plants move, too

Adapted:PSLE Science Partner A Complete Guide to L&U Block© Singapore Asia Publishers Pte Ltd. All rights reserved.Reproducible for home/classroom use only.STRICTLY NOT FOR SALE.Look for other useful resources: www.sapgrp.com

Plants Move, Too

Plants move too. Their movements are, however, slow, so they are not obvious within a short period of time. We can only see their movements after a period of time.

These are some of the ways in which we will notice that plants move. As a plant grows, it moves upwards, downwards and sideways.

As more leaves grow, the plant becomes broader sideways.

The stem grows upwards so that the plant becomes taller and is able to get more sunlight.

The roots grow downwards into the soil to absorb water and hold the plant firmly to the ground.The growth of a plant

Plants will also tend to move towards the sunlight as they grow. This is seen by the way they bend themselves towards a source of light. This activity of plants bending towards light is called phototropism.

The plant bends towards the light streaming in from the window.

Some plants are sensitive to touch. The leaves of the mimosa plant close by themselves when touched.

leaves are open leaves are closed

The leaves of the mimosa plant display a touch-induced movement.

The leaves of the Venus flytrap plant close when a living thing such as a fly lands on them. In this way, the fly is trapped and cannot escape. It becomes food for the plant.

ciliaa trapped insect

The leaves of the Venus flytrap shuts in less than a second, trapping an insect.

Moving helps plants to

• grow and become bigger and taller, • respond to changes around them e.g. the mimosa plant closes its leaves when they are

touched, and • obtain food as in the case of the Venus flytrap.

Online Resources_Con.Sci.Pro.Skills for PSLE.indd 1 2019/8/28 16:38:36


Plants Grow in Different Places

Plants may grow on land or in water.

Plants that grow on land are known as land plants. The rose plant, angsana tree and lalang are all land plants.

Plants that grow in water are known as water or aquatic plants.

Water / Aquatic plants

Floating plants(floats on the water

surface)

duckweed

water hyacinth

water lettuce

mosquito fern

Partially submerged plants

(roots and part of the plant are underwater but the top half of the plant is

above the water)

cattail

arrowhead

water lotus

water lily

Submerged plants(the whole plant grows

underwater)

cabomba

hydrilla

tape grass



Different Types of Body Coverings

Hair / Fur• They are known as

mammals.• Those living in cold

climates have a thick, dense coat of fur which traps air to keep them warm (e.g. polar bear).

• The fur coat of an animal has colours or patterns. They serve as a camouflage to help the animal blend in with its surroundings.

Feathers• They are known as

birds.• Feathers give a

bird a protective covering which is lightweight, thus enabling it to fly easily.

• The feathers of some birds are coloured for camouflage purposes or to help it attract mates.

Shell• This is a hard

covering which protects an animal’s fragile inner body from sustaining injuries due to an impact (e.g. tortoise, turtle, clam, lobster).

• The animal may hide inside its shell and play dead to escape from its enemies.

• The patterns on the shells serve as a form of camouflage.

Different types of body coverings

Moist skin• The skin is porous.

This means the skin has many tiny holes invisible to the naked eye.

• When the skin is kept wet, dissolved oxygen in the water can enter the animal’s skin to help it to breathe

(e.g. frog).

Exo-skeleton• Animals that

do not have a backbone are called invertebrates (e.g. jellyfish).

• This is actually the skeleton located outside the animal’s body.

Scales• They can be fish / reptiles.• Scales protect the animal’s

body. They do not have the quality to absorb water which will make it too heavy for the animal when it is in the water. Therefore, the animal can swim.

• The scales may have colours or patterns which act as a camouflage or to help the animals attract their respective mates.

• Some reptiles have thick, rough scales to fend off their enemies (e.g. crocodile).



Bacteria

• Bacteria

Many types of bacteria are harmful as they can enter the body and cause illnesses. Bad bacteria are called germs.

There are also good bacteria in our body. They help the body to carry out its functions more effectively.

Cultured milk drinks contain the good bacteria L.Casei Shirota. They help in the digestion of food by killing the bad bacteria present in the digestive tract.

A special type of bacteria is added into milk and boiled. After keeping the mixture warm for several hours, the milk becomes thick and the sugar in it becomes slightly acidic. The acid in the milk prevents any harmful bacteria growing in it. This is the yoghurt that we take when we buy it from the supermarket. The bacteria in yoghurt are good for our digestive system.

