Year 8 Science Notes - Wikispaceshchomework.wikispaces.com/file/view/Year+8+Science...Huanui College Year 8 Science Page 1 of 15 Year 8 Science Notes Biology Plants Plant Reproduction

Huanui College Year 8 Science

Page 1 of 15

Year 8 Science Notes

Biology Plants Plant Reproduction

Sex cells are produced by the reproductive organs. In plants, these are contained inside flowers. Sex cells are used for sexual

reproduction which needs two parents. The offspring from sexual reproduction are different from the parents; they are new

varieties.

The pollen grains need to be carried to the stigma of another flower. They can be carried by insects or the wind. The carrying of

pollen from an anther to a stigma is called pollination.

Once on the stigma, a pollen grain grows a pollen tube which enters the ovule containing an egg cell. The nucleus from the pollen

grain then joins with the nucleus inside the egg cell. This is called fertilisation.

Photosynthesis

This is a chemical reaction and so can be written as a word equation:

water + carbon dioxide (+ light energy) glucose + oxygen

Light energy and chlorophyll are needed for photosynthesis to happen. The light energy is changed into chemical energy which is

stored in the glucose that is made.

Getting the water

Water is taken out of the soil by the roots.

Roots are branched and spread out to help them

absorb water from a large volume of soil. They

also have root hair cells which are adapted to

their function – they have a large surface area to

help them absorb water quickly. The water

flows up xylem tubes (made of hollow cells) to

the leaf.


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Water is also needed because mineral salts are dissolved in it, which are needed to keep plants healthy. Water also stops plants

wilting and can keep their leaves cool.

Getting the carbon dioxide

Air, containing carbon dioxide, diffuses

into leaves through small holes called

stomata. Leaves are thin so that the

carbon dioxide does not need to go very

far before reaching the cells that need it.

Photosynthesis can often be speeded up by

increasing the amount of carbon dioxide

around a plant.

Getting the light

Many leaves are wide so that they have a big surface area to trap as much sunlight as possible. Most photosynthesis happens in the

palisade cells which are found near the upper surface of leaves. Palisade cells are packed with chloroplasts. Chloroplasts contain

chlorophyll which absorbs light energy. Photosynthesis can often be speeded up by increasing the amount of light.

Respiration Plant cells release the energy stored in glucose using aerobic respiration (another chemical reaction):

glucose + oxygen carbon dioxide + water (+ energy)

All living cells need energy and so all living cells respire. Respiration happens all the time but photosynthesis can only happen

when there is light.

Uses of glucose

Glucose is a type of sugar. It is used for three things:

• respiration

• making other substances that act as stores of energy (eg starch)

• making new materials for growth.

Glucose is turned into cellulose (for cell walls), fats and proteins. To make proteins, mineral salts called nitrates are needed.

New substances made by a plant are carried around the plant in phloem tubes. New substances help to build up a plant’s biomass

(the mass of all the materials in the plant except water).

Humans as Organisms

Food and digestion

We need to eat a wide variety of foods to provide our bodies with all the substances that are needed. When we do this, we are said

to have a balanced diet.

Substance needed Examples Why it’s needed Good sources

carbohydrate starch, sugars for respiration to release energy pasta, bread

protein for growth and repair meat, beans

vitamins vitamin C for health fruits and vegetables – oranges

contain a lot vitamin C

minerals calcium for health fruits, vegetables and dairy products

– milk contains a lot of calcium

fibre for health; helps to keep our

intestines clean stop them getting

blocked up (constipation)

wholemeal bread

water for health; water is important

solvent in the body

We can do tests to find out which substances are in foods. For example, starch makes iodine solution go a blue–black colour.


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Nutrition information labels on foods tell us what the food contains. The labels also tell us how much chemical energy is stored

in the food. The amount of energy is measured in kilojoules (kJ).

Eating too much of some foods can cause problems. Too much fat may cause heart disease.

To make use of the food, our bodies need to break it up into smaller sized molecules. This is called digestion. Digestion turns

large insoluble substances into small soluble ones. The organs of the digestive system help us digest food. Many of them produce

enzymes (chemicals that break up food).

To help absorb the digested food, the small intestine is covered with villi. These increase the surface area.

The digested food substances are carried around the body in the blood. The blood travels through blood vessels. Arteries carry

blood away from the heart and veins carry blood towards the heart. The smallest blood vessels are capillaries. Substances enter

and leave the blood through capillaries. Cells get the substances they need from the blood in capillaries.

