PSLE Science · Primary 5 and 6
Life Cycles of Plants and Animals
Every living thing goes through a life cycle. It starts as an egg or a seed, grows, reproduces and eventually dies. In PSLE Science this topic belongs to the Cycles theme. You need to know the stages in the life cycles of common animals and flowering plants, and how flowering plants reproduce through pollination, fertilisation, seed dispersal and germination. Exam questions often show an animal or plant you have never seen, so learn the patterns, not just the examples.
What the syllabus expects
- Describe the stages in the life cycles of animals such as the butterfly, mosquito, beetle, cockroach, frog and chicken
- Compare life cycles with three stages and life cycles with four stages
- Describe the life cycle of a flowering plant from seed to adult plant
- Explain pollination and fertilisation and describe how fruits and seeds form
- Describe how seeds are dispersed by wind, water, animals and splitting, and explain why dispersal is important
- State the conditions needed for germination and explain a fair test that investigates them
Animal life cycles
A life cycle is the series of changes a living thing goes through from the start of its life until it becomes an adult that can reproduce. Some animals have four stages: egg, larva, pupa and adult. The butterfly, mosquito and beetle have this kind of life cycle. The larva of a butterfly is the caterpillar, which eats a lot and grows quickly. In the pupa stage it does not feed, and its body changes completely into the adult. Other animals have three stages. The cockroach and grasshopper go through egg, nymph and adult. A nymph looks like a small adult without fully developed wings. The chicken goes through egg, chick and adult. The frog goes through egg, tadpole, young frog and adult frog. A tadpole lives in water and breathes through gills, while an adult frog breathes through its lungs and its moist skin. In some life cycles the young look like the adult, and in others they look very different.
The life cycle of a flowering plant
A flowering plant goes through the stages seed, seedling and adult plant. The adult plant produces flowers, the flowers develop into fruits containing seeds, and the cycle starts again. A seed has a seed coat that protects it, a young plant (embryo) and one or two seed leaves that store food. Germination is when a seed starts to grow into a seedling. Seeds need water, air and warmth to germinate. Light is not needed for germination, because the young plant uses the food stored in the seed leaves. Once green leaves appear, the seedling makes its own food by photosynthesis, so from then on it needs light to keep growing healthily. As the seedling grows, the seed leaves shrink and drop off because their stored food has been used up.
Pollination, fertilisation and fruits
A flower has male and female parts. The male part is the stamen, made up of the anther and the filament. The anther produces pollen grains, which contain the male reproductive cells. The female part is made up of the stigma, the style and the ovary, and the ovary contains ovules, which contain the female reproductive cells. Pollination is the transfer of pollen grains from the anther to the stigma. Flowers pollinated by insects are usually large, brightly coloured and scented, with nectar to attract insects. Flowers pollinated by wind are usually small and dull, with large, feathery stigmas that hang out of the flower to catch pollen. After pollination, fertilisation takes place when a male reproductive cell from the pollen grain joins with a female reproductive cell in an ovule. After fertilisation, the ovary develops into the fruit and each fertilised ovule develops into a seed.
Seed dispersal
If all the seeds fell just under the parent plant, the young plants would compete with one another and with the parent for space, water, sunlight and mineral salts, and many would not survive. Dispersal carries seeds away from the parent plant, so more of them can survive and the plant can grow in new places. Fruits dispersed by wind are light and have wing-like or hair-like structures, such as the angsana fruit and the dandelion. Fruits dispersed by water, such as the coconut, have a fibrous husk that traps air so they float, and a waterproof outer covering. Fruits dispersed by animals are either juicy and brightly coloured, so animals eat them and pass out the seeds in their droppings, or have hooks that catch on animal fur. Some fruits, such as the balsam, split open when they are ripe and fling their seeds away. This is dispersal by splitting.
Worked example
Mei Ling put 10 green bean seeds on cotton wool in each of three containers. A had dry cotton wool and was placed on a table near a window. B had wet cotton wool and was placed next to A. C had wet cotton wool and was placed in a dark cupboard in the same room. After five days, the seeds in B and C had germinated but those in A had not. (a) Which two set-ups should she compare to find out if water is needed for germination? Explain. (b) What do the results of B and C show? (c) Why did she use 10 seeds in each container instead of one?
- Find the one variable that differs in each pair. A and B differ only in whether there is water. B and C differ only in whether there is light.
- For (a), compare A and B. Everything else is the same (type and number of seeds, cotton wool, light, room), so water is the only variable changed.
- Seeds in B (with water) germinated and seeds in A (without water) did not, so water is needed for germination.
- For (b), compare B and C. Seeds germinated both with light and without light, so light is not needed for germination.
- Give the reason: the young plant uses food stored in the seed leaves, so it does not need light to make food yet.
- For (c), using more seeds makes the results more reliable, because a few seeds might not germinate even when conditions are suitable.
Answer: (a) A and B, because water is the only variable that is different between them. Only the seeds with water germinated, so water is needed. (b) Light is not needed for germination. (c) To make the results more reliable, in case some seeds cannot germinate.
Common mistakes
- Saying seeds need light to germinate. They need water, air and warmth. Light is only needed later, when the leaves start making food.
- Mixing up pollination and fertilisation. Pollination is the transfer of pollen to the stigma. Fertilisation is the joining of male and female reproductive cells in the ovule, and it comes after pollination.
- Saying the ovule becomes the fruit. The ovary becomes the fruit and the fertilised ovules become the seeds.
- Calling a young cockroach a larva. Three-stage life cycles have a nymph. Larva and pupa belong to four-stage life cycles.
- Saying a mosquito spends its whole life in water. The egg, larva and pupa stages are in water, but the adult lives on land and flies.
- Writing 'so that the plant can spread' for dispersal without a reason. Explain that it reduces competition for space, water, sunlight and mineral salts.
Exam tips
- In experiment questions, name the one variable that is different between the two set-ups you compare, and say that everything else was kept the same.
- For dispersal questions, give the feature and how it helps, for example 'the wing-like structure allows the fruit to be carried further by the wind'.
- When you are given an unfamiliar life cycle, count the stages and check whether the young look like the adult before you answer.
Practice questions
1. Easy: Name the four stages in the life cycle of a mosquito. In which stages does it live in water?
Show answer
Egg, larva, pupa and adult. The egg, larva and pupa stages are in water, which is why removing stagnant water stops mosquitoes from breeding.
2. Medium: A fruit contains 6 seeds. How many ovules were fertilised, and which part of the flower developed into the fruit?
Show answer
6 ovules were fertilised, because each seed develops from one fertilised ovule. The ovary developed into the fruit.
3. Harder: Fruit X is light and has many fine hairs. Fruit Y is large, with a waterproof skin and a thick fibrous husk. State how each fruit is likely to be dispersed and explain why dispersal is important to the plant.
Show answer
X is dispersed by wind: it is light and its hairs allow it to be carried far by the wind. Y is dispersed by water: its fibrous husk traps air so it floats, and its waterproof skin stops water getting in. Dispersal carries seeds away from the parent plant, so the young plants compete less for space, water, sunlight and mineral salts and more of them survive.
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