• Microscopic aquatic organisms

Amoeba and paramecium are two examples of microscopic creatures that live in the water. Although you will not be able to see these organisms when you look into the pond, they form an integral part of the pond community.

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Exploring Materials

Look at the non-living things around us. They are made of different materials, such as plastics, wood, rubber, glass, fabrics, ceramics and metals.

Different materials have different physical properties. They include:

• strength : A strong material can hold heavy loads without breaking. • flexibility : A flexible material will bend without breaking. • elasticity : An elastic material will stretch when we pull it and return to its original shape

when we let it go. • transparency : A transparent object will allow most light to pass through and we will be

able to see what is inside the object. • waterproof : A waterproof material will not absorb water. • ability to f loat / sink in water : Objects that are lighter than water will float while those that

are heavier than water will sink.

Some materials are natural materials. These materials are made from plant or animal parts, or from substances derived from the ground, such as metal ores.

Some materials are man-made or synthetic.

Sometimes, more than one material may seem to be equally suitable to make an object. For example, a chair can be made of plastic, metal or wood. However, there will be some differences in the three chairs made.

Materials used to make a chair

Plastic Metal Wood

Cheap and lightweight

However, a plastic chair is not as durable as a wood or a metal one, especially for outdoor or long-term use.

Strong

However, it will be heavy and less convenient to move around. If it is made of iron, the chair may rust after long periods of outdoor use.

Strong, but not as strong as metal

Does not rust. Suitable for both indoor and outdoor use.

As you can see, a chair made of either one of the materials has its pros and cons. It is up to the individual to decide which chair suits his / her needs best.

When deciding which material to use to make a certain object, we have to consider the characteristics of the material and whether or not it suits the intended purpose. The table below provides a list of materials, their characteristics, origins and common uses.



Life Cycles of Plants

Flowering plants produce flowers. The flowers later become fruit. Inside the fruit, we can find seeds. The fruit protect the seeds. Seeds can develop into new plants. The life cycle of flowering plants follows the three stages of seed-seedling-adult.

Seed

Stage 1: The plant begins its life as a seed. With enough air, food, water and the right temperature, the seed will begin to germinate or grow.

Seedling (Young plant)

Stage 2: The young plant that grows is known as a seedling.

At first, the seedling obtains its food from the seed leaves.

When the true leaves start to grow, the plant is ready to make its own food by the process of photosynthesis. The seed leaves will shrivel and fall off.

Roots grow deep down into the ground to obtain water and mineral salts needed for the plant.

The plant will grow towards the sunlight.

Adult plant

Stage 3: As the plant grows, it develops flowers which later become fruit.

There are seeds inside the fruit.

The seeds will fall to the ground and develop into new plants. The life cycle then repeats itself.

The shoot grows upwards to obtain maximum sunlight. When the true leaves appear, the plant is ready to make its own food.

Seed leaves provide the plant with food before the true leaves develop.

Roots grow downwards to absorb water and mineral salts from the ground. They hold the plant firmly in the soil.

Life cycle of a flowering plant



The Three States of Matter

Matteris anything that has weight and takes up space.

SOLID• has a definite shape The shape of a solid

will not change unless we do something to it, e.g. moulding a ball of plasticine, folding a piece of paper etc.

• has a definite volume It occupies a fixed

amount of space and cannot be compressed.

• has weight / mass We can find out how

heavy an object is by weighing it on an electronic balance or beam balance.

Matter exists in three different states.

LIQUID• has a definite volume

The plunger cannot be pushed further into the syringe because the water inside the syringe has a definite volume and cannot be compressed.

• has no definite shape

Water takes the shape of the container it is in. The water in each part of the communicating vessel above takes the shape of the container but the water level remains the same throughout.

• has weight / mass We can find out how heavy

a liquid is by weighing the empty container on an electronic balance or beam balance before weighing the container filled with liquid.

The difference in the two readings is the weight of the liquid only.

plunger

water

GAS• has no definite volume

The plunger can be pushed further into the syringe because the air inside the syringe has no definite volume and can be compressed.

• has no definite shape

The air inside the three balloons takes the shape of the balloons.

• has weight / mass

When the balloon on one side bursts, the rod tilts because the balloon that is still filled with air is heavier. This shows that gases have weight.

air



Methods of Dispersal

Fruit and seeds may be scattered (dispersed) in different ways depending on their environment and characteristics.