Cells need food substances to:

• release energy

• make new substances.

Cells use a chemical reaction called respiration to release energy from a sugar called glucose.

Respiration

All living cells need to respire to release energy. Energy is needed by organisms to help them move, grow and make new

substances to help them stay alive.


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Respiration normally requires oxygen and so it is called aerobic (with air) respiration. It is a series of chemical reactions which

can be summarised in a word equation:

glucose + oxygen carbon dioxide + water (+ energy)

Glucose and oxygen are the reactants. Carbon dioxide and water are the products. Energy is released but it is not a chemical

substance so we can either miss it out of the equation or put it in brackets.

Glucose is supplied by the digestion of carbohydrates. It is carried around the body dissolved in the plasma of the blood. The

blood travels through blood vessels and is pumped by the heart. The heart and the blood vessels form the circulatory system.

The oxygen is absorbed from the air by the lungs. The lungs are part of the breathing system.

The alveoli give the lungs a large surface area so that oxygen can quickly diffuse from the air inside the lungs into the blood

contained in capillaries. The walls of the alveoli and the walls of the capillaries are only one cell thick which also makes it easy

for oxygen to diffuse into the blood. The oxygen is carried by the red blood cells.

Tissue fluid comes out of other capillaries around the body and bathes the tissues in the body. Tissue fluid contains oxygen and

glucose. The cells take the oxygen and glucose that they need from the tissue fluid and put the carbon dioxide that is produced

back into the tissue fluid. The tissue fluid soaks back into other capillaries and the carbon dioxide dissolves in the blood plasma.


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In the lungs the dissolved carbon dioxide diffuses out of the blood and into the air in the lungs. That is why we breathe out

(exhale) more carbon dioxide than we breathe in (inhale). The carbon dioxide is excreted by the lungs. Carbon dioxide can be

tested for by using limewater which turns from clear to cloudy. Oxygen diffusing into the blood and carbon dioxide diffusing out

of the blood is called gas exchange.

Composition of inhaled and exhaled air

Inhaled air Exhaled air

nitrogen gas 78% 78%

oxygen gas 21% 16%

carbon dioxide gas 0.03% 4%

water vapour variable more

.When you exercise, your breathing rate (number of breaths in one minute) and your pulse rate (number of times your heart

beats in one minute) increase. This is because your cells need more oxygen and glucose for respiration.

In some diseases or when there is little air (e.g. at the top of a mountain) the body cannot get enough oxygen. People in these

situations often feel short of breath and tired. If too little oxygen gets to cells, the cells cannot release energy from food and so

they die.

Drugs are chemicals that affect how the body works. Cigarettes contain a drug called nicotine which is addictive (you feel that

you cannot do without it). The chemicals in cigarette smoke can cause diseases.

Poison in cigarette

smoke

Harm it causes

nicotine Makes arteries narrower, causes heart disease.

tar Clogs up the lungs and stops the cilia working, causes cancer and bronchitis.

carbon monoxide Stops red blood cells carrying so much oxygen.

A balanced diet is essential to health. It is made up of the correct amounts of the following seven food substances: carbohydrates,

fats, proteins, vitamins, minerals, water and fibre.

A shortage of a particular substance can cause a specific deficiency disease.

Eating too much of a particular substance can also cause problems. Too much fat can cause obesity and heart disease.


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Chemistry Atoms and elements

Elements

An element is a simple substance that cannot be split into anything simpler by chemical reactions. Atoms are the smallest

particles of an element that can exist. Atoms of one element are all the same, and are different from atoms of all the other

elements.

There are over 100 different elements. All the elements are shown in the Periodic Table. Each element has a chemical symbol,

which is usually one or two letters. A symbol is written with the first letter as a capital, and the second letter is small.

carbon C oxygen O

nitrogen N hydrogen H

gold Au silver Ag

copper Cu aluminium Al

Metals and non-metals

The properties of a substance are the words that we use to describe it, or measurements that we can make on it. Metals and non-

metals have different properties.

