Reproduction - Primary 6 PSLE Science Study Notes

Key Concepts

1. Why Living Things Reproduce

Reproduction is the process by which living things produce offspring (young of the same kind)
It ensures the continuity of the species — without reproduction, a species would die out (become extinct)
There are two main types of reproduction:
- Sexual reproduction: Involves two parents (male and female); offspring are not identical to either parent
- Asexual reproduction: Involves only one parent; offspring are genetically identical to the parent

2. Sexual Reproduction in Plants

Parts of a Flower (Know These for PSLE!)

A typical flower has both male and female reproductive parts:

Male parts (stamen):

Anther — produces pollen grains (contains the male gametes)
Filament — stalk that supports the anther

Female parts (pistil):

Stigma — sticky tip that receives pollen
Style — tube connecting stigma to ovary
Ovary — contains the ovule(s)
Ovule — contains the female gamete (egg cell); develops into a seed after fertilisation

Other parts:

Petals — attract pollinators (insects, birds)
Sepals — protect the flower bud
Nectary — produces nectar to attract insects

Remember: Ovule → Seed | Ovary → Fruit (after fertilisation)

Pollination

Pollination is the transfer of pollen grains from the anther to the stigma.

Important: Pollination is NOT fertilisation. Pollination must happen BEFORE fertilisation can occur.

Two types of pollination:

Type	Description
Self-pollination	Pollen is transferred from the anther to the stigma of the same flower or another flower on the same plant
Cross-pollination	Pollen is transferred from the anther of one plant to the stigma of a different plant of the same species

Cross-pollination is more common and produces greater variation in offspring.

Agents of pollination:

Insect-pollinated flowers:

Large, brightly coloured petals (to attract insects)
Sweet scent and nectar (to attract insects)
Sticky or spiky pollen (sticks to insect’s body)
Sticky stigma (to trap pollen)
Examples: hibiscus, orchid, rose

Wind-pollinated flowers:

Small petals, often no petals (insects not needed)
No scent or nectar
Large, feathery stigma (to catch pollen from the air)
Pollen that is light, smooth, and produced in large quantities (easily carried by wind)
Long filaments that hold anthers outside the flower
Examples: grass, maize, rice

Fertilisation in Plants

After pollination, fertilisation takes place:

A pollen tube grows from the pollen grain, through the stigma and down the style, towards the ovule in the ovary
The male gamete (sperm nucleus) travels down the pollen tube
The male gamete fuses with the female gamete (egg cell) inside the ovule
This fusion is called fertilisation and forms a zygote
The zygote develops into an embryo, which becomes the seed
The ovule develops into the seed
The ovary develops into the fruit

Key point: Fertilisation = fusion of male and female gametes to form a zygote.

Seed Dispersal

Seeds need to be dispersed (spread away) from the parent plant to:

Reduce competition for water, sunlight, minerals, and space
Increase the chances of survival of the offspring
Allow the species to spread to new areas

Methods of seed dispersal:

1. Wind dispersal

Seeds are light with special structures to catch the wind
Examples:
- Dandelion — has a parachute-like structure (pappus) of fine hairs
- Maple (angsana) — has wings that cause it to spin and slow its fall
- Cotton — has fluffy fibres

2. Water dispersal

Seeds/fruits that float on water; often have a waterproof or fibrous outer layer
Examples:
- Coconut — large fibrous husk traps air, allowing it to float; waterproof shell protects the seed
- Sea bean — hard waterproof coat

3. Animal dispersal (external — on body)

Seeds/fruits have hooks, spines, or barbs that attach to fur or feathers of passing animals
Examples:
- Burdock — has hooked fruits that catch on fur/clothing
- Sword grass — barbed seeds

4. Animal dispersal (internal — eaten)

Fruits are fleshy and attractive to eat; seeds have hard seed coats that survive digestion
Animals eat the fruit, and seeds are passed out in droppings far from the parent plant
Examples:
- Mango, rambutan, berries, tomato

5. Self-dispersal (explosive mechanism)

Fruits dry out and split open forcefully, flinging seeds away
Examples:
- Impatiens (touch-me-not) — pods coil up and burst when touched
- Rubber plant — pods explode with a loud crack
- Saga — pods twist and flick seeds out

Germination

Germination is the process by which a seed develops into a seedling (young plant).

Conditions needed for germination:

Water — softens the seed coat and activates enzymes; needed for chemical reactions
Warmth — provides the right temperature for enzymes to work
Oxygen — needed for the seed to respire and release energy for growth

Common mistake: Seeds do NOT need sunlight to germinate. The seed contains stored food (in the cotyledons) that the embryo uses for energy until the seedling develops leaves and can photosynthesise.