1 By Wind

• Characteristics

- light and dry - may have wing-like structures so that the seeds / fruit can be easily carried away

by the wind

s

angsana shorea lalang yellow flame

2 By Water

• Characteristics

- has a fibrous husk that traps air, enabling it to float on water and be carried over long distances

- has a waterproof covering

s

coconut pong pong lotus nipah mangrove

3 By Animals

• Characteristics

- may have hook-like structures which hook on to the fur of animals and drop off at another location

- may have thick, juicy flesh which is eaten by animals, leaving the seed behind - some edible fruit may have small, indigestable seeds that are swallowed by animals

and passed out in their waste later

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s

• edible fruit

mango rambutan apple orange

• fruit with hook-like structures

spikelet

mimosa love grass

4 By Explosive Action

• Characteristics

- fruit split open forcefully (explosive action) when they are ripe, scattering the seeds

s

balsam cotton lady’s finger rubber saga flame of the forest

The African tulip fruit have pods that split open, but not forcefully enough for the seeds to be dispersed on their own. The seeds are small and light so that they can be carried away by the wind.

Another example is the kapok.

African tulip and seed kapok

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Factors Affecting Evaporation

How fast evaporation takes place depends on several factors.

Factor Rate of evaporation

1 Temperature Of The Surroundings / Substance

The higher the temperature, the faster the rate of evaporation. Wet clothes tend to dry faster on a sunny day.

A B

The temperature of water in bowl A has increased because of the lamp shining above it. Thus, evaporation takes place at a much faster rate in bowl A than bowl B.

2 Presence Of Wind

If there is wind, the wind blows away the water vapour as it is formed, so more water vapour can rise into the air. The rate of evaporation increases as the wind speed increases

A B

The water vapour is removed from the surface of the water in bowl A faster because of the presence of the fan blowing at it. Thus, evaporation takes place at a much faster rate in bowl A than bowl B.

3 Exposed Surface Area Of Water

Since evaporation takes place from the surface of the liquid, the greater the exposed surface area, the faster the rate of evaporation.

A B

Both containers A and B contain the same volume of water. The water in bowl B is poured out onto the table. With a larger surface area, more heat and wind can assist in the evaporation of the water when the water is on the table. Therefore, evaporation takes place at a much faster rate on the table than in bowl A.

4 Humidity (amount of water vapour in the air)

On a humid day when there is already a lot of water vapour in the surrounding air, evaporation takes place more slowly.

A BBowl A is placed in a container and it is covered with a lid. As the water evaporates into water vapour, it takes up the space inside the container. Soon, there is a lot of water inside the container. This is referred to as humidity. When humidity is high, water is not able to evaporate as quickly as before. Therefore, evaporation takes place at a much faster rate in bowl B than bowl A.



Our Body Systems

Let’s look at the five main systems in the human body.

System Organs Functions

Circulatory system s hearts blood vessels

s food, minerals, water and oxygen are delivered to the parts of the body that require them

s carbon dioxide and waste materials are delivered to the excretory organs to be given out by the body

Digestive system s mouths gullets stomachs small intestines large intestines anus

s breaks down the food we eat into simpler substances that can be absorbed by the body to provide us with energy

Respiratory system s noses mouths windpipes lungss diaphragm

s takes in oxygen which is used by the body for respiration and gives out carbon dioxide and water vapour as a waste product

Skeletal system s all the 206 bones in our body

s supports the body and gives it its shape and structure

s protects the vital organs from injury (the skull protects the brain; the ribcage protects the heart and lungs etc)

s bones contain bone marrow inside; bone marrow helps produce red and white blood cells

s joints between the bones allow movements of body parts

Muscular system s all the muscles in the body

s attached to the bones to enable the body to move



The Process of Photosynthesis

PHOTOSYNTHESIS

The process by which carbon dioxide and water are taken in by plants and converted into glucose and oxygen in the presence of sunlight and chlorophyll.

GLUCOSE

• This is a type of sugar which is food for the plant.

• The glucose made in the leaves is carried to all parts of the plant which need it.

• The excess glucose is stored as starch in other parts of the plant. When plant eaters eat these plant parts, the energy is passed on into their bodies. When animal eaters eat the plant eaters, the energy is then passed on to them.

OXYGEN

Oxygen is a component of air.

Oxygen is required for all living things for breathing.

By giving out oxygen, plants help to purify the air. They provide more oxygen for us to breathe in.

PHOTO = light

SYNTHESIS =putting together

carbon dioxide (from the air)

chlorophyll

+

sunlight water

++



Exhaled Air

Let’s find out if exhaled air contains carbon dioxide.

Limewater can be used to test for the presence of carbon dioxide.