Metals Non-metals

good conductors of heat and electricity poor conductors of heat and electricity

shiny dull

solids with a high melting point (except for mercury) most are solids or gases

found on the left-hand side of the Periodic Table found on the right-hand side of the Periodic Table

three metals are magnetic no non-metals are magnetic

metals can burn to form alkaline oxides non-metals can burn to form acidic oxides

flexible brittle

Compounds

Elements can join together to make compounds. The name of the compound tells you the elements that are in it. Compounds made

from two elements always have a name which ends in ‘-ide’.

These elements join together … … to make these compounds

carbon, oxygen carbon dioxide

sodium, chlorine sodium chloride

magnesium, oxygen magnesium oxide


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A chemical formula tells you the name and number of atoms in a compound. The smallest particle of many compounds is called a

molecule. Molecules are made up of atoms. Some elements are also made of molecules. For example, a molecule of oxygen

contains two oxygen atoms joined together. The formula is O2.

Elements Compounds Mixtures

atoms of helium (He)

molecules of carbon dioxide (CO2)

a mixture of helium and oxygen

molecules of oxygen (O2)

molecules of water (H2O)

a mixture of carbon dioxide

and oxygen

a lump of carbon (C)

a lump of sodium chloride (NaCl)

a lump of bronze

(an alloy of copper and tin)

Chemical reactions

Compounds can react chemically by mixing them with other chemicals, or by using heat or electricity. You can tell that a

chemical reaction has occurred if there is a colour change or when a gas is given off.

Most chemical reactions also involve an energy change. This is usually in the form of heat, but can also involve light being given

off, for example, in burning (combustion).

In a chemical reaction a new substance is always formed. Most chemical reactions are not easily reversed (they are irreversible).

Some chemical reactions take place just by mixing. When you make a solid by mixing two liquids, the solid is called a

precipitate.

Other chemical reactions need energy to start them off. This energy can be in the form of heat, light or electricity. When you use

energy to split up compounds they are decomposed.

We can write word equations to show a chemical reaction. The chemicals that you start with are called the reactants. The

chemicals at the end are called the products. For example:

magnesium + oxygen magnesium oxide

reactants product


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Physics Magnets and electromagnets

Magnetism is a non-contact force. Magnets attract magnetic materials. Iron, nickel and cobalt are magnetic materials. Mixtures,

like steel, that include a magnetic material will also be attracted to a magnet. Other metals, like aluminium, are not magnetic and

will not be attracted to a magnet. Iron oxide is a compound that is a magnetic material. It is used to make video and music

cassettes and computer discs. Magnetic materials can also block magnetism.

You can make a magnet from a piece of iron or steel.

• The two ends of a bar magnet are called the north seeking pole and the south seeking pole or north pole and south pole

for short.

• A north pole and a south pole attract each other.

• Two north poles or two south poles will repel each other.

• The space around a magnet where it has an effect is called its magnetic field.

This is the shape of the magnetic field of a bar magnet.

You can find the shape of the mag netic field using iron filings or using a plotting compass.

The Earth has a magnetic field. A compass is a small magnet that always points north. But magnetic materials placed near a

compass can change the direction that it points.

Magnets can be used to sort iron and aluminium cans for recycling. Only the iron cans are attracted to the magnet. Magnets can

also be used for holding fridge doors shut, and in compasses that sailors or walkers use.

A wire with electricity flowing through it has a magnetic field around it. An electromagnet is a coil of wire with an electric

current flowing through it.

You can make an electromagnet stronger by:

• increasing the number of coils of wire

• increasing the size of the current (by increasing the voltage)

• using an iron core.

Electromagnets can be used for lifting things. They are also used in electric bells, relays and in video and music recording.

Electromagnets are used to make bells work.


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A reed switch has two thin pieces of iron inside it. If a magnet is held near the switch, the pieces of iron are magnetised and touch

each other. A reed switch can also be switched on using an electromagnet. Any switch that is worked by electricity is called a

relay.

Relays are used to make things safer. For example, the starter motor in a car uses a high current and needs thick wires for the

current to flow through. A relay is used in a car so that the driver does not have to touch any part of the circuit that has a high

current.

Light

Objects which create light are luminous sources. Light travels in straight lines.

Light waves travel through transparent objects but not through opaque objects. Shadows are made because light cannot travel

through opaque objects. Translucent objects show a glow of light through them.

Transmission and absorption

Transparent materials let light pass straight through. We say they transmit light. Opaque surfaces can absorb light. Black

surfaces absorb light very well and reflect very little. This is why they look so dark.