Stages of germination:

Seed absorbs water and swells
Seed coat splits open
Radicle (embryonic root) emerges first and grows downward
Plumule (embryonic shoot) emerges and grows upward toward light
Seedling develops its first true leaves
Seedling begins photosynthesis and no longer depends on stored food

3. Sexual Reproduction in Animals

Involves a male and a female parent
Male produces sperm cells (male gametes)
Female produces egg cells (female gametes)
Fertilisation: A sperm cell fuses with an egg cell → forms a zygote → develops into an offspring

External Fertilisation

Fertilisation takes place outside the female’s body
Common in aquatic animals (water supports the eggs and prevents them from drying out)
Large numbers of eggs and sperm are released because most will not be fertilised or will not survive
Examples:
- Fish: Female lays eggs in water, male releases sperm over them
- Frogs: Female lays eggs in water (spawn), male releases sperm at the same time

Internal Fertilisation

Fertilisation takes place inside the female’s body
Sperm is transferred from male to female during mating
Fewer offspring are produced, but parental care increases their chances of survival
Examples: mammals, birds, reptiles

Oviparous vs Viviparous

Term	Meaning	Examples
Oviparous	Gives birth by laying eggs; embryo develops outside the mother’s body	Fish, frogs, lizards, snakes, turtles, birds, platypus
Viviparous	Gives birth to live young; embryo develops inside the mother’s body	Most mammals (dogs, cats, horses, humans, whales)

Note: Most mammals are viviparous, but the platypus and echidna are exceptional — they are oviparous mammals (egg-laying mammals).

Comparison of external vs internal fertilisation:

	External Fertilisation	Internal Fertilisation
Location	Outside the body (usually in water)	Inside the female’s body
Number of eggs	Many (thousands)	Few
Parental care	Usually little or none	Usually more parental care
Survival rate of offspring	Low	Higher
Examples	Fish, frogs	Birds, reptiles, mammals

4. Asexual Reproduction

Only ONE parent is needed
No gametes (sperm or egg cells) are involved
Offspring are genetically identical to the parent (clones)
Faster than sexual reproduction — allows rapid increase in numbers
Disadvantage: No genetic variation, so the entire population is vulnerable to the same disease

Types of Asexual Reproduction

Binary fission (in bacteria):

The single-celled organism simply splits into two identical cells
Extremely fast — can happen every 20 minutes under ideal conditions

Budding (in yeast and hydra):

A small bud grows out of the parent organism
The bud eventually detaches and becomes an independent organism

Vegetative propagation (in plants):

A new plant grows from a vegetative part (root, stem, or leaf) of the parent plant
All offspring are genetically identical to the parent
Farmers and gardeners use this to rapidly produce many identical plants

Method	Part used	Example
Runners (stolons)	Horizontal stem that grows along the ground; new plants form at nodes	Strawberry, spider plant
Bulbs	Underground fleshy leaves that store food; new bulb grows from the parent	Onion, garlic, tulip
Tubers	Swollen underground stems that store food; buds (eyes) grow into new plants	Potato, yam
Cuttings	A section of stem is cut and planted; roots grow from the cut end	Rose, hibiscus, sugarcane
Rhizomes	Horizontal underground stems	Ginger, ferns

5. PSLE Key Vocabulary

Term	Definition
Gamete	A reproductive cell — either a sperm cell (male) or an egg cell (female)
Fertilisation	The fusion of a male gamete (sperm) with a female gamete (egg cell) to form a zygote
Zygote	The cell formed immediately after fertilisation; it develops into the offspring
Pollination	The transfer of pollen from the anther to the stigma (NOT the same as fertilisation)
Germination	The process by which a seed begins to grow and develop into a seedling
Ovule	The structure inside the ovary that contains the female gamete; develops into a seed after fertilisation
Ovary	The part of the flower that contains the ovule(s); develops into a fruit after fertilisation
Anther	The part of the stamen that produces pollen (male gametes)
Stigma	The sticky tip of the pistil that receives pollen during pollination
Pollen tube	The tube that grows from a pollen grain through the stigma and style to the ovule
Oviparous	Describes animals that reproduce by laying eggs
Viviparous	Describes animals that give birth to live young
Vegetative propagation	A form of asexual reproduction in plants using roots, stems, or leaves
Self-pollination	Transfer of pollen from the anther to the stigma of the same flower or same plant
Cross-pollination	Transfer of pollen from the anther of one plant to the stigma of a different plant of the same species

Worked Examples

Question 1

“A bee visits flower X and then flower Y of the same species. Explain how this leads to fertilisation in flower Y.”

Model Answer: When the bee visits flower X, pollen grains from the anther of flower X stick to the bee’s body. When the bee then visits flower Y, some of these pollen grains are brushed off onto the stigma of flower Y. This is called cross-pollination.

After pollination, a pollen tube grows from the pollen grain, through the stigma and style of flower Y, towards the ovule in the ovary. The male gamete travels down the pollen tube and fuses with the female gamete (egg cell) in the ovule. This fusion forms a zygote, which then develops into a seed. This process is called fertilisation.

(Award marks for: pollen on bee’s body → transferred to stigma of flower Y → pollen tube grows → male gamete fuses with female gamete → zygote formed)

Question 2

“State two differences between wind-pollinated and insect-pollinated flowers.”