Take a glass and fill it up with some limewater. Place a straw inside it. Then take a deep breath and blow gently into the straw. The limewater turns milky. This goes to show that the carbon dioxide present in the exhaled air turns the limewater chalky.

The limewater turns from clear and colourless to chalky.

Air is blown into the limewater through the straw.

Experiment to show that exhaled air contains carbon dioxide

Let’s find out if exhaled air contains water vapour.

When you place a mirror in front of your mouth and exhale into it, you will notice that water droplets can be seen on the mirror surface. This is because water vapour is given out during exhalation (breathing out). Upon coming into contact with the cool surface of the mirror, the water vapour condenses into tiny water droplets.



Blood Vessels

Blood vessels are rubbery tubes that carry the blood around the body.

The three types of blood vessels are arteries, veins and capillaries.

Blood vessel Function

artery

thick, elastic wall

small lumen

• thick-walled blood vessel• carries blood rich in oxygen from the heart to other parts of

the body (aorta)

There is an artery that carries blood containing little oxygen from the heart to the lungs to pick up more oxygen to be circulated round the body. It is called the pulmonary artery.

capillary

single cell wall

• very thin-walled, narrow blood vessel which connects blood flowing from the arteries to the veins

The thin walls enable• oxygen, digested food and water to pass from the capillary

into the body cells• carbon dioxide and waste materials to pass from the body

cells into the bloodstream to be removed

vein

thin wall

large lumen

• thin-walled blood vessel• carries blood rich in carbon dioxide back to the heart to be

removed (vena cava)

There is a vein that carries blood containing oxygen from the lungs to the heart. It is called the pulmonary vein.



Reproduction of Cells

Cells cannot live forever. Although different cells have different lifespans which range from a few days to a few months, all cells will eventually die.

When cells die, new cells have to replace them.

Cells divide to form new cells.

One cell divides into two separate cells. The two cells in turn divide to form four cells and so on. This is known as cell division. Cell division is necessary to replace new cells when they are destroyed or damaged. It is also necessary so that the organisms can grow.

At first,1 cell

After 1st division,2 cells

After 2nd division,4 cellsand so on

parent cell

Cell division

The newly-divided cells are identical to their parent cells.

Some single-celled organisms such as bacteria and yeast reproduce in a special way. This is known as budding.

bud bud grows bigger

bud separates into a new organism

parent cell

Reproduction by budding The parent cell develops a small bud. The bud grows bigger and bigger until it attains the right size. It then breaks away from its parent and forms a new, identical cell.

Although cells normally divide to form new identical cells, sometimes an abnormal division may take place. The cells may divide uncontrollably to form tumours which can then lead to cancer.



Using Electricity in Different Ways

The brightness of a bulb depends on the amount of electrical current flowing through the circuit. The amount of electric current that is able to flow through the bulbs in a circuit is affected by some ways.

1 Increasing The Number Of Batteries In Series

We can adjust the brightness of a bulb in a circuit by increasing or decreasing the number of batteries used.

To make the bulb light up more brightly, we place more batteries in the electrical circuit. The batteries should be arranged in a series, meaning the positive terminal of one battery touches the negative terminal of another battery. This will increase the amount of electric current flowing through the circuit.

The bulb is bright when one battery is used.

The bulb is brighter when two batteries are used.

The bulb is brightest when three batteries are used.

Batteries connected in a series

Alternatively, to make the bulb light up less brightly, we place fewer batteries in the electrical circuit. The electric current flowing through the circuit decreases, so the bulb does not light up as brightly as before.

However, the bulb will blow when we keep increasing the number of batteries in a circuit. This happens because there is too much electrical current flowing through the circuit. The voltage of the bulb cannot handle the increase in electrical current and the bulb fuses.

2 Increasing The Number Of Bulbs In Series

For bulbs arranged in series in a circuit with a fixed number of batteries, the greater the number of bulbs, the dimmer each of the bulbs will be as the electric current has to be shared between all the bulbs. If one bulb fuses, there will be a gap in the circuit and all the other bulbs will not light up.

The brightness of a bulb is compromised when there are many bulbs in a series.

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3 Arrangement Of Bulbs In Series And In Parallel

When we change the arrangement of the bulbs in a circuit, the brightness of the bulbs will be affected.

When two bulbs are connected in series, there is only one path for electricity to flow through the two bulbs. The amount of electric current that is able to cross each device depends on its resistance of each device and the current flowing through the circuit. When adding more components in a series circuit, the current flow decreases. Therefore, the bulbs light up but not so brightly.