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Reflection

Light rays are scattered by

rough surfaces, and a reflection

cannot be seen.

A plane mirror is a flat mirror.

Light is reflected evenly by a

plane mirror.

The angle of incidence is equal to the angle of

reflection.

When light shines on to an object viewed in a

mirror, the rays are reflected into the eye. They

seem to come from a position behind the mirror.

The image is the same size as the object and the

same distance from the mirror. In the image left is

right and right becomes left.

Refraction

When light hits something transparent it changes direction. This is called refraction.

Refraction takes place at the interface between two substances. When light is transmitted through glass it slows down and bends

towards the normal. When it travels back out it speeds up again and bends away from the normal.

Colour

White light is a mixture of colours. White light can be split up using a prism to give a spectrum of seven colours (red, orange,

yellow, green, blue, indigo, violet).

The splitting of colour into a spectrum is called dispersion.

A rainbow is produced when water droplets in the air refract sunlight.

Different colours can be made by mixing light of the three primary colours (red, green and blue).

Coloured light can be made using a filter. A red filter lets red light through, but absorbs all the other colours.

We are able to see colours because objects do not reflect all the colours in light:

White objects reflect all the colours.

A red object only reflects red and all other colours are absorbed.

This idea applies to all colours except black.

Black objects absorb all colours.


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Sound and hearing

Sound vibrations and waves

Sound is a form of energy. Sounds are made when things vibrate. The vibrations are passed on by particles in solids, liquids or

gases. Sound needs a substance to pass on the vibrations, so it can travel through solids, liquids and gases but not through a

vacuum.

The speed of sound is faster through solids than liquids, and slowest through gases. This is because the particles are very close

together in solids and so the energy is more likely to be passed from one particle to the next. The sound travels in all directions

because the particles move in all directions unless something stops them.

Sound waves can be shown on an oscilloscope.

The frequency of a wave is the number of vibrations each second. The unit for frequency is hertz (Hz). If you listen to a sound

with a frequency of 100 Hz, one hundred waves reach your ear every second. High pitched sounds have a high frequency, and

low pitched sounds have a low frequency.

The distance between the waves is called the wavelength. It can be measured between any point on a wave and the same point of

the next wave. It is often more convenient to measure it between the top of one wave and the next.

Half the height of the wave is called the amplitude. The

loudness of a sound depends on the amplitude. Louder notes

have more energy and the wave has a bigger amplitude.


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Hearing and the ear

Sound waves travel through the air and into the

ear. They cause the eardrum to vibrate. The

vibrations are passed on to the cochlea in the

inner ear, where they are changed to electrical

signals called impulses. A nerve takes this

message to the brain. When the message

reaches the brain we hear the sound.

Sound can damage the ears if it is too loud or

goes on for too long. Loud sounds can damage the eardrum or the cochlea. Unpleasant sound is often called noise.

We can measure how loud a sound is by using a sound intensity meter. This is an instrument which measures the loudness of a

sound in decibels (dB). The threshold of hearing is the quietest sound we can hear and we say this is 0 dB.

Soft materials can absorb sound. Soft materials are used in soundproofing and for making ear protectors. Double glazed

windows and soft materials like curtains help to reduce sound levels.

Sound and light

One major difference between light and sound energy is that light can travel through space (a vacuum) but sound cannot.

Light also travels much faster than sound. It is nearly a million times faster. Light travels at 300 million metres per second (or 300

000 km/s) and sound travels at about 330 metres per second.

Both light waves and sound waves can be reflected. We hear a reflected sound wave as an echo.

Energy Energy is needed to make things happen. There are different kinds of energy, such as light energy and heat energy that we get

from the Sun, and electrical energy.

We need fuels to provide energy in our homes, factories and for transport. A fuel is something which can release heat energy.

Fossil fuels

Fossil fuels:

• are made from plants and animals which were trapped in mud and rocks millions of years ago

• include coal, oil and natural gas

• are non-renewable (they take millions of years to form, and so our supplies will run out)

• produce gases which cause pollution when they are burnt

• are relatively cheap to obtain

• contain chemical energy which changes to heat energy when they are burnt

• originally got their energy from the Sun. The plants that became coal got their energy from the Sun, and the animals that

became oil got their energy from plants which got their energy from the Sun.

Electricity is not a fuel. It has to be generated using other energy resources.

How coal is formed.


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How oil and natural gas are formed.

