Primary 6 PSLE Science Life Cycles Quiz

Primary 6 PSLE Science Quiz - Life Cycles (Answer Key)

Total Marks: 40

Section A: Multiple-Choice Questions (10 marks)

1. Answer: (3) Grasshopper
Explanation: Grasshoppers have a three-stage life cycle (egg → nymph → adult), which is incomplete metamorphosis. Butterflies, mosquitoes, and beetles have four-stage life cycles (complete metamorphosis).
Mark: 1

2. Answer: (4) Adult frog
Explanation: Frog eggs and tadpoles live in water. Tadpoles breathe through gills. As they develop into adults, they grow lungs and lose gills. Only the adult frog breathes through lungs (and also through skin).
Mark: 1

3. Answer: (2) Fertilisation
Explanation: In flowering plants, pollination (transfer of pollen to stigma) is followed by fertilisation (fusion of male and female gametes in the ovule). Fruit formation occurs after fertilisation.
Mark: 1

4. Answer: (2) 84
Working: Egg (10) + Larva (60) + Pupa (14) = 84 days. The adult stage duration (90 days) is not included in "development from egg to adult".
Mark: 1

5. Answer: (2) Seed dispersal prevents overcrowding and competition among young plants.
Explanation: Seed dispersal spreads seeds away from the parent plant, reducing competition for light, water, nutrients, and space. Not all seeds are wind-dispersed (1 is false). Water-dispersed seeds have air spaces/fibrous husks, not wings (3 is false). Animals disperse seeds by eating fruits AND by carrying hooked/spiny fruits on fur (4 is false).
Mark: 1

6. Answer: (1) The larva has a siphon for breathing; the pupa has respiratory trumpets.
Explanation: Mosquito larva breathes through a siphon at the rear; pupa breathes through two respiratory trumpets on the thorax. The larva is elongated, pupa is comma-shaped (2 is reversed). Both larva and pupa move (3 is false). Neither has wings (4 is false).
Mark: 1

7. Answer: (4) Root → Seed leaves → Shoot
Explanation: During germination, the radicle (embryonic root) emerges first to anchor and absorb water. Then the hypocotyl elongates, pulling the seed leaves (cotyledons) above ground. The plumule (shoot) then grows from between the seed leaves.
Mark: 1

8. Answer: (3) Anther
Explanation: The anther is the male reproductive part of the flower that produces pollen grains containing male gametes. The stigma receives pollen, the ovary contains ovules (female gametes), and the style connects stigma to ovary.
Mark: 1

9. Answer: (3) Butterfly
Explanation: Butterflies undergo complete metamorphosis (egg → larva → pupa → adult). Chickens have no metamorphosis. Cockroaches and grasshoppers undergo incomplete metamorphosis (egg → nymph → adult).
Mark: 1

10. Answer: (4) Week 3 to Week 4
Working: Calculate growth per week:
Week 0–1: 3–2 = 1 cm
Week 1–2: 5–3 = 2 cm
Week 2–3: 12–5 = 7 cm
Week 3–4: 20–12 = 8 cm (greatest increase)
Week 4–5: 23–20 = 3 cm
Week 5–6: 25–23 = 2 cm
Mark: 1

Section B: Structured Questions (12 marks)

11. (a) Egg and Pupa (Chrysalis) [1]
Explanation: The egg stage is dormant. The pupa (chrysalis) is a non-feeding, transformative stage. The larva (caterpillar) feeds voraciously, and the adult feeds on nectar.

11. (b) The larva crawls using legs/prolegs; the adult flies using wings. [1]
Explanation: Caterpillars have true legs and prolegs for crawling on plants. Adult butterflies have two pairs of wings for flight. Accept: "Larva moves by crawling; adult moves by flying."

12. (a) Set-up A only [1]
Explanation: Germination requires water, oxygen, and suitable temperature (warmth). Set-up A has all three. Set-up B lacks oxygen. Set-up C lacks water. Set-up D has unsuitable temperature (too cold).

12. (b) Set-up B has no oxygen. Oxygen is needed for respiration to release energy for germination. [1]
Explanation: Seeds respire aerobically during germination to obtain energy for growth. Without oxygen, aerobic respiration cannot occur, so no energy is released for metabolic processes.

13. (a) Animal dispersal (by attachment / epizoochory) [1]
Explanation: Hooked spines catch on animal fur/feathers for transport.

13. (b) The fruit has hooked spines / stiff hairs that catch onto animal fur. [1]
Explanation: Structural adaptation for external attachment. Accept: "Hooks on fruit surface" or "Spiny fruit wall".

14. (a) X → Radicle [1]
Explanation: The radicle is the embryonic root, first to emerge during germination.

14. (b) The endosperm stores food (starch, proteins, oils) for the developing embryo during germination. [1]
Explanation: Provides energy and building materials until the seedling can photosynthesise.

15. (a) As temperature increases, the number of days to reach the adult stage decreases. / Higher temperature leads to faster development. [1]
Explanation: Within the viable range, metabolic rate increases with temperature, speeding up growth and development.

15. (b) Prediction: Less than 40 days (e.g., 30–35 days). Reasoning: The trend shows decreasing days with increasing temperature, so at 35°C development would be even faster. [1]
Explanation: Extrapolating the trend. Note: At excessively high temperatures, development may slow or stop due to enzyme denaturation, but within the range shown, the trend holds. Accept reasonable prediction with correct reasoning.

16. (a) Fertilisation (in a flowering plant) [1]
Explanation: The diagram shows pollen tube growth and entry of male nuclei into the ovule — the process of fertilisation.