A series circuit with one battery and two bulbs

In a series circuit, if a bulb fuses or a component is disconnected or damaged, there is a gap in the circuit. It becomes an open circuit, disrupting the flow of electric current.

In a parallel circuit, all the components have access to the full amount of electrical current available. If any one of the components of a parallel circuit stops working, the other components will still be able to carry on working. The bulbs stay bright even if more bulbs are added in parallel.

In a parallel circuit, each bulb is directly connected to the power source. Therefore, the bulbs arranged in parallel light up more brightly than when they are arranged in series.

A parallel circuit with one battery and two bulbs

4 Arrangement Of Batteries Connected In Parallel

When the batteries are arranged in parallel, the bulbs in all three circuits will be of the same brightness. This is because the current flows through one loop at a time until the chemical potential in the battery has been used up before flowing through the next loop to make use of current from the second battery.

same brightnesssame brightness same brightness

Batteries connected in a parallel

However, the bulb in the third circuit shown above will remain lit three times as long as the bulb in the first circuit because it has three batteries to obtain its power from.

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Can Magnetism Pass Through Everything?

The magnetic force (or magnetism) can pass through thin sheets of non-magnetic objects such as paper, glass or wood. However, if the magnet is too weak and the material is too thick, the magnetic force may not be able to pass through.

By putting a magnet on the outside surface of the glass tank filled with oil and slowly moving the magnet upwards, you can guide the steel ball bearing that is in the tank of oil upwards until it is above the surface of the water. The magnetic force is able to pass through the glass and attract the steel ball bearing on the other side.

In this way, the steel ball bearing can be removed from the tank without getting your hand oily.

glass tank filled with oil

steel ball bearing

magnet

N S

Removing a steel ball bearing from a glass tank using a magnet

If the same steel ball bearing is dropped into an iron tank filled with oil, the magnet sticks to the iron tank. This is because iron is a magnetic material.

The steel ball bearing cannot be attracted to the magnet.

iron tank filled with oil

steel ball bearing

magnet

NS

The steel ball bearing cannot be removed from an iron tank using a magnet.



Effects of a Force

When a force acts on an object, one or more of the following situations can happen to the object.

1 A Force Can Cause A Stationary Object To Start Moving

A goalkeeper places the soccer ball on the ground. Then he moves a few steps back before running towards it and giving it a hard kick. The soccer ball moves by flying through the air.

2 A Force Can Stop A Moving Object

A soccer player who sees the soccer ball rolling towards him can use his foot to stop it.

A goalkeeper jumps to catch the soccer ball to prevent it from entering the net.

A goalkeeper makes a brilliant save.

3 A Force Can Cause A Moving Object To Move Faster Or Slower

While kicking the soccer ball to make it move, a footballer can choose to kick it with more force to make it move faster, or to kick it with less force so that its speed is reduced.

4 A Force Can Cause A Moving Object To Change Its Direction Of Motion

When the soccer ball moves, it travels in a certain direction. As the ball approaches a football player, he can decide to kick it in a different direction so that a player from the opposing team does not get the soccer ball. He applies a force to change the initial direction of the football.

The direction of the ball is changed.

5 A Force Can Cause An Object To Bend, Change Its Shape Or Break

A blacksmith heats a piece of steel to a high temperature (between 649 – 816°C). When he removes it from the super-hot oven called a forge, he hammers along the edge of the steel so that it curves to form the blade of the sword. If the blacksmith removes the steel when it is not yet hot enough (it appears bluish in colour), the steel can shatter upon hammering.

A force is used to bend an object.

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A baker has to knead the dough by pressing, pushing and pulling it. Then he shapes the dough into different shapes before baking them in the oven.

A force is used to change the object’s shape.

A plastic ruler can be bent by using force. Its shape changes. However, when too much force is applied, the plastic ruler breaks into two.

A force is used to break an object.

Destructive Forces

Although forces can help us to do work such as cutting fruit or pushing the trolley in the supermarket, there are some forces which can harm us.

The force of a hurricane can uproot trees and damage houses.

A house is damaged when a tree is uprooted.

In 2004, a massive underwater earthquake off the coast of Sumatra generated massive tsunami waves. It inflicted massive damage along the coastal lands of ten countries including Indonesia and Sri Lanka. More than 280,000 people were killed in the natural disaster.

People running for their lives away from the tsunami

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Factors Affecting the Environment

1 Temperature

The higher the temperature, the greater the amount of water lost by the organisms. Losing too much water may result in plants wilting or animals dying from dehydration.

