Primary 6 PSLE Science Systems Quiz

Primary 6 PSLE Science Quiz - Systems

ANSWER KEY

Total Marks: 40

Section A: Multiple Choice

1. Answer: B — Oesophagus (1 mark)

• The oesophagus is a muscular tube that transports food from mouth to stomach by peristalsis. It does not produce any digestive enzymes.
• The mouth produces saliva containing amylase (starch digestion).
• The stomach produces pepsin and hydrochloric acid (protein digestion).
• The small intestine produces multiple enzymes: maltase, lactase, sucrase, peptidase, and receives bile and pancreatic enzymes.
• Common mistake: Students confuse the oesophagus with organs of chemical digestion. The oesophagus is purely for transport (mechanical movement only).

2. Answer: A — Legs → vena cava → right atrium → right ventricle → pulmonary artery → lungs (1 mark)

• This is the correct path of deoxygenated blood returning to the heart and then to the lungs for oxygenation.
• Step-by-step reasoning:
  1. Deoxygenated blood from legs enters the inferior vena cava
  2. Enters right atrium (top right chamber)
  3. Passes to right ventricle (bottom right chamber)
  4. Pumped via pulmonary artery to lungs (only artery carrying deoxygenated blood)
• Why other options wrong:
  • B: Blood does NOT go to left side before lungs — this would skip oxygenation.
  • C: Aorta carries blood AWAY from heart to body, not toward it.
  • D: Pulmonary vein carries oxygenated blood FROM lungs TO heart, wrong direction.

3. Answer: C — To exchange gases between the body and the environment (1 mark)

• The respiratory system (lungs, trachea, bronchus, alveoli) performs gas exchange: taking in oxygen and releasing carbon dioxide.
• A describes the circulatory system (blood transport).
• B describes the digestive/excretory system.
• D describes the circulatory system (heart function).

4. Answer: C — Sensitivity (1 mark)

• Sensitivity (or irritability/response to stimuli) is the ability of living things to detect and respond to changes in their environment.
• The plant shoot grows toward light — this is phototropism, a response to light stimulus.
• Other characteristics: reproduction (making offspring), respiration (energy release), excretion (waste removal).

5. Answer: B — P: carry oxygen; Q: fight disease; R: clot blood; S: transport nutrients and waste (1 mark)

Section B: Structured Questions

6. (a) — 1 mark

Two functions of the waxy cuticle:

• Prevents water loss / reduces transpiration (any one for 1 mark)
• Protects leaf from physical damage / pathogens (either acceptable)

6. (b) — 2 marks

The palisade layer (X) contains the most chloroplasts because:

• It is the upper layer of mesophyll, closest to the upper epidermis and light source (1 mark)
• Its cells are column-shaped and tightly packed to maximize light absorption and hold many chloroplasts efficiently (1 mark)

Teaching note: Chloroplasts contain chlorophyll, the green pigment that traps light energy for photosynthesis. The palisade layer is adapted for maximum light capture.

6. (c) — 1 mark

Carbon dioxide enters through the stoma (Z) by diffusion — moving from an area of higher concentration (outside air) to lower concentration (inside leaf where CO₂ is used in photosynthesis).

7. (a) — 1 mark

Fallopian tube / oviduct (structure B)

7. (b) — 2 marks

The uterus (C) has a thick muscular wall because:

• It needs to contract strongly during childbirth to push the baby out through the birth canal (1 mark)
• The muscular layer (myometrium) expands during pregnancy and contracts rhythmically during labour (1 mark)

7. (c) — 1 mark

The ovary (A) is adapted by containing many follicles that can develop into egg cells, and it produces hormones (oestrogen and progesterone) to regulate the menstrual cycle.

8. (a) — 1 mark

The Sun / solar energy / heat from the Sun

8. (b) — 2 marks

Condensation (Q) happens at higher altitudes because:

• Temperature decreases with altitude / air is cooler higher up (1 mark)
• Cooler air cannot hold as much water vapour, so water vapour condenses into liquid water droplets forming clouds (1 mark)

8. (c) — 1 mark

Rainwater infiltrates / seeps into the ground to become groundwater, which may eventually flow into rivers, lakes, or aquifers, or be absorbed by plant roots.

9. (a) — 1 mark

The spinal cord

9. (b) — 3 marks

Sequence of reflex arc (1 mark for correct order, 2 marks for explanation):

1. Receptor in the skin detects heat/pain and generates nerve impulse (½ mark)
2. Sensory neurone transmits impulse to the spinal cord (CNS) (½ mark)
3. Relay neurone in the spinal cord passes impulse to motor neurone (½ mark)
4. Motor neurone carries impulse to effector (muscle) causing it to contract and pull hand away (½ mark + ½ mark for "muscle contracts")
5. Brain is informed after the reflex has occurred (½ mark for awareness/monitoring)

Teaching note: The reflex arc bypasses the brain for speed — the brain receives the message later so you are aware of what happened.

9. (c) — 1 mark

A reflex action is faster because it does not travel to the brain; it goes only to the spinal cord and back, protecting the body from injury more quickly.

10. (a) — 1 mark

Any two from: cell wall, chloroplasts, large permanent vacuole (½ mark each, 1 mark total)

10. (b) — 2 marks

Structure S (chloroplast) contains chlorophyll which (1 mark):

• Traps/absorbs light energy from the sun (½ mark)
• Uses this energy to convert carbon dioxide and water into glucose and oxygen during photosynthesis (½ mark)

10. (c) — 1 mark

Without the cell wall (P), the cell would lose its rigid shape / become flaccid and may burst if too much water enters by osmosis (there is no wall to provide structural support or protection against excessive water uptake).