16. (b) One male nucleus fuses with the egg cell to form the zygote (which develops into the embryo). The other male nucleus fuses with the two polar nuclei to form the triploid nucleus (which develops into the endosperm). [1]
Explanation: This is double fertilisation, unique to flowering plants. Accept: "One forms the embryo, the other forms the endosperm/food store."

Section C: Open-Ended Questions (18 marks)

17. (a) Both life cycles begin with an egg stage. / Both have four stages. / Both undergo metamorphosis. [1]
Explanation: Any valid similarity. Most direct: both start from eggs laid by the adult female.

17. (b) Difference 1: Mosquito has a larval and pupal stage (complete metamorphosis); grasshopper has nymph stages that resemble the adult (incomplete metamorphosis).
Difference 2: Mosquito larva and pupa live in water (aquatic); grasshopper nymphs and adults live on land (terrestrial).
Difference 3: Mosquito has 4 stages; grasshopper has 3 stages (egg, nymph, adult). [2]
Explanation: Award 1 mark per distinct difference. Key contrasts: type of metamorphosis, habitat of young stages, number of stages.

17. (c) The larva has a breathing tube (siphon) at the end of its abdomen that it extends to the water surface to take in air. [1]
Explanation: Mosquito larvae are air-breathers; they hang at the water surface with the siphon breaking the surface tension.

17. (d) Mosquitoes lay eggs in stagnant water. The larvae and pupae develop in water. Removing stagnant water removes the breeding site, so eggs cannot hatch and larvae/pupae cannot survive, reducing the population. [2]
Mark breakdown: 1 mark for identifying water as breeding site for eggs/larvae/pupae; 1 mark for explaining removal breaks the life cycle.
Common mistake: Only saying "mosquitoes breed in water" without linking to life cycle stages.

18. (a) Characteristic 1: Large, brightly coloured petals to attract bees visually.
Characteristic 2: Sweet-smelling nectar / nectar guides to attract bees by scent and guide them to nectar. [2]
Explanation: Insect-pollinated flowers have visual and olfactory attractants. Accept: "Sweet nectar as food reward", "Pollen as food", "Nectar guides".

18. (b) When the bee visits the flower to collect nectar, pollen from the anther brushes onto its hairy body. When the bee visits another flower of the same species, some of this pollen rubs off onto the stigma, achieving pollination. [2]
Mark breakdown: 1 mark for pollen transfer to bee's body; 1 mark for transfer from bee to stigma of another flower.
Explanation: Emphasise cross-pollination (between flowers) and role of bee's hairy body.

18. (c) Change 1: The zygote (fertilised egg) divides and develops into the embryo (with radicle and plumule).
Change 2: The integuments (outer layers of ovule) develop into the seed coat (testa) to protect the embryo.
Change 3: The ovary develops into the fruit (though this is outside the ovule).
Change 4: The endosperm may develop (from fusion of second male nucleus with polar nuclei) to store food. [2]
Mark breakdown: 1 mark per valid change within the ovule. Best answers: embryo formation and seed coat formation.

19. (a) Advantage: Seeds can be dispersed far from the parent plant over a wide area. / No reliance on animals.
Disadvantage: Dispersal is unpredictable / dependent on wind direction and strength. / Many seeds may land in unsuitable habitats (e.g., water, concrete). / Seeds must be light/small, limiting food reserves. [2]
Explanation: Wind dispersal = wide but random; animal dispersal = targeted but requires mutualism.

19. (b) Advantage: Seeds are deposited in specific habitats (e.g., where animals defecate/roost) often with fertiliser (dung). / Seeds can be carried longer distances by mobile animals. / Seeds protected inside fruit during transport.
Disadvantage: Dependent on presence of animal dispersers. / If animals eat fruit but destroy seeds (e.g., seed predators), no dispersal. / Fruits require energy to produce flesh. [2]

19. (c) Plant the crop near natural habitats / hedgerows to attract birds. / Avoid using pesticides that harm seed-dispersing animals. / Provide perching sites / water sources for birds. [1]
Explanation: Any practical method to encourage animal dispersers.

19. (d) Seed dispersal reduces overcrowding and competition among seedlings and the parent plant for light, water, nutrients, and space. It also allows colonisation of new habitats. [1]
Explanation: Core concept: survival of species through reduced competition and range expansion.

20. (a) Set-up 2 only [1]
Explanation: Set-up 2 has water (wet cotton wool), oxygen (open to air), and suitable temperature (room temp). Set-up 1 lacks water. Set-up 3 lacks oxygen (oil layer prevents air from dissolving in water).

20. (b) The oil layer prevents oxygen from dissolving in the water / cuts off air supply to the seeds, creating anaerobic conditions. [1]
Explanation: Oil is impermeable to gases; it seals the water surface.

20. (c) Water (moisture), Oxygen (air), and Suitable Temperature (warmth). [2]
Mark breakdown: 1 mark for listing all three correctly; 1 mark for correct terminology (accept "warmth" for temperature).
Common mistake: Listing "sunlight" or "soil" — not required for germination.

20. (d) The seedling uses the stored food (in cotyledons/endosperm) for respiration to release energy for growth before it can photosynthesise. Since no new food is made yet, the dry mass (stored food) decreases as it is broken down and respired (releasing CO₂ and H₂O). [2]
Mark breakdown: 1 mark for identifying use of stored food for respiration; 1 mark for explaining loss of dry mass as CO₂/H₂O release.
Explanation: Dry mass = organic matter. Respiration converts organic matter → CO₂ + H₂O + energy. CO₂ and H₂O are lost to air, so dry mass drops until photosynthesis begins.

End of Answer Key