Different organisms are suited to survive at different temperatures. For example, plants in a tropical climate like Singapore thrive in a temperature of about 30°C while some plants grown in colder climates thrive at much lower temperatures.

The amount of light and the temperature of the environment may be interrelated since the greater the amount of sunlight received by the habitat, the hotter the environment will be.

The temperature affects the availability of water since on a hot day, more water will evaporate to form water vapour.

The temperature affects the type of soil present. On a hot day, the soil will be drier since there is less water in the soil.

The temperature affects the availability of food since temperature will have an impact on plant growth. This affects both plant and animal eaters. If there were fewer plants, there would be fewer plant eaters, resulting in less food for the animal eaters as well.

2 Presence Of Other Organisms In The Habitat

Advantages:

• Plants provide food and shelter for the other organisms.

• Decomposers help to break down dead organisms and return them back to the soil to enrich it.

• Underground animals such as earthworms help to loosen the soil to introduce air so as to make the soil more absorbent to nutrients. This leads to healthier and more abundant plant growth.

• Some animals may become the prey of other animals.

Disadvantages:

Too many organisms living in the same habitat may lead to overcrowding and competition for neccessities such as air, food, water and even sunlight.

3 Amount Of Air

Plants and animals require oxygen for respiration.

Green plants require carbon dioxide for photosynthesis. If carbon dioxide is limited, the plants will not be able to make food and will eventually die.

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4 Availability Of Food

All living things need food to carry out essential life processes for survival.

Ideally, the habitat should have sufficient food for the inhabitants.

5 Availability Of Water

Depending on the amount of water an organism needs for its survival, it will seek out a habitat that provides sufficient water for its needs.

6 Amount Of Light Available

Green plants need light to photosynthesise.

Some animals prefer to live in places with ample sunlight while others like to live in dark places, such as in a leaf litter. The darkness may serve to help the prey to hide from its predator.

7 Type Of Soil

Plants survive well in rich, fertile soil. Without soil, the roots of the plants cannot be anchored firmly in the ground and the plants will die.

If the soil does not contain sufficient water and nutrients for the plant, it will not grow well and may die.

Earthworms and some insects also survive in the soil, especially soil rich in the remains of dead organisms.

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Changes to Population Size of an Organism

The population size of an organism can either increase or decrease due to many reasons.

Factors That Lead To A Decrease In The Population Size

1 An Increase In The Number Of Predators

When the number of predators increase, there is an increase in the demand for food. Hence, the population size of the prey will decrease.

2 A Change In The Surroundings

When environmental conditions become unfavourable for members of the population to survive, its population size will decrease.

• changes in weather (drought, floods, typhoons, hurricanes, storms, heat waves etc.) • changes caused by natural disasters (e.g. earthquakes, forest fires, landslides due to

heavy rains, eruptions of nearby volcanoes etc.) • changes caused by gradual climate change (e.g. global warming leading to the melting

of the polar ice caps and raising of the world’s temperatures in the future) • changes caused by man (e.g. deforestation, deliberate starting of forest fires, pollution)

3 An Outbreak Of Disease

Diseases can affect the population size of an organism. If the organisms succumb to illnesses especially in an epidemic, the decrease in its population size is substantial.

Factors That Lead To An Increase In Population Size

1 A Decrease In The Number Of Predators

When the number of predators decrease, the number of prey increases because there are fewer predators to eat them.

2 Presence Of A New Population Of Prey

When another population of prey moves in, it gives the predators more choice in terms of its food as there is an increase in an overall food supply for them.

3 Changes In Weather Conditions

When the surroundings become more favourable for survival and reproduction, the population size of an organism increases.

4 An Increase In The Amount Of Food Available

Where there is the availablity of food, the population size of an organism will naturally increase.

The food relationship in the real world is more complicated than what we see in a food chain.

Since many animals have more than one source of food, food chains can be interconnected to show how a group of organisms depend on each other to survive.



Adaptations of Aquatic Animals

All organisms need oxygen to survive. Without oxygen, organisms will die. Aquatic organisms have adapted themselves to a life in the water by various means. They take in dissolved oxygen that is in the water or come up to the surface of the water to take in air.

Adaptations For Breathing In The Water

• Gills

Fish and other aquatic creatures like prawns, crabs, mussel and tadpoles have gills to help them breathe under water.

gills

gills

water flows over the gills, then out of the fish

water flows in through the mouth

gill

Gills of a fish Gills of a prawn The mussel breathes

using its gills.

