigen, like a lock and key. When antibodies bind to the antigens, they destroy the pathogens or cause them to clump together so phagocytes can destroy them more easily. The lymphocytes can divide rapidly to produce many identical cells that all produce the same antibody.

The immune system provides long-term protection because some lymphocytes become memory cells after the infection is cleared. These memory cells remain in the blood for many years, sometimes for life. If the same pathogen enters the body again, the memory cells recognise its antigens immediately and respond much faster by producing antibodies in larger quantities. This secondary response is so rapid that the person doesn't become ill, which is why they are immune to that disease. This is the principle behind vaccination.

Mark: 8/8

Examiner commentary: This is an exemplary response that demonstrates comprehensive understanding of all three aspects of the immune response. The candidate uses precise scientific terminology throughout (antigens, non-self, phagocytosis, complementary, secondary response) and explains the mechanisms clearly. The response is logically structured, following the sequence of immune response from detection through to long-term immunity, with clear links between stages. The lock and key analogy shows conceptual understanding, and the reference to vaccination demonstrates application of knowledge.

Grade C (pass) answer

Pathogens have antigens which are detected by white blood cells.

White blood cells fight infection in different ways. Some white blood cells engulf pathogens and digest them. Other white blood cells produce antibodies. Antibodies attach to the antigens on the pathogen and destroy them. The antibodies are specific to each pathogen so they only work on one type. When a pathogen is killed, the white blood cells remember it.

This gives long-term protection because if the same pathogen enters the body again, the white blood cells can produce antibodies faster. This means the person won't get ill from the same disease twice. This is called immunity.

Mark: 5/8

Examiner commentary: This response demonstrates sound understanding of the main concepts but lacks the detail and precision required for top marks. The candidate correctly identifies antigens, describes phagocytosis (though without using the term), and explains antibody specificity. However, the answer does not mention lymphocytes by name, does not explain what antigens are, and the description of memory cells is oversimplified ("remember it" rather than explaining memory cells remain in blood). The mechanism of antibody-antigen binding lacks detail (no mention of complementary shape). The response would benefit from more precise terminology and fuller explanations of mechanisms.

Grade E (near miss) answer

White blood cells detect germs in the body because they are foreign.

White blood cells kill the germs by eating them. They also make antibodies which are chemicals that kill the germs. When you get ill, your body makes antibodies and this means you become immune to the disease. The antibodies stay in your blood forever so if you get the same disease again the antibodies kill it before you get ill.

Mark: 2/8

Examiner commentary: This response shows basic awareness of the immune response but contains significant misconceptions and lacks scientific terminology. The candidate uses "germs" rather than pathogens, doesn't mention antigens at all (which is fundamental to detection), and incorrectly suggests antibodies remain in the blood rather than memory cells. The description of phagocytosis as "eating" is too imprecise. The response fails to distinguish between different types of white blood cells or explain the mechanism of antibody action. To improve, the candidate needs to learn key terminology (pathogen, antigen, lymphocyte, memory cell) and understand that memory cells, not antibodies, provide long-term immunity.

Question 6(c) — Sample Answers

Grade A (high distinction) answer*

The increasing atmospheric carbon dioxide has significant impacts on all three areas mentioned, with both beneficial and harmful effects that must be carefully considered.

For global ecosystems, the impacts are predominantly harmful. Rising CO₂ is the main cause of climate change and global warming. This leads to melting polar ice caps and rising sea levels, threatening coastal ecosystems and low-lying habitats. Coral reefs are particularly vulnerable, experiencing bleaching due to increased ocean temperatures and acidification from dissolved CO₂. Many species face extinction as their habitats disappear or shift geographically faster than they can adapt or migrate. Food chains are disrupted as the timing of seasonal events like flowering, breeding and migration become mismatched. However, there is a minor beneficial effect in that some plant communities in cooler regions may experience enhanced growth due to warmer temperatures and increased CO₂ availability.

For agricultural food production, the effects are mixed and regionally variable. The harmful impacts include more frequent extreme weather events such as droughts and floods which destroy crops and reduce yields. Changing rainfall patterns make traditional farming practices unreliable. Pests and diseases are spreading to new regions as temperature zones shift, affecting crops that previously had natural protection. This threatens global food security, particularly in developing nations. However, there are some beneficial effects. In some temperate regions, longer growing seasons and the CO₂ fertilisation effect could increase crop yields if water and nutrients are sufficient. Some areas may become suitable for crops that previously couldn't grow there, potentially increasing agricultural diversity.

