Secondary 4 Combined Science Biology Preliminary Examination Paper 5

TuitionGoWhere Practice Paper - Combined Science Biology Secondary 4

PRELIMINARY EXAMINATION - Version 5: ANSWER KEY & MARKING SCHEME

Total Marks: 65

Section A: Structured Questions (50 marks)

Question 1: Cell Structure and Function (8 marks)

(a)(i) [2 marks]

• A: Nucleus [1]
• D: Mitochondrion [1]

(a)(ii) [1 mark]

• B (Cell membrane): Controls the movement of substances into and out of the cell / selectively permeable barrier / protects the cell contents. [1]
• Accept any one correct function.

(a)(iii) [2 marks]

• Muscle cells require more energy/ATP for contraction [1]
• Mitochondria are the site of aerobic respiration where ATP is produced [1]
• Award [1] for stating energy demand, [1] for linking to mitochondrial function.

(b)(i) [1 mark]

• Red blood cells lack a nucleus and other organelles / red blood cells are specialised for oxygen transport and do not carry out aerobic respiration / red blood cells obtain energy through anaerobic respiration only. [1]

(b)(ii) [2 marks]

• Heart muscle cells contract continuously and require more energy/ATP than skin cells [1]
• Therefore, heart muscle cells have more mitochondria to carry out more aerobic respiration to meet their higher energy demands [1]
• Award [1] for identifying difference in energy demand, [1] for linking to mitochondrial function.

Question 2: Movement of Substances (7 marks)

(a)(i) [2 marks]

• Iodine molecules diffused from the beaker into the dialysis tubing [1]
• Iodine reacted with starch inside the tubing to form a blue-black complex [1]

(a)(ii) [2 marks]

• Process: Diffusion [1]
• Explanation: Iodine molecules moved from a region of higher concentration (in the beaker) to a region of lower concentration (inside the dialysis tubing) down the concentration gradient / the dialysis tubing is permeable to iodine but not starch, so iodine can pass through [1]

(b)(i) [1 mark]

• As temperature increases, the distance diffused by the dye increases / there is a positive correlation between temperature and distance diffused. [1]

(b)(ii) [2 marks]

• At higher temperatures, molecules have more kinetic energy [1]
• Therefore, molecules move faster, increasing the rate of diffusion / dye molecules spread through the agar more quickly [1]
• Award [1] for kinetic energy, [1] for linking to rate of diffusion.

Question 3: Enzymes and Biomolecules (8 marks)

(a)(i) [2 marks]

• Location: Stomach [1]
• Explanation: Enzyme P has an optimum pH of 2, which matches the acidic environment of the stomach (due to hydrochloric acid) [1]

(a)(ii) [2 marks]

• Enzyme Q has an optimum pH of 8 (alkaline) [1]
• At pH 2, the acidic conditions cause the enzyme to denature / the shape of the active site changes, so the substrate can no longer bind / the enzyme loses its catalytic function [1]
• Award [1] for identifying pH mismatch, [1] for explaining denaturation.

(b)(i) [2 marks]

• Reducing sugar (glucose/fructose/maltose) — indicated by the brick-red precipitate with Benedict's test [1]
• Lipids (fats/oils) — indicated by the cloudy white emulsion with the ethanol emulsion test [1]
• Award [1] for each correct biomolecule identified with supporting evidence.

(b)(ii) [1 mark]

• The Biuret test detects proteins, and the sample did not contain proteins / the solution remained blue, indicating the absence of peptide bonds. [1]

(b)(iii) [1 mark]

• Any one correct function of lipids:
  • Long-term energy storage
  • Thermal insulation
  • Protection of vital organs
  • Component of cell membranes (phospholipids)
  • Source of fat-soluble vitamins (A, D, E, K)
• Accept any one valid function.

Question 4: Photosynthesis and Plant Transport (9 marks)

(a) [1 mark]

• Carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll) [1]
• Accept the word equation with correct reactants and products. Conditions (light, chlorophyll) may be written above the arrow.

(b)(i) [1 mark]

• As the distance of the lamp from the plant increases, the number of oxygen bubbles produced per minute decreases / the rate of photosynthesis decreases with increasing distance. [1]

(b)(ii) [2 marks]

• As the distance increases, the light intensity reaching the plant decreases [1]
• Light is required for the light-dependent reactions of photosynthesis / light provides energy for photosynthesis / lower light intensity reduces the rate of photosynthesis, so less oxygen is produced [1]

(b)(iii) [1 mark]

• Any one correct limiting factor:
  • Carbon dioxide concentration
  • Temperature
  • Availability of water
  • Amount of chlorophyll
• Accept any one valid factor.