Insects like the damselfly nymphs, stonefly nymphs and mayfly nymphs have gills which enable them to live at the bottom of streams and ponds. Stonefly nymphs have gills on their thorax and mayfly nymphs have gills on their abdomen.

A stonefly nymph A mayfly nymph

• Gill chambers

Some aquatic organisms do not stay in the water all the time. They come out of the water and move around on land but only for short periods of time. In addition to gills for them to breathe under water, they also have gill chambers.

A crab is one aquatic organism that has a gill chamber. A gill chamber is a special space in its body that allows it to store water. The filaments of its gills dangle down into the base of the chamber into the pool of water. Therefore, as long as the crab keeps its gill filaments moist, it is able to extract oxygen from the surrounding air in the gill chamber.

gills

A crab and its gill chamber

A mudskipper also has a gill chamber because it does not stay underwater indefinitely. It comes out of the water to move around on land. In the same way as the crab, it does

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not move far away from a water source. This is because it needs to regularly keep its gill chamber with water.

A mudskipper on land

• Breathing tubes

Some aquatic insects take in oxygen using a breathing tube (also called a siphon) found at the end of their abdomens. They live just below the water surface. They stick their breathing tubes out through the surface of the water to take in oxygen from the surroundings.

Examples of such insects are the young of the mosquito (the larva / wriggler and the pupa), the water stick insect and the water scorpion.

A wriggler comes to the surface of the water and pushes its breathing tube out to get air.

A water stick insect A water scorpion

• Air bubbles

The great diving beetle and the water spider carry an air bubble on their body. This ‘mini-oxygen tank’ helps them to breathe even while under water.

layer of air

A water spider getting air from the surface so that it can live underwater.

A water spider uses its legs to scrape off the air bubble on its back.

air bubble

• Special nostrils

Some mammals that live in the water have special nostrils that allow them to breathe in air. They have to come up to the surface of the water to take a breath.

When manatees come up to the surface of the water, they exhale very hard. This allows them to breathe in, changing up to 90% of the air in their lungs. They are then able to stay underwater for longer periods of time between breaths.

2



A manatee can only breathe through its nostrils so it has a special structural adaptation in the form of a flap that covers tightly over its nostrils when it is underwater.

A manatee under water

When seals and sea lions are underwater, their nostrils are shut automatically.

A seal swims underwater. A sea lion dives underwater after taking in air.

• Blowholes

Dolphins, whales and porpoises have blow-holes at the top of the heads. When they need to breathe, they come up to the surface of the water, expel the carbon dioxide and take in a breath of fresh air.

blow-holeblow-hole

The blow-holes are found at the top of the heads of the porpoise and the dolphin.

• Skin

Some aquatic animals breathe using their skin.

Some salamanders have no lungs or gills. Instead, they breathe through their moist skins.

A frog can live on land and in water. An extensive number of blood vessels can be found throughout the frog’s skin. Dissolved oxygen passes through its membranous skin and directly enters into the bloodstream when the frog is in the water.

3



Depletion of the Ozone Layer

release CFCs (chlorofluorocarbons) into the atmosphere

UV rays from the sun are unable to pass through the ozone layer to reach the earth.

• photo-damaged skin (skin damaged by prolonged exposure)• eye damage like cataract (caused by exposure of the retina in the eyes to

UV rays)• skin cancer

The ozone is a protective barrier located above the earth to prevent harmful UV (ultraviolet) rays from reaching the earth.

a ‘hole’ in the ozone layer

harmful UV rays• aerosol sprays• refrigerator coolants• burning of styrofoam

products

escaping radiation

radiation absorbed by greenhouse gases

The ‘hole’ in the ozone layer above the Antarctic is the result of extensive pollution since the 1980s. Many people call it the ‘ozone hole’ to indicate that the ozone layer is thinner and depleted in that area more than the other areas. Do not imagine it as a real hole in the ozone layer because the ozone layer is a gas and a hole cannot be formed in a state like gas.



How We See Objects

The moon does not give out light of its own. We can see the moon because light rays from the sun which shine on the moon are reflected off the surface of the moon into our eyes.

In the same way, a boy in a dark room is able to see the flower because the light from the torch which shines on the flower is ref lected by the flower into his eyes.

A boy has object M. When he shines a torch at object M, a bright spot is seen on the wall. This shows that object M must have reflected the light that was shone on it onto the wall. Object M is a mirror.

