SCIENCE
REVISION
The contents on this page will be especially helpful for those who are confused or need extra revision for your class lessons.
EXPLANATIONS IN PRIMARY SCIENCE
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Read more about the scientific processes in Primary Science and deepen your understanding.
The processes covered on this page are:
Photosynthesis, Respiration
Pollination, Fertilisation, Germination, Seed dispersal
Evaporation, Condensation, Boiling, Freezing, Melting
As you read about the processes, bear these questions in mind.
1) What is it about?
2) When does it occur?
3) What does it require?
Not all of the questions will be applicable for all of the processes.
Photosynthesis
Photosynthesis is the process where plants make food for themselves in the presence of light. Chlorophyll present in the plant absorbs light energy and uses this energy to convert carbon dioxide and water into food (sugar) for the plant, giving off oxygen at the same time.
Carbon dioxide + water -> Sugar (food) + oxygen
Important points:
1) Photosynthesis takes place only when there is light (hence it occurs only during the day).
2) Oxygen is produced and given off.
3) Food produced in the leaves is transported to all parts of the plant through the food carrying tubes.
4) Water needed for photosynthesis is absorbed by the roots and transported to the leaves.
5) Carbon dioxide needed for photosynthesis enters through the stomata in the leaves.
Respiration
Respiration is the process where living things produce energy to carry out life processes (various activities that the living things do). It occurs in the cells of the living things and takes place throughout the whole living thing where there are live cells.
During respiration, sugar (glucose) and oxygen is converted to energy and carbon dioxide and water are given off.
Sugar (glucose) + oxygen -> energy + carbon dioxide + water
Important points:
1) Respiration occurs all the time.
2) When more energy is needed for activities, more oxygen and sugar needs to be supplied to the cells to produce more energy. Hence the breathing rate of an organism needs to increase to take in more oxygen and the heart rate also needs to increase to pump more oxygen and sugar to all parts of the body to produce more energy.
FERTILIZATION
Fertilisation occurs when a male sex cell fuses with a female sex cell.
In humans (and other animals), the sperm fuses with the egg (ovule).
For plants, the male sex cell present in the pollen grain fuses with an ovule in the ovary.
Important point:
1) After fertilisation, the fertilised egg will develop into a baby for humans. For plants, the fertilised ovules will develop into seeds while the ovary will develop into a fruit.
GERMINATION
Germination is the process where the seed develops into a young plant. For a seed to germinate, warmth, oxygen (air) and water is required.
POLLINATION
Pollination refers to the transfer of pollen grains from the anther to the stigma of a flower.
There are two methods of pollination, cross pollination and self pollination.
Cross pollination occurs when the pollen grains of one flower is transferred to the stigma of another flower.
Self pollination occurs when the pollen grains of a flower is transferred to the stigma of the same flower.
Agents of pollination are things that help flowers to transfer the pollen grains.
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The agents of pollination that you learnt are:
1) Wind
2) Animals
3) Humans
Pollen grains are small and light so that they can be carried by the wind or by insects.
Seed Dispersal
Seed dispersal refers to how plants ensure that seeds are dispersed away from the parent plant. Seed dispersal is important to avoid overcrowding so that the young plants do not need to compete for sunlight, space, water and nutrients.
There are four methods of seed dispersal.
1) Wind
2) Water
3) Animals
4) Splitting
Characteristics of fruits dispersed by each of the four methods.
By Wind: 1) light,
2) has wing light structure or hairs
By Water: 1) water proof outer covering
2) fibrous husk (traps air allowing the fruit to float)
By Splitting: 1) lines of weakness on fruit
By Animals:
Type A: (fruits are attached to the fur of animals and transported to other places)
1) has hooks
2) light
Type B: (fruits that are eaten by animals)
1) juicy and fleshy
note: The seeds can be either big or small. If it is small, the animals typically eat the whole fruit and pass out the indigestible seeds in their faeces. If the seeds are big, the animals will throw away the seeds after eating the flesh.
EVAPORATION
Evaporation occurs only at the surface of a liquid. During evaporation, a substance gains heat energy from its surroundings and changes from a liquid to a gas.
Important points:
1) Heat energy is gained by a liquid during evaporation.
2) Evaporation happens all the time (as long as the substance is in liquid state).
3) The factors that affect rate of evaporation are:
a) exposed surface area
b) temperature
c) amount of wind
BOILING
When a substance boils, it changes from a liquid to a gaseous state. Boiling occurs throughout the liquid and bubbles of gas are formed in the liquid. The temperature when a substance boils is called its boiling point.
Important points:
1) Boiling occurs at a specific temperature for different substances. For pure water, it is at 100 oC.
2) The boiling point of a substance is the highest temperature that substance can exist in its liquid state.
3) Heat energy is gained during boiling.
Condensation
Condensation occurs when a gas changes into a liquid after losing heat energy when it comes into contact with a cold surface.
Important points:
1) Heat energy is lost by the gas (eg water vapour) to a cold surface for condensation to occur.
2) For water, during condensation, tiny water droplets are formed.
3) Clouds are tiny water droplets that have condensed in the cold air at high altitudes.
4) The "white cloud" above a pot of boiling water is made of tiny water droplets that have condensed from the hot water vapour coming out of the water.
FREEZING
Freezing occurs when a liquid changes into a solid.
The temperature when this occurs is called the freezing point of a substance. For pure water, it occurs at 0 oC.
Important points:
1) Heat energy is lost by a liquid for it to change into the solid state.
MELTING
Melting occurs when a solid changes into a liquid.
The temperature when this occurs is called the melting point of a substance. For pure water, it occurs at 0 oC.
Important points:
1) Heat energy is gained by a solid for it to change into the liquid state.
