Primary 6 PSLE Science Life Cycles Quiz

Primary 6 PSLE Science Quiz - Life Cycles: ANSWER KEY

Total Marks: 40

Section A: Multiple Choice (Questions 1–10, 1 mark each)

1. Answer: ( A )

Explanation: Complete metamorphosis involves four distinct stages: egg → larva (caterpillar) → pupa (chrysalis) → adult butterfly. The larva looks completely different from the adult and has different feeding habits. This is different from incomplete metamorphosis (like grasshoppers) where the young resemble small adults without wings.

Key concept: Complete metamorphosis = dramatic body change through pupa stage.

2. Answer: ( C )

Explanation: The life cycle of a flowering plant is: seed → seedling → young plant → mature plant (with flowers) → fruit and seeds. The pupa is a stage in the life cycle of insects with complete metamorphosis (butterflies, moths, beetles), not in plants.

Common mistake: Confusing animal and plant life cycles. Remember: plants don't have pupae!

3. Answer: ( D )

Explanation: While each stage involves growth, the metamorphosis from tadpole to adult frog is the most dramatic change. The tadpole lives in water, breathes with gills, has a tail, and eats plant matter. The adult frog lives on land and water, breathes with lungs and skin, has four legs, and eats insects. This complete body restructuring during metamorphosis is the most dramatic change.

Key concept: Metamorphosis = complete change in body form and lifestyle.

4. Answer: ( B )

Explanation: After fertilisation, the ovary swells and develops into the fruit. The ovules inside the ovary develop into seeds. The flower's other parts (petals, sepals, stamens, stigma, style) wither and fall away.

Teaching note: Remember this sequence: Ovary → Fruit; Ovule → Seed. The ovary is like a protective container that becomes the fruit wall.

5. Answer: ( A )

Explanation: Mosquitoes undergo complete metamorphosis: Egg → Larva (wriggler, lives in water) → Pupa (tumbler, also in water) → Adult (flying insect). The larva and pupa are aquatic, while the adult is terrestrial/aerial.

Key distinction: Complete metamorphosis always has a pupa stage; incomplete metamorphosis does not.

6. Answer: ( B )

Explanation: Stem cuttings are a form of asexual reproduction (vegetative propagation). The new plant grows from a part of the parent plant and has identical genetic material (same DNA). This produces a clone — genetically identical to the parent. Seeds from sexual reproduction combine genetic material from two parents, so offspring vary.

Key concept: Asexual reproduction = no genetic variation = identical offspring.

7. Answer: ( A )

Explanation: Ferns reproduce by spores, not seeds. They do not produce flowers, fruits, or cones. Spores develop in tiny cases (sporangia) on the underside of fronds. Ferns can perform photosynthesis (they are green plants); they just don't use seeds for reproduction.

Common mistake: Option C says "spores found in cones" — that's gymnosperms (conifers), not ferns. Ferns have sporangia, not cones.

8. Answer: ( B )

Explanation: Grasshoppers have incomplete metamorphosis (hemimetabolism): egg → nymph → adult. The nymph looks like a small adult but lacks fully developed wings and cannot reproduce yet. It goes through multiple moults, gradually developing wings and growing larger until it reaches adult form.

Key difference from complete metamorphosis: No pupa stage; gradual change, not dramatic transformation.

9. Answer: ( C )

Explanation: Both reproductive strategies are evolutionarily successful because they suit different niches:

• Many small eggs, no care (Species X): Good in unstable environments or where adult survival is uncertain. High quantity compensates for low survival.
• Few large eggs, with care (Species Y): Good in stable environments where investing heavily in each offspring pays off. High quality over quantity.

Neither is "better" absolutely — success depends on environment, predation pressure, resource availability, and parental survival.

Key concept: Reproductive strategies represent evolutionary trade-offs, not simple superiority.

10. Answer: ( C )

Explanation: The pollen grain contains the male gametes (male sex cells). When pollen lands on a compatible stigma, it germinates and grows a pollen tube down to the ovule for fertilisation. The ovule (A) contains the female gamete; stigma (B) receives pollen; ovary (D) protects ovules and becomes fruit.

Teaching tip: Remember: Pollen = male; Ovule = female.

Section B: Short Answer (Questions 11–16, 2 marks each)

11. Two advantages of seed dispersal: (2 marks)

Marking scheme — any two of:

Sample answer:

• Seeds dispersed away from the parent plant reduce competition for light, water, and nutrients (1 mark)
• This allows the species to colonise new areas and increases survival chances (1 mark)

12. (a) Fertilisation / pollination and fertilisation (1 mark)

Note: Accept "pollination followed by fertilisation" or just "fertilisation." Pollination alone is not sufficient — fertilisation (fusion of male and female nuclei) must occur for fruits and seeds to develop.

(b) Why seeds need dispersal away from parent plant: (1 mark)

Key point: To reduce competition for resources (light, water, nutrients, space) with the parent plant, which would reduce survival of seedlings.