For the rate of photosynthesis, the primary effect of increasing CO₂ is beneficial since carbon dioxide is a raw material for photosynthesis. According to the equation, more CO₂ should increase the rate of photosynthesis and therefore plant growth, assuming light intensity and temperature are adequate. This CO₂ fertilisation effect has been demonstrated in controlled environments. However, this benefit is limited by other factors. In many natural environments, photosynthesis becomes limited by temperature, water availability, or nutrient supply before CO₂ concentration becomes limiting. Additionally, the negative impacts of climate change, such as drought stress or extreme heat, may counteract any benefit from increased CO₂.

In conclusion, while increased CO₂ can enhance photosynthesis rates in ideal conditions, the harmful effects on ecosystems and agriculture resulting from climate change are likely to outweigh these benefits in most scenarios.

Mark: 8/8

Examiner commentary: This is an outstanding response that demonstrates sophisticated understanding and evaluation. The candidate addresses all three areas comprehensively, consistently considering both beneficial and harmful effects as required. The answer shows excellent scientific knowledge (coral bleaching, ocean acidification, CO₂ fertilisation effect, limiting factors) and makes clear links to the increasing CO₂ shown in Fig. 6.1. The response is exceptionally well-structured with topic sentences and a concluding synthesis. The evaluation is balanced and nuanced, recognising regional variation and the interplay between factors. The candidate demonstrates the critical thinking skills expected at the highest level by recognising that beneficial effects may be offset by limiting factors or other impacts.

Grade C (pass) answer

Increasing carbon dioxide in the atmosphere has several effects on ecosystems, agriculture and photosynthesis.

For global ecosystems, the main problem is climate change. Rising CO₂ causes global warming which melts ice caps and causes sea levels to rise. This destroys habitats for many animals like polar bears and penguins. Many species will become extinct because they can't survive in the warmer temperatures. There are also more extreme weather events like hurricanes and droughts which damage ecosystems. However, some plants might grow better in warmer weather.

For agriculture, climate change causes problems for farmers. Droughts mean crops don't get enough water and die, reducing food production. Floods also destroy crops. The changing climate means farmers can't predict the weather as well as before, making farming difficult. However, in some countries the warmer weather might mean they can grow crops they couldn't grow before, and the growing season might be longer which could increase food production.

For photosynthesis, more CO₂ is good because it's needed for photosynthesis. The equation for photosynthesis shows that CO₂ is a reactant, so more CO₂ means faster photosynthesis and more glucose produced. This means plants will grow faster. However, photosynthesis also needs light and the right temperature, so if these aren't right then more CO₂ won't help.

Overall, increasing CO₂ has mostly harmful effects even though it can help photosynthesis.

Mark: 5/8

Examiner commentary: This response covers all three required areas and attempts to discuss both beneficial and harmful effects, meeting the basic requirements of the question. The candidate shows reasonable understanding of climate change impacts and the role of CO₂ in photosynthesis. However, the response lacks the depth and scientific precision of a top-level answer. Specific ecosystems and mechanisms are mentioned only briefly (coral reefs, food chains not discussed). The agricultural discussion is somewhat general and doesn't mention specific mechanisms like pest distribution or CO₂ fertilisation effect by name. The photosynthesis section correctly identifies CO₂ as a limiting factor but doesn't develop the evaluation fully. The conclusion is present but brief. To improve, the candidate should include more specific examples, use more precise scientific terminology, and develop the evaluation with more detailed reasoning.

Grade E (near miss) answer

Carbon dioxide increasing in the atmosphere is bad for the environment. It causes global warming which is when the Earth gets hotter. This is bad for animals because their habitats get destroyed. Polar bears are losing their homes because the ice is melting.

For farming, global warming makes it harder to grow crops because of the weather. There are more droughts so crops die. This means less food for people. Global warming also causes floods which destroy the crops.

For photosynthesis, more carbon dioxide is good because plants need carbon dioxide to photosynthesise. This means plants can grow faster when there is more carbon dioxide in the air. Plants make oxygen during photosynthesis which is good for humans and animals to breathe.

In conclusion, increasing carbon dioxide is mainly bad because of global warming but it helps plants grow.

Mark: 2/8

Examiner commentary: This response demonstrates basic awareness that CO₂ increase has consequences but fails to meet the requirements of evaluation expected in an extended response. The candidate mentions environmental and agricultural impacts but only describes harmful effects, not evaluating or considering beneficial aspects as required. The treatment of each area is superficial with very limited scientific content. Key terminology is absent (climate change mentioned, but not ecosystem disruption, food chains, species extinction explained). The agriculture section doesn't distinguish between different types of impact. The photosynthesis section shows a common misconception that more CO₂ automatically means more growth without considering limiting factors. The response also contains irrelevant material (oxygen production for breathing). To reach a passing grade, the candidate must develop their scientific knowledge significantly, learn to structure evaluative responses that consider multiple perspectives, and avoid vague generalisations in favor of specific, developed points with scientific reasoning.