11. (a) — 1 mark

Urine

11. (b) — 2 marks

The bladder (C) can expand and contract because:

• Its wall contains smooth muscle (detrusor muscle) that can contract to push urine out (1 mark)
• The lining is folded/rugae and the wall is elastic, allowing it to stretch and store urine before emptying (1 mark)

11. (c) — 1 mark

If the ureter (B) is blocked, urine cannot drain from the kidney to the bladder. This causes urine to back up in the kidney, potentially causing kidney damage, infection, or swelling (hydronephrosis).

12. (a) — 2 marks

Oxygen decreases in exhaled air because:

• Oxygen is absorbed into the blood in the lungs at the alveoli (1 mark)
• It is used by body cells for aerobic respiration to release energy (1 mark)

12. (b) — 2 marks

During exercise:

• Muscles need more energy, so they need more oxygen for increased aerobic respiration (1 mark)
• Increased breathing rate means more air (and oxygen) is taken in per minute, and more carbon dioxide (waste product) is removed more quickly (1 mark)

13. (a) — 1 mark

Cabbage (plant)

13. (b) — 2 marks

Only about 10% of energy transfers to the next level because:

• Most energy is lost as heat during respiration for life processes (movement, maintaining body temperature) (1 mark)
• Some energy is lost in indigestible parts / waste (faeces), and some energy remains in uneaten parts / dead organisms (1 mark)

13. (c) — 1 mark

Decomposers (bacteria and fungi) break down dead organisms and waste, releasing nutrients/minerals back into the soil for the producer (cabbage) to absorb and reuse.

14. (a) — 1 mark

The cornea (A) refracts/bends light entering the eye, helping to focus light onto the retina.

14. (b) — 2 marks

To focus on near objects:

• The ciliary muscles contract (1 mark)
• This makes the suspensory ligaments slacken, allowing the lens to become thicker/more convex (1 mark), which increases its refractive power to focus light from close objects onto the retina.

14. (c) — 1 mark

The lens (C) — this person is myopic/short-sighted; the lens may be too curved or the eyeball too long, causing images to focus in front of the retina instead of on it.

15. (a) — 1 mark

Hinge joint (or synovial joint)

15. (b) — 2 marks

To bend the arm:

• The biceps muscle contracts and pulls on the radius bone (1 mark)
• At the same time, the triceps muscle relaxes, allowing this movement to occur (1 mark)
• This is antagonistic muscle action — muscles work in opposite pairs.

15. (c) — 1 mark

Ligaments hold bones together at the joint, providing stability and preventing dislocation while allowing controlled movement.

Section C: Application and Data Analysis

16. (a) — 1 mark

35°C

16. (b) — 2 marks

Above 35°C:

• The enzymes involved in photosynthesis begin to denature (1 mark)
• This means their active site changes shape, so they can no longer bind efficiently to substrate molecules, slowing the reaction rate (1 mark)

16. (c) — 3 marks

Evaluation of the claim:

Full marks for nuanced evaluation: The student should recognize the measurement shows zero, but explain this as enzyme denaturation — a physical/chemical failure of biological catalysts rather than photosynthesis "choosing" to stop.

17. (a) — 2 marks

Tube A is the control (1 mark) because it has all factors present (water, air, warmth) so it shows normal germination conditions for comparison with tubes where one factor is removed (1 mark).

17. (b) — 3 marks

Analysis of whether water or temperature is more important:

Conclusion: Water is more critical than optimal temperature for germination to occur at all, though warmth greatly increases the rate/success (3 marks for reasoned conclusion with data comparison, 2 marks for partial analysis, 1 mark for attempt)

17. (c) — 1 mark

Possible reasons: those 2 seeds may have different genetic tolerance to cold, or the fridge temperature fluctuated/was not consistently cold, or seed dormancy was broken by other factors for those particular seeds.

18. (a) — 2 marks

Calculation:

$\text{Percentage change} = \frac{\text{Final} - \text{Initial}}{\text{Initial}} \times 100\% = \frac{4.3 - 5.0}{5.0} \times 100\% = \frac{-0.7}{5.0} \times 100\% = -14\%$

Answer: -14% (or decreased by 14%) — decrease must be indicated (2 marks with working, 1 mark partial)

18. (b) — 2 marks

In distilled water (0% glucose):

• Water concentration is higher outside the potato than inside the potato cells (1 mark)
• Water moves into the potato cells by osmosis (down the water potential gradient), causing cells to swell and the potato to increase in length/turgidity (1 mark)

18. (c) — 2 marks

19. (a) — 1 mark

Open — during atrial systole, blood is pushed from atria into ventricles, so the atrioventricular valves (bicuspid/tricuspid) must be open.

19. (b) — 3 marks

During ventricular systole:

19. (c) — 2 marks

Calculation:

$\text{Heart rate} = \frac{60 \text{ seconds per minute}}{0.8 \text{ seconds per beat}} = 75 \text{ beats per minute}$

Answer: 75 bpm (or 75 beats/min) — 2 marks with working, 1 mark for correct method with error.

20. (a) — 1 mark

One advantage: higher blood pressure / faster blood flow to body tissues, allowing more efficient delivery of oxygen and nutrients and better removal of waste; OR oxygenated and deoxygenated blood do not mix; OR can maintain different pressures in pulmonary and systemic circuits.

20. (b) — 2 marks

Two chambers are sufficient because:

• Fish have lower metabolic demands than mammals / are cold-blooded (ectothermic), so they need less oxygen than humans (1 mark)
• Water contains less dissolved oxygen than air, so gills are adapted with counter-current exchange to maximise extraction; the simpler heart is adequate for their lower energy needs and aquatic environment (1 mark)

20. (c) — 2 marks

Possible adaptations in low-oxygen environments:

Any 2 reasonable suggestions with explanation: 2 marks (1 mark each)

END OF ANSWER KEY