Primary 6 PSLE Science Systems Quiz

Primary 6 PSLE Science Quiz - Systems (Answer Key)

Total Marks: 50

Section A: Multiple-Choice Questions (20 marks)

1. Answer: (2) [2 marks]

Explanation: During inhalation, the diaphragm contracts and flattens (moves down), and the external intercostal muscles contract, pulling the ribs up and out. This increases the volume of the thoracic cavity, decreasing pressure so air rushes in. Option (1) describes exhalation. Option (3) has diaphragm moving up when contracted (wrong). Option (4) has diaphragm relaxing but moving down (wrong).

2. Answer: (2) [2 marks]

Explanation: During respiration, glucose is broken down with oxygen to release energy, producing carbon dioxide and water vapour as waste products. Therefore, exhaled air has more CO₂ (4% vs 0.04%) and more water vapour (saturated). Oxygen decreases (21% → 16%) because it is used up. Nitrogen remains unchanged (78%) as it is inert and not used in respiration.

3. Answer: (3) [2 marks]

Explanation: Oxygenated blood (rich in O₂, low in CO₂) returns from the lungs via the pulmonary vein into the left atrium. It then passes to the left ventricle to be pumped to the body via the aorta. The right atrium and right ventricle handle deoxygenated blood.

4. Answer: (3) [2 marks]

Explanation: The ileum (final section of the small intestine) has villi and microvilli that provide a huge surface area for absorption of digested nutrients (glucose, amino acids, fatty acids, glycerol, vitamins, minerals). The stomach mainly digests proteins; the duodenum receives digestive juices but absorption is limited; the large intestine absorbs water and minerals, not most nutrients.

5. Answer: (3) [2 marks]

Explanation: Chloroplasts contain chlorophyll for photosynthesis and are found only in plant cells (and some protists). Animal cells lack chloroplasts. Both cells have a nucleus (1 is wrong). Both have a cell membrane (2 is wrong). Plant cells have a large central vacuole; animal cells have small vacuoles (4 is wrong).

6. Answer: (2) [2 marks]

Explanation: The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs to pick up oxygen. The aorta carries oxygenated blood to the body. The pulmonary vein carries oxygenated blood from lungs to left atrium. The vena cava carries deoxygenated blood from body to right atrium.

7. Answer: (2) [2 marks]

Explanation: As distance increases (10 → 50 cm), light intensity decreases, and bubbles per minute decrease (48 → 2). This shows rate of photosynthesis decreases as light intensity decreases (or increases as light intensity increases). Light intensity is inversely proportional to distance squared.

8. Answer: (3) [2 marks]

Explanation: Water fleas are the food source for small fish. If water fleas decrease, small fish have less food, so their population will decrease. This is a direct trophic relationship: Producer → Primary Consumer → Secondary Consumer.

9. Answer: (3) [2 marks]

Explanation: Urea is a nitrogenous waste product from protein metabolism, filtered by kidneys and excreted in urine. Glucose is normally reabsorbed (not in urine unless diabetic). Oxygen is carried by blood, not excreted. Proteins are too large to be filtered (normally absent in urine).

10. Answer: (3) [2 marks]

Explanation: The interneuron (relay neuron) in the spinal cord receives impulses from the sensory neuron, processes them, and passes them to the motor neuron. It enables a rapid response without involving the brain. Sensory neuron carries impulse to spinal cord (2). Motor neuron carries impulse to effector (4). Receptor detects stimulus (1).

Section B: Structured Questions (20 marks)

11. [4 marks]

(a) Diffusion [1 mark]
Accept: Diffusion of oxygen / gaseous diffusion

(b) Two structural features: [2 marks, 1 each]

1. One-cell-thick walls (alveolar and capillary walls) → short diffusion distance
2. Large surface area (millions of alveoli) → more space for gas exchange
   Also accept: Moist lining (dissolves gases), Good blood supply (maintains concentration gradient)

(c) Oxygen combines with haemoglobin in red blood cells to form oxyhaemoglobin, which is bright red. [1 mark]
Key concept: Haemoglobin + O₂ → Oxyhaemoglobin (bright red); Deoxyhaemoglobin (dark red)

Common mistake: Saying "blood turns red because of oxygen" without mentioning haemoglobin.

12. [4 marks]

(a) Y (Small intestine) [1 mark]
Most chemical digestion occurs in the duodenum (first part of small intestine) where bile, pancreatic juice, and intestinal juices act.

(b) Villi and microvilli: [2 marks]

• Increase surface area tremendously (folding + microscopic projections)
• Thin walls (one cell thick) for short diffusion distance
• Rich blood capillary network to transport absorbed nutrients away, maintaining concentration gradient
• Lacteals for fat absorption
  Any two well-explained points for 2 marks.

(c) Absorbs water and mineral salts from undigested food; forms and stores faeces temporarily before egestion. [1 mark]
Accept: Absorption of water; formation of faeces.

