Secondary 4 Combined Science Biology Preliminary Examination Paper 3

TuitionGoWhere Practice Paper - Combined Science Biology Secondary 4

PRELIMINARY EXAMINATION — ANSWER KEY AND MARKING SCHEME

TuitionGoWhere Secondary School (AI)

Subject: Combined Science Biology (5087/5088)
Level: Secondary 4
Paper: Biology Paper 2 (Structured and Free Response)
Version: 3 of 5
Total Marks: 65

SECTION A: STRUCTURED QUESTIONS (40 marks)

Question 1: Cell Structure and Function (6 marks)

(a) [2 marks]

• Cell P: Animal cell [1]
• Cell Q: Plant cell [1]

(b) [2 marks]

• Structural feature: Cell wall OR Large central vacuole OR Chloroplasts [1]
• Function (accept any one matching the feature stated):
  • Cell wall: Provides structural support and protection / prevents cell from bursting
  • Large central vacuole: Stores water and solutes / maintains turgor pressure
  • Chloroplasts: Site of photosynthesis / contains chlorophyll for light absorption [1]

(c) [2 marks]

• Cell P (animal cell) requires more energy/ATP for metabolic activities [1]
• Mitochondria are the site of aerobic respiration where ATP is produced [1]
• (Accept: Animal cells are more metabolically active than plant cells which also obtain energy from chloroplasts)

Question 2: Movement of Substances (7 marks)

(a) [2 marks]

• Working: Percentage change = (Change in mass ÷ Initial mass) × 100 [1]
• = (0.2 ÷ 5.2) × 100 = 3.85% (accept 3.8% or 3.9%) [1]
• Award [1] for correct formula, [1] for correct answer with units

(b) [3 marks]

• 0.0 mol/dm³ sucrose solution is distilled water / has higher water potential than potato cells [1]
• Water moves from region of higher water potential (solution) to region of lower water potential (potato cells) [1]
• By osmosis / down the water potential gradient [1]
• Therefore, potato cells gain water and mass increases

(c) [2 marks]

• The potato strip would lose mass / decrease in mass [1]
• Because the 1.0 mol/dm³ sucrose solution has lower water potential than the potato cells, so water moves out of the potato cells by osmosis [1]

Question 3: Organelles and Cell Specialisation (5 marks)

(a) [2 marks]

• Different cell types have different numbers of mitochondria [1]
• Cardiac muscle cells have the most mitochondria (5000), followed by liver cells (1500), and skin cells have the fewest (200) [1]
• (Accept: Cells with higher energy demands have more mitochondria)

(b) [3 marks]

• Cardiac muscle cells contract continuously/rhythmically throughout life to pump blood [1]
• This requires a large amount of energy/ATP [1]
• Mitochondria are the site of aerobic respiration where ATP is produced, so more mitochondria are needed to meet the high energy demand [1]

Question 4: Gas Exchange and Transport (8 marks)

(a) [3 marks]

• A: Alveolus / Alveoli [1]
• B: Pulmonary vein [1]
• C: Aorta [1]

(b) [1 mark]

• Diffusion [1]

(c) [4 marks] Award marks for correct sequence and named structures:

1. Oxygen diffuses from alveolus into pulmonary capillary [1]
2. Oxygen binds to haemoglobin in red blood cells [1]
3. Blood travels via pulmonary vein → left atrium → left ventricle → aorta [1]
4. Oxygen diffuses from systemic capillary into tissue fluid, then into muscle cell [1]

• (Accept any four correct steps with named structures)
• Deduct [1] if structures are not named
• Deduct [1] if diffusion is confused with active transport

Question 5: Enzymes and Digestion (6 marks)

(a) [3 marks]

• Bile salts emulsify fats / break large fat droplets into smaller fat droplets [1]
• This increases the surface area of fats [1]
• For more efficient digestion by lipase enzyme / increases rate of fat digestion [1]

(b) [2 marks]

• Without bile salts, fats would remain as large droplets with smaller surface area [1]
• Lipase would have less surface area to act on, so rate of fat digestion would be slower [1]

(c) [1 mark]

• Any one of: Deamination of excess amino acids / Detoxification of harmful substances / Regulation of blood glucose concentration / Storage of glycogen / Production of plasma proteins / Breakdown of haemoglobin
• Accept any valid function of the liver

Question 6: Active Transport (4 marks)

(a) [1 mark]

• Active transport [1]

(b) [3 marks]