Smooth, shiny surfaces are able to reflect light very well. When you look into a mirror, you are able to see a clear image of yourself. The mirror reflects almost all the light that shines on it back into your eyes in a regular manner.

Barry is behind Andy. However, Andy is able to see Barry’s reflection in the mirror and know that he is there. This is because light from the sun falls onto Barry (Ray A) and is reflected into the mirror (Ray B). The mirror then reflects the light into Andy’s eyes (Ray C).

earth

moon

Light rays from the moon are reflected into the man’s eyes on earth.

sun

Light rays from the sun shine on the moon.

Light rays from a torch shine on the flower.

Light rays from the flower are reflected into the boy’s eyes so that he can see it.

Light rays from object M is reflected onto the wall. It is seen as a bright patch.

Light rays from the torch shine on object M.

Object M

Andy

Barry

Ray C

Ray B

Ray A



Our Sense of Touch — An AccurateJudge of Temperature?

1

Let’s find out if our sense of touch is accurate in judging temperature.

Step 1

Place your left hand in a container of hot water and your right hand in a container of cold water.

cold water(5°C)

hot water(50°C)

lefthand

righthandBrrr ... this

feels COLD!

Ooh ... thisfeels HOT!

We can use our hands to feel whether the water is hot or cold.

Step 2

Take out both hands and immediately place them into a third container which contains tap water at room temperature.

tap water(25°C)

righthand

lefthandNow, this feels

COLD!

No, it feels HOT!

Interestingly, the left hand (which was previously in hot water) now feels cold and the right hand (which was previously in cold water) now feels warm!

Both hands which are now in the SAME container of tap water are feeling different degrees of heat.

From the results of this simple demonstration, we can conclude that our sense of touch is NOT an accurate judge of temperature.



Explanation

Before placing both hands into the container of tap water, the left hand was already feeling hot while the right hand was already feeling cold.

Our skin is sensitive to changes in the temperature of the surroundings.

To the hand that was already feeling cold, subjecting it to the sudden increase in temperature (from 5°C to 25°C) makes it feel hot.

The feeling of hotness comes from the experience of a rise in temperature of the water as compared to the previous case.

The nerves in the right hand sends signals to the brain that the water is now HOTTER than it was previously, hence, the feeling of hotness.

tap water(25°C)

righthand

No, it feels HOT!

To the hand that was already feeling hot, subjecting it to the sudden decrease in temperature (from 50°C to 25°C) makes the hand feel cold.

The cold feeling comes from the experience of a fall in temperature of the water as compared to the previous case.

The nerves in the left hand sends signals to the brain that the water is now COLDER than previously, hence, the feeling of coldness.

tap water(25°C)

lefthand

Now, this feels COLD!

In conclusion, although our sense of touch can give us a rough indication of whether an object is hot or cold, we cannot rely totally on our sense of touch to accurately gauge the temperature of an object.

A similar experiment can be carried out by holding a mouthful of iced water in your mouth for 30 seconds. After that, swallow the water and drink some water at room temperature. You will feel the water at room temperature seems hotter than the usual tap water. The explanation for this effect is the same as that in the experiment above.

In order to give an accurate measurement of how hot or cold something is, we can use a thermometer.

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Using Energy from Moving Water

Hydroelectric power stations are built near water sources like large rivers with a high volume of water.

Moving water has kinetic energy. But moving water that is forced by gravity to flow downwards has even more kinetic energy.

In a hydroelectric power station, moving water moves through a narrow tube called a penstock. The downward movement of water passes through the blades of the turbine, turning them.

The turbines are connected to the generator. The generator generates electrical energy that is sent to homes and factories.

dam

reservoir

penstock

power line

generator

turbine

outflow

The inside workings of a hydroelectric station

The ocean tides provide a regular source of power generation. The movement of water flowing in and out of the coastal region can turn turbines.

Wave control stations are built into a rock face on the shoreline. It has an opening that allows seawater to freely enter and leave a chamber.

The level of water rises inside the chamber when seawater enters it. The air at the top of the chamber becomes compressed when water enters the chamber.

The compressed air is then forced through a hole. At the end of the hole are the turbines. The movement of the air turns the turbines.

When the waves in the sea outside draw back, the water inside the chamber recedes too. The air is sucked back under pressure into the chamber. The constant movement of water results in a constant stream of air in both directions. This produces enough movement in the turbines to drive a generator.

air is forced through the hole and out

Air is compressed and decompressed as the seawater enters and draws back.waves

rock face

Seawater enters the chamber.

The inside workings of a wave power station


plants move, too

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