Sample answer: If seeds fall directly under the parent, they would compete with the parent for sunlight, water, and minerals, and most would not survive to grow into healthy plants.

13. (a) Animal P (Frog) — lowest survival rate (1 mark)

Reason: It produces the largest number of offspring but provides no parental care. With many eggs and no protection, most offspring will be eaten by predators or die from environmental conditions. This is a high-quantity, low-investment strategy with inherently low survival rates.

Note: Must state both high number AND no parental care to explain low survival.

(b) One advantage and one disadvantage of Animal R's strategy: (1 mark)

Note: Any one valid advantage and one valid disadvantage accepted. Must identify which is which.

14. (a) Adolescent stage / teenage years / puberty (1 mark)

Accept: "Between child and adult" or approximate age range 12–18 years.

(b) Why reproduction is important for species continuation: (1 mark)

Key concept: Reproduction produces new individuals to replace aging/dead members, ensuring the species does not die out (extinction). It passes genetic information to the next generation.

Sample answer: Reproduction ensures that new individuals are produced to take the place of those that die, allowing the species to continue existing and preventing extinction.

15. (a) Methods of seed dispersal: (1 mark — both correct for 1 mark, no half marks)

(b) Features suited to dispersal method: (1 mark — one for each plant)

16. Fertilisation and development: (2 marks)

Step-by-step answer:

Teaching note: After fertilisation: zygote → embryo → seed. The ovule becomes the seed; the ovary becomes the fruit.

Section C: Structured and Application Questions (Questions 17–20, 3 marks each)

17. Seed germination experiment: (3 marks)

(a) Setup Y will show germination (1 mark)

Reason: Setup Y has all three necessary conditions for germination:

• Water — absorbed for metabolic processes and enzyme activation
• Air/oxygen — needed for aerobic respiration to release energy for growth
• Suitable temperature (room temperature, 25°C) — enzymes work best at moderate temperatures

(1 mark for correct identification, 1 mark for correct explanation of all three conditions)

Why others fail:

• W: No water — seeds remain dormant, metabolic reactions cannot start
• X: Too cold (4°C) — enzymes work too slowly, metabolic reactions halted
• Z: No oxygen — oil prevents air reaching seeds; seeds cannot respire aerobically to get energy

(b) Purpose of Setup W: (1 mark)

As a control. To show that seeds with air and suitable temperature but without water cannot germinate. This proves that water is necessary for germination by comparison with Setup Y.

18. Moth life cycle: (3 marks)

(a) Correct order: A → B → C → D (Egg → Larva → Pupa → Adult) (1 mark)

Must be in correct sequence. Accept written description if letters unclear.

(b) Two differences from grasshopper life cycle: (2 marks)

Any two valid differences accepted, 1 mark each.

19. Apple tree reproduction strategies: (3 marks)

(a) Why grafting produces more consistent fruit quality: (1 mark)

Grafting is asexual reproduction/vegetative propagation. The new tree has identical genetic material/DNA to the parent plant (it is a clone). Therefore, it will produce fruit with the same characteristics (size, colour, taste, ripening time) as the parent with desirable traits.

Keywords: identical genes/DNA, clone, same desirable characteristics.

(b) One disadvantage of grafting: (1 mark)

• Lack of genetic variation — all trees are genetically identical, so if a new disease or pest appears that can attack one tree, all trees are equally vulnerable (no resistance variation)
• OR: Expensive and requires skill — needs trained workers, specific timing, correct rootstock selection
• OR: Dependent on existing rootstock — may have limited root characteristics (disease susceptibility, soil preference)

(c) Why commercial farmers still prefer grafting: (1 mark)

• Guaranteed quality and market requirements — consumers expect consistent apple size, colour, taste; grafting ensures this predictability
• Faster fruit production — grafted trees bear fruit in 2–3 years; seed-grown trees take 5–8 years or more
• Proven disease resistance of rootstock — can combine good fruit genes (scion) with strong root genes (rootstock)

Any valid economic or agricultural reason accepted.

20. Forest ecosystem changes: (3 marks)

(a) Ecological succession (1 mark)

Accept: "succession" or "secondary succession" (since it started from grassland, not bare rock).

(b) Why animal types changed with plant changes: (1 mark)

Animals are dependent on plants for food and habitat/shelter. As plant types changed:

• Grasses → grasshoppers/butterflies (herbivores feeding on grasses, nectar)
• Shrubs → shrub-nesting birds (nesting sites, berries/seeds)
• Tall trees → squirrels (tree-dwelling, nuts) and birds of prey (hunting from high perches, prey availability increased)

Key concept: Different animals have specific adaptations suited to particular plant structures and food sources.

(c) Two effects of forest clearing: (1 mark for any two valid points)

Marking: Any two valid effects with brief explanation = 1 mark (0.5 each). Must be specific to "organisms established in mature forest."

END OF ANSWER KEY