• Active transport moves substances against the concentration gradient / from low to high concentration [1]
• This requires energy in the form of ATP [1]
• Carrier proteins in the cell membrane use ATP to change shape and move glucose molecules across the membrane [1]

Question 7: Cell Identification and Function (4 marks)

(a) [1 mark]

• Red blood cell / Erythrocyte [1]

(b) [2 marks] Award [1] for each correct structural feature:

• Biconcave disc shape → increases surface area for faster diffusion of oxygen
• No nucleus → more space for haemoglobin to carry oxygen
• Flexible/elastic membrane → can squeeze through narrow capillaries
• Small size → allows close contact with capillary walls for efficient gas exchange (Accept any two with correct functional explanation)

(c) [1 mark]

• Red blood cells do not contain mitochondria because they transport oxygen rather than use it for aerobic respiration / they rely on anaerobic respiration to avoid using the oxygen they carry [1]

SECTION B: DATA-BASED QUESTIONS (15 marks)

Question 8: Osmosis Investigation (8 marks)

(a) [4 marks] Graph plotting marks:

• Correct axes with labels and units: y-axis "Change in mass (g)", x-axis "Salt concentration (%)" [1]
• Appropriate scales on both axes [1]
• All five points plotted correctly [1]
• Line of best fit drawn (should be a straight line or smooth curve passing through or near all points) [1]

(b) [1 mark]

• 4% salt concentration (where change in mass = 0 g) [1]

(c) [3 marks]

• The 8% salt solution has lower water potential than the carrot cells [1]
• Water moves out of the carrot cells into the salt solution by osmosis / down the water potential gradient [1]
• Therefore, the carrot cylinder loses mass [1]

Question 9: Cell Structure Comparison (7 marks)

(a) [4 marks]

(Accept any valid function and matching structural adaptation)

(b) [2 marks]

• Root hair cells are located underground in the soil [1]
• They do not receive light, so chloroplasts are not needed for photosynthesis [1]

(c) [1 mark]

• Mitochondria produce ATP through aerobic respiration to provide energy for the sperm's movement/swimming towards the egg [1]

SECTION C: FREE RESPONSE QUESTIONS (10 marks)

Question 10: Transport in Humans (10 marks)

(a) [4 marks] Award marks for structure-function links:

• Thick, muscular wall → withstands high blood pressure from heart pumping [1]
• Elastic fibres in wall → allow artery to stretch and recoil, maintaining blood pressure [1]
• Narrow lumen → maintains high pressure for efficient blood flow [1]
• Smooth endothelial lining → reduces friction for smooth blood flow [1] (Accept any four valid structure-function pairs)

(b) [3 marks]

• Capillary walls are one cell thick / composed of a single layer of endothelial cells → short diffusion distance [1]
• Capillaries have a narrow lumen → red blood cells pass in single file, maximising contact with capillary wall [1]
• Capillaries form extensive networks / are highly branched → large total surface area for exchange [1]

(c) [3 marks]

• Anaemia results in fewer red blood cells / less haemoglobin [1]
• Less oxygen can be transported from lungs to body cells [1]
• During exercise, muscle cells require more oxygen for aerobic respiration to produce ATP; insufficient oxygen leads to fatigue and breathlessness [1]

Question 11: Enzymes and Metabolism (10 marks)

(a) [4 marks]

• The lock-and-key hypothesis states that the active site of an enzyme has a specific shape [1]
• This shape is complementary to the shape of the substrate molecule [1]
• The substrate fits into the active site like a key fits into a lock, forming an enzyme-substrate complex [1]
• The reaction occurs at the active site, and products are released; the enzyme remains unchanged and can be reused [1]

(b) [3 marks]

• As temperature increases, the rate of enzyme-catalysed reaction initially increases [1]
• This is because enzyme and substrate molecules have more kinetic energy, increasing the frequency of effective collisions [1]
• However, above the optimum temperature, the enzyme is denatured: the active site loses its specific shape, and the substrate can no longer bind, so the reaction rate decreases sharply [1]

(c) [3 marks]

• At pH 7 (neutral), amylase is at its optimum pH, so the reaction occurs rapidly [1]
• At pH 2 (acidic), amylase is denatured because the pH is far from its optimum [1]
• The active site of amylase loses its specific shape, so starch can no longer bind, and no reaction occurs [1]

END OF ANSWER KEY

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PRELIMINARY EXAMINATION — Version 3 of 5