Plants — Photosynthesis & Reproduction
Photosynthesis equation, leaf structure, pollination, seed dispersal — with interactive PSLE-style questions and model answers.
Photosynthesis — The Big Picture
Plants are the only organisms that can make their own food using sunlight. This process is called photosynthesis and takes place mainly in the leaves, inside structures called chloroplasts.
| What goes IN (Reactants) | Where it comes from | What comes OUT (Products) | What happens to it |
|---|---|---|---|
| Carbon dioxide (CO₂) | From the air through stomata (tiny pores on leaves) | Glucose | Stored as starch OR used for energy by the plant |
| Water (H₂O) | Absorbed by roots from soil; transported up by xylem | Oxygen (O₂) | Released into the air through stomata — the oxygen we breathe! |
| Light energy | From sunlight, absorbed by chlorophyll in chloroplasts |
Parts of a Leaf & Their Roles
| Leaf Part | Function | PSLE Link |
|---|---|---|
| Chloroplasts | Contain chlorophyll — the green pigment that absorbs light energy for photosynthesis | More chloroplasts = more photosynthesis |
| Stomata | Tiny pores (mostly on underside of leaf). Let CO₂ in; let O₂ and water vapour out | Open in light, close in dark to save water |
| Palisade cells | Long, tightly packed cells near the top of the leaf; full of chloroplasts — main site of photosynthesis | Near top for maximum light exposure |
| Xylem | Transports water and dissolved minerals UP from roots to leaves | Water is a reactant for photosynthesis |
| Phloem | Transports glucose (food) made in leaves DOWN to the rest of the plant | Glucose is a product of photosynthesis |
| Spongy layer | Loosely packed cells with air spaces — allows CO₂ and O₂ to diffuse between cells and reach stomata | Air spaces needed for gas exchange |
Reproduction — Pollination to Germination
| Stage | What Happens | Singapore Example |
|---|---|---|
| Pollination | Pollen transferred from anther (male) to stigma (female) — same or different flower | Bees pollinate bougainvillea in HDB gardens |
| Fertilisation | Pollen tube grows down style → pollen nucleus fuses with ovule → fertilised egg (zygote) | Occurs inside the flower after pollination |
| Seed & Fruit formation | Ovule → seed; Ovary → fruit (fruit protects and aids dispersal) | Chiku fruit, mango, rambutans |
| Seed dispersal | Seeds spread away from parent plant to reduce competition | Angsana seeds spin in wind; sea beans float on sea |
| Germination | Seed sprouts into seedling with water, warmth, and oxygen (NOT light initially) | Bean seeds germinating in a pot of soil |
Fill in the Blanks
Word Bank: photosynthesis · chlorophyll · chloroplasts · stomata · glucose · oxygen · pollination · fertilisation · dispersal · germination · xylem · phloem
1. The process by which plants make their own food using sunlight is called .
2. The green pigment inside chloroplasts that absorbs light energy is called .
3. Photosynthesis takes place inside structures in leaf cells called .
4. Carbon dioxide enters and oxygen leaves the leaf through tiny pores called .
5. The two products (outputs) of photosynthesis are (a sugar) and (released into the air).
6. The transfer of pollen from an anther to a stigma is called .
7. When a pollen nucleus fuses with an ovule, occurs, producing a fertilised egg that develops into a seed.
8. Seeds being carried away from the parent plant by wind, water, or animals is called seed . This reduces competition between parent and offspring.
9. When a seed sprouts and begins to grow into a new plant, this is called .
10. Water is transported from the roots up to the leaves through tubes called . Glucose is transported from the leaves to the rest of the plant through .
True or False
Multiple Choice Questions
1. A plant is placed in a dark cupboard for one week. Which of the following would you observe?
2. The Angsana tree (found along Orchard Road) has seeds with wing-like structures that spin as they fall. What type of seed dispersal is this?
3. Which part of the flower directly produces pollen?
4. Which of these correctly describes why dispersal of seeds is important to a plant?
5. A scientist tests two leaves from the same plant — Leaf A was kept in bright light for 6 hours; Leaf B was kept in darkness for 6 hours. She then tests both leaves for starch using iodine solution. What result would she observe?