Model Answer:

Feature	Wind-pollinated	Insect-pollinated
Pollen	Light, small, and produced in large quantities	Sticky or spiky, produced in smaller quantities
Petals	Small or absent; no bright colours	Large and brightly coloured
Stigma	Large and feathery (to catch airborne pollen)	Small and sticky (to trap pollen from insect)
Scent/Nectar	No scent; no nectar	Often has sweet scent and nectar

(State any two differences clearly, one for each flower type)

Question 3

“A farmer wants to grow more strawberry plants. Suggest a method of asexual reproduction he could use and explain why he would choose this method over growing from seeds.”

Model Answer: The farmer could use runners (stolons). Strawberry plants naturally produce long horizontal stems (runners) that grow along the ground. When a node on the runner touches the soil, it takes root and grows into a new strawberry plant.

The farmer would choose this method because:

The new plants are genetically identical to the parent plant, so the farmer can be sure the offspring will have the same desirable characteristics (e.g., large, sweet fruits) as the parent
It is faster than growing from seeds — the new plants establish quickly and produce fruit sooner
It is simpler — no need for pollination or waiting for seeds to form and germinate

Question 4

“A seed was placed in moist soil in a dark cupboard. After one week, the seed had germinated and a seedling was growing. Explain why the seed was able to germinate without sunlight.”

Model Answer: The seed does not need sunlight to germinate because it does not photosynthesise at this stage. Instead, the seed uses food stored in the cotyledons (seed leaves) to provide the energy needed for the embryo to grow. The food stored in the seed is broken down through respiration to release energy for germination. Sunlight is only needed once the seedling has developed green leaves and can begin to make its own food through photosynthesis.

Common Mistakes

1. Confusing pollination with fertilisation

Pollination = transfer of pollen from anther to stigma (this is a physical process)
Fertilisation = fusion of male and female gametes inside the ovule (this produces a zygote)
Pollination must happen BEFORE fertilisation can occur

2. Thinking seeds need sunlight to germinate

Seeds germinate using stored food energy — they do NOT need sunlight
Sunlight is only needed after the seedling develops leaves (for photosynthesis)
The three conditions for germination are: water, warmth, and oxygen

3. Forgetting the ovary → fruit, ovule → seed transformation

After fertilisation: ovule develops into seed; ovary develops into fruit
The fruit protects the seeds and often helps in seed dispersal

4. Confusing oviparous and viviparous

Oviparous = egg-laying (O for egg, like Ovum)
Viviparous = live birth (think of “vivid” life being born)
Remember: birds and fish are oviparous; most mammals are viviparous

5. Saying “seeds need food” rather than “seeds have stored food”

The seed contains stored food (in the endosperm or cotyledons)
The seedling does not need an external food source until it can photosynthesise

6. Confusing self-pollination and cross-pollination

Self-pollination: pollen stays on the same plant
Cross-pollination: pollen moves to a different plant of the same species (not a different species!)

7. Forgetting to name the agent of pollination AND explain the adaptation

For seed dispersal questions, always: name the method → name the structural feature → explain how the feature helps dispersal

Exam Tips

Flower diagram: Be able to draw and label a complete flower — anther, filament (together = stamen), stigma, style, ovary, ovule (together = pistil/carpel), petals, and sepals. Know the function of each part.
Pollination vs fertilisation sequence: Always state the correct order — pollination FIRST, then pollen tube grows, then fertilisation occurs.
Seed dispersal answers: Use the format: “The [structure] allows the seed to be dispersed by [agent] because [how the structure helps].” For example: “The coconut has a fibrous husk that traps air, allowing it to float on water and be dispersed far from the parent plant.”
Germination conditions: When asked to state conditions, name all three: water, warmth, and oxygen. Do not include sunlight — it is a common trap.
Asexual reproduction advantages: For structured-answer questions, be ready to explain two advantages: (1) offspring are identical to parent so desirable traits are preserved; (2) it is faster / does not require a mate.
Comparison tables: PSLE often asks you to compare two things (e.g., sexual vs asexual, wind vs insect pollination). Always make the contrast explicit — do not just describe one side.
For “explain” questions: Always link the structure to the function. Simply naming the structure is not enough for full marks.

Quick Summary

Reproduction ensures the continuity of species; it can be sexual (two parents, variation) or asexual (one parent, identical offspring).
Pollination = transfer of pollen from anther to stigma; can be by wind or insects (and other agents). It is NOT fertilisation.
Fertilisation in plants = pollen tube grows from pollen grain to ovule → male gamete fuses with female gamete → zygote forms → seed develops.
Ovule → seed; Ovary → fruit after fertilisation.
Seed dispersal reduces competition; methods include wind (light/winged seeds), water (floating seeds), animals (hooks/fleshy fruits), and self-dispersal (explosive pods).
Germination needs water, warmth, and oxygen — NOT sunlight; the seed uses its own stored food.
Fertilisation in animals can be external (in water, e.g. fish and frogs) or internal (inside female’s body, e.g. birds and mammals).
Oviparous animals lay eggs (fish, frogs, reptiles, birds); viviparous animals give birth to live young (most mammals).
Asexual reproduction involves one parent, produces identical offspring, and is faster than sexual reproduction. Methods in plants include runners, bulbs, tubers, and cuttings.
Key vocabulary to master: gamete, fertilisation, zygote, pollination, germination, ovule, ovary, oviparous, viviparous, vegetative propagation.