13. [4 marks]

(a) Aorta [1 mark]

(b) Left ventricle pumps blood to the whole body (systemic circulation), requiring high pressure to overcome long distance and resistance. Right ventricle pumps only to the lungs (pulmonary circulation), a shorter distance with lower resistance. Thicker wall = stronger contraction = higher pressure. [2 marks]
Key comparison: Systemic vs pulmonary circulation; pressure difference.

(c) Valves prevent backflow of blood, ensuring one-way flow through the heart. [1 mark]
Accept: Ensure blood flows in one direction; prevent mixing of oxygenated and deoxygenated blood.

14. [4 marks]

(a) To provide carbon dioxide for photosynthesis / to vary the concentration of CO₂. [1 mark]
NaHCO₃ dissolves → releases CO₂ in water.

(b) Two constant variables: [2 marks, 1 each]

1. Light intensity (lamp distance fixed at 20 cm)
2. Temperature (maintained at 25°C)
3. Type/size of plant (same Elodea)
4. Volume of water (500 ml)
5. Duration of each trial (20 minutes)
   Any two valid.

(c) As the mass of sodium hydrogen carbonate increases, the volume of oxygen collected increases, but the rate of increase slows down at higher masses (plateaus at 4 g). [1 mark]
Accept: Increases then levels off; positive correlation but not directly proportional; CO₂ becomes non-limiting at high concentrations.

15. [4 marks]

(a) Example food chain (4 organisms): [1 mark]
Grass → Grasshopper → Frog → Snake
Or: Grass → Rabbit → Fox → Hawk; Trees → Deer → Fox → Hawk; Shrubs → Grasshopper → Frog → Hawk, etc.
Must start with producer, end with tertiary consumer, arrows from food to consumer.

(b) Snakes eat frogs. If snakes are removed, frog population increases (less predation). More frogs eat more grasshoppers, so grasshopper population decreases. [2 marks]
Step-by-step: Snakes ↓ → Frogs ↑ → Grasshoppers ↓. This is a trophic cascade.

(c) Decomposers (bacteria, fungi) break down dead organisms and waste, recycling nutrients back into the environment for producers to use. [1 mark]
Key words: Break down dead matter, recycle nutrients, return to soil/air.

16. [4 marks]

(a) Medulla [1 mark]
Controls involuntary actions: heartbeat, breathing, blood vessel diameter, swallowing, vomiting.

(b) Difference: [1 mark]

Any one clear difference.

(c) Advantage: Rapid response to protect the body from harm (e.g., pulling hand from hot object) without waiting for brain processing, preventing or reducing injury. [2 marks]
Key points: Speed, protection, bypasses brain.

Section C: Open-Ended Questions (10 marks)

17. [3 marks]

(a) Chloroplast / Cell wall / Large central vacuole [1 mark]
Any one correct structure.

(b) Function: [1 mark]

• Chloroplast: Contains chlorophyll, traps light energy for photosynthesis
• Cell wall: Provides structural support, maintains shape, prevents bursting
• Large central vacuole: Stores water/substances, maintains turgor pressure, supports cell
  Match function to structure chosen in (a).

**(c) Partially permeable means the membrane allows some substances (like water, oxygen, carbon dioxide) to pass through freely, but restricts or prevents others (like large molecules, ions) from passing. [1 mark]
Also accept: Selectively permeable; controls movement of substances in/out of cell.

18. [3 marks]

(a) Sample 2 [1 mark]
Pulmonary vein carries oxygenated blood from lungs to left atrium → High O₂, Low CO₂.

(b) The pulmonary artery carries blood from the right ventricle to the lungs. This blood has just returned from the body where oxygen was used up in respiration and carbon dioxide was produced. So it is deoxygenated (low O₂) and high in CO₂. [2 marks]
Key trace: Body cells → Vena cava → Right atrium → Right ventricle → Pulmonary artery → Lungs. At body cells: O₂ used, CO₂ produced.

19. [3 marks]

(a) Setup P is the control — it has all conditions needed for photosynthesis (light, CO₂ from air, chlorophyll, water) to show that photosynthesis occurs normally and starch is produced. [1 mark]
Purpose of control: Baseline for comparison; proves starch production is due to experimental conditions.

(b) Potassium hydroxide absorbs carbon dioxide, so no CO₂ is available for photosynthesis. Without CO₂, photosynthesis cannot occur, so no starch is produced. Iodine test remains yellow-brown (no starch). [2 marks]
Step-by-step: KOH absorbs CO₂ → No CO₂ for photosynthesis → No glucose/starch made → Iodine negative.

20. [3 marks]

(a) Palisade mesophyll [1 mark]
Also accept: Palisade layer; Palisade cells. Reason: Tightly packed, columnar cells with most chloroplasts to trap maximum light.

(b) Air spaces increase surface area for gas exchange and allow rapid diffusion of gases (CO₂ in, O₂ out) between the atmosphere (via stomata) and the photosynthesising cells. [1 mark]
Key: Large surface area, short diffusion path, connects to stomata.

(c) Guard cells control the opening and closing of stomata to regulate gas exchange (CO₂ in, O₂ out) and water loss (transpiration). [1 mark]
Mechanism: Turgid → stomata open; Flaccid → stomata closed.

End of Answer Key