Flower Parts & Pollination
| Feature | Insect-Pollinated Flower | Wind-Pollinated Flower |
|---|---|---|
| Petals | Large, brightly coloured, scented — to attract insects | Small, dull, no scent — insects not needed |
| Pollen | Sticky and heavy — clings to insects' bodies | Light, smooth, produced in huge quantities — carried by wind |
| Stigma | Small, sticky — inside flower to receive pollen from insects | Large, feathery, hanging outside flower — catches pollen from air |
| Nectar | Produced — rewards insects for visiting | None — no need to attract insects |
| Singapore examples | Bougainvillea, orchid, hibiscus | Grass, coconut palm, maize/corn |
Open-Ended Questions (PSLE Style)
Q1. [2 marks] A plant is put in a dark cupboard for one week. Explain what happens to photosynthesis and what you would observe if you tested a leaf for starch at the end of the week.
If we test the leaf with iodine solution, it would remain brown/orange (negative starch test) — showing no starch is present. The leaf may also turn yellow as chlorophyll breaks down without light.
Q2. [3 marks] A dandelion seed has a feathery 'parachute' attached to it. (a) What type of seed dispersal is this? (b) Explain how the parachute helps dispersal. (c) Why is dispersal important for the plant?
(b) The feathery parachute (pappus) is very light and has a large surface area. When the seed falls, the parachute catches the air and slows the seed's descent, keeping it airborne for longer. Wind can then carry the lightweight seed over long distances away from the parent plant.
(c) Seed dispersal is important because it reduces competition between the parent plant and its offspring (and between seeds themselves) for sunlight, water, minerals, and growing space. Seeds that land far from the parent have a better chance of finding adequate resources and surviving.
Q3. [2 marks] Compare an insect-pollinated flower and a wind-pollinated flower. Give TWO differences and explain why each difference exists.
2. Pollen: Insect-pollinated flower pollen is sticky and heavy so it clings to the body of visiting insects and is transferred from flower to flower. Wind-pollinated flower pollen is light, smooth, and produced in enormous quantities so it can float easily in the air and reach other flowers by chance.
Q4. [Extended] [4 marks] Describe the complete journey of reproduction in a flowering plant, from pollination to a new seedling. Use the words: pollination, pollen tube, fertilisation, seed, fruit, dispersal, germination.
2. Pollen tube: The pollen grain germinates on the stigma and grows a pollen tube down through the style to the ovary, where it reaches the ovule.
3. Fertilisation: The male nucleus from the pollen travels down the pollen tube and fuses with the female nucleus in the ovule. This forms a fertilised egg (zygote).
4. Seed and Fruit: The fertilised ovule develops into a seed. The surrounding ovary develops into a fruit, which protects the seed and aids dispersal.
5. Dispersal: The fruit/seed is carried away from the parent plant by wind, water, animals, or self-dispersal mechanisms. This reduces competition between parent and offspring.
6. Germination: Given the right conditions (water, warmth, oxygen), the seed sprouts and grows into a seedling, starting the life cycle again.
Key Facts to Remember
- Photosynthesis: CO₂ + H₂O → Glucose + O₂ (needs light energy and chlorophyll)
- Reactants: carbon dioxide (from air via stomata) + water (from soil via roots/xylem) + light energy
- Products: glucose (stored as starch) + oxygen (released — what we breathe!)
- Plants do BOTH photosynthesis (day) AND respiration (day and night)
- Starch test: iodine turns blue-black if starch present (leaf was in light)
- Male = Anther (pollen) + Filament = Stamen. Female = Stigma + Style + Ovary + Ovule = Pistil
- Ovule → seed; Ovary → fruit after fertilisation
- Seed dispersal reduces competition between parent and offspring
- Germination needs water, warmth, oxygen — NOT light (initially)
- Wind-pollinated: light pollen, large feathery stigma, no petals/nectar. Insect-pollinated: sticky pollen, colourful petals, nectar
Great Work! 🌱
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