(c)(i) [3 marks]

• Palisade mesophyll: Contains many chloroplasts / chloroplasts are densely packed [1] because these cells are located near the upper epidermis and receive the most direct light for photosynthesis [1]
• Spongy mesophyll: Contains fewer chloroplasts [0.5] because these cells receive less light / are located deeper in the leaf / the primary function is gas exchange through large air spaces [0.5]
• Award [1] for palisade description, [1] for palisade explanation, [0.5] for spongy description, [0.5] for spongy explanation.

(c)(ii) [1 mark]

• Any one correct function:
  • Allow diffusion of carbon dioxide and oxygen for photosynthesis and respiration
  • Facilitate gas exchange between the leaf cells and the atmosphere
  • Provide a large surface area for gas exchange
• Accept any one valid function.

Question 5: Human Transport System (9 marks)

(a)(i) [2 marks]

• F: Left ventricle [1]
• G: Aorta [1]

(a)(ii) [1 mark]

• The left ventricle pumps oxygenated blood to all parts of the body (except the lungs) via the aorta / the left ventricle has thick muscular walls to generate high pressure to pump blood throughout the systemic circulation. [1]
• Accept any one correct function.

(b)(i) [2 marks]

• The pulmonary artery carries deoxygenated blood from the heart to the lungs (low oxygen concentration) [1]
• In the lungs, gas exchange occurs at the alveoli: oxygen diffuses from the alveoli into the blood, so the pulmonary vein carries oxygenated blood back to the heart (high oxygen concentration) [1]

(b)(ii) [1 mark]

• The aorta receives oxygenated blood directly from the left ventricle, which has just received oxygenated blood from the pulmonary vein via the left atrium / there is no change in oxygen concentration as blood passes through the left side of the heart. [1]

(c) [3 marks]

• Pathway:
  1. Oxygen enters the lungs during inhalation and diffuses across the alveolar epithelium into the blood capillaries [1]
  2. Oxygen binds to haemoglobin in red blood cells and is transported via the pulmonary vein to the left atrium → left ventricle → aorta [1]
  3. Oxygenated blood travels through arteries and arterioles to capillaries in the leg muscle, where oxygen diffuses from the blood into the muscle cells [1]
• Award [1] for each major stage of the pathway. Must name at least three structures (e.g., alveoli, pulmonary vein, left atrium, left ventricle, aorta, capillaries).

Question 6: Respiration and Gas Exchange (9 marks)

(a) [2 marks]

• Any two correct differences:
  1. Aerobic respiration requires oxygen; anaerobic respiration does not require oxygen [1]
  2. Aerobic respiration produces carbon dioxide and water; anaerobic respiration produces lactic acid (in humans) [1]
  3. Aerobic respiration releases more energy/ATP (38 ATP per glucose); anaerobic respiration releases less energy/ATP (2 ATP per glucose) [1]
  4. Aerobic respiration occurs in the mitochondria; anaerobic respiration occurs in the cytoplasm [1]
• Award [1] for each correct difference. Accept any two.

(b)(i) [1 mark]

• Lactic acid [1]

(b)(ii) [2 marks]

• The athlete has built up an oxygen debt during the sprint [1]
• Heavy breathing provides extra oxygen to break down/oxidise the lactic acid accumulated in the muscles / to repay the oxygen debt / to convert lactic acid back to pyruvate or glucose in the liver [1]

(c)(i) [1 mark]

• Diffusion [1]

(c)(ii) [2 marks]

• Any two correct features with explanations:
  1. Thin walls (one cell thick): The alveolar epithelium and capillary endothelium are only one cell thick, providing a short diffusion distance for gases [1]
  2. Large surface area: The alveoli are numerous and highly folded, providing a large surface area for gas exchange [1]
  3. Moist surface: The alveolar surface is moist, allowing gases to dissolve before diffusing [1]
  4. Rich blood supply: The alveoli are surrounded by a dense network of capillaries, maintaining a steep concentration gradient for rapid diffusion [1]
• Award [1] for each correct feature with explanation. Accept any two.

(c)(iii) [1 mark]

• Carbon dioxide diffuses from the blood (higher concentration) into the alveolus (lower concentration) down its concentration gradient / carbon dioxide is a waste product of respiration and must be removed from the body. [1]

Section B: Free Response Question (15 marks)

Question 7: Cells, Transport, and Enzymes

(a) [6 marks]

• Structure of cell membrane (3 marks):
  • The cell membrane is composed of a phospholipid bilayer [1]
  • Phospholipids have hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward [1]
  • Proteins are embedded in the phospholipid bilayer (fluid mosaic model) / the membrane also contains cholesterol and glycoproteins [1]
• Control of movement (3 marks):
  • The cell membrane is selectively permeable / partially permeable [1]
  • Small, non-polar molecules (e.g., oxygen, carbon dioxide) can diffuse directly through the phospholipid bilayer [1]
  • Larger or charged molecules (e.g., glucose, ions) require transport proteins (carrier proteins or channel proteins) to cross the membrane / active transport uses carrier proteins and ATP to move substances against the concentration gradient [1]
• Award marks for accurate description of structure and clear explanation of how structure relates to function.

(b) [4 marks]

• Lock-and-key hypothesis:
  • The active site of an enzyme has a specific three-dimensional shape that is complementary to the shape of its specific substrate [1]
  • The substrate fits into the active site like a key fits into a lock, forming an enzyme-substrate complex [1]
  • This binding lowers the activation energy required for the reaction to occur [1]
  • The reaction takes place, products are formed, and the products are released from the active site; the enzyme remains unchanged and can be reused [1]
• Award [1] for each key point. Accept diagrams if clearly labelled and explained.

(c)(i) [2 marks]

• As temperature increases from 10°C to 40°C, the kinetic energy of enzyme and substrate molecules increases [1]
• This increases the frequency of successful collisions between enzyme and substrate molecules / more enzyme-substrate complexes form per unit time, increasing the rate of reaction [1]

(c)(ii) [2 marks]

• Above 40°C (the optimum temperature), the enzyme begins to denature [1]
• The high temperature breaks the hydrogen bonds and other bonds maintaining the enzyme's tertiary structure / the active site changes shape, so the substrate can no longer bind / the enzyme loses its catalytic function [1]

(c)(iii) [1 mark]

• Any one correct suggestion:
  • Repeat the investigation at each temperature and calculate the mean/average volume of oxygen produced
  • Control other variables such as pH, enzyme concentration, or substrate concentration
  • Use a water bath to maintain a constant temperature throughout each trial
• Accept any one valid suggestion for improving reliability.

Question 8: Photosynthesis, Plant Transport, and Ecology

(a) [6 marks]

• Leaf adaptations for photosynthesis:
  1. Large surface area: The leaf blade is broad and flat to capture maximum light energy [1]
  2. Thin structure: The leaf is thin, allowing carbon dioxide to diffuse quickly to photosynthetic cells and oxygen to diffuse out [1]
  3. Palisade mesophyll cells: These cells are elongated, tightly packed, and contain many chloroplasts; they are located near the upper epidermis to receive the most direct light [1]
  4. Chloroplasts: Contain chlorophyll, which absorbs light energy for photosynthesis [1]
  5. Stomata: Pores on the lower epidermis allow carbon dioxide to enter and oxygen to exit; guard cells regulate stomatal opening and closing [1]
  6. Network of veins (xylem and phloem): Xylem transports water to photosynthetic cells; phloem transports sugars away from the leaf [1]
• Award [1] for each correct adaptation with explanation. Accept any six valid points.

(b) [4 marks]

• Transport of water:
  • Water enters root hair cells by osmosis from the soil [1]
  • Water moves through the root cortex to the xylem vessels [1]
  • Water is transported up the xylem through the stem to the leaves by transpiration pull (cohesion-tension theory) / transpiration creates a negative pressure that pulls water up the xylem [1]
• Transport of mineral salts:
  • Mineral salts are absorbed by root hair cells by active transport (against the concentration gradient, requiring ATP) [0.5]
  • Mineral salts are dissolved in water and transported up the xylem along with water to all parts of the plant [0.5]
• Award [1] for each key point. Must cover both water and mineral transport.

(c)(i) [2 marks]

• Plants absorb carbon dioxide from the atmosphere during photosynthesis [1]
• Carbon dioxide is converted into glucose and other organic compounds, which are stored in the plant biomass (carbon sequestration) / growing large masses of plants removes significant amounts of carbon dioxide from the atmosphere [1]

(c)(ii) [2 marks]

• Any two correct limitations:
  1. Land used for carbon farming may compete with land needed for food production / may lead to deforestation or loss of biodiversity [1]
  2. When plants die and decompose, or are burned, the stored carbon is released back into the atmosphere as carbon dioxide [1]
  3. Carbon farming requires large amounts of water, fertilisers, and other resources, which may have their own environmental impacts [1]
  4. The rate of carbon dioxide absorption may decrease as plants mature or if environmental conditions are unfavourable [1]
• Accept any two valid limitations.

(c)(iii) [1 mark]

• Burning the plants releases the same amount of carbon dioxide that the plants absorbed during photosynthesis / there is no net removal of carbon dioxide from the atmosphere if the plants are burned / the carbon cycle remains balanced (carbon neutral) rather than achieving net carbon sequestration. [1]

END OF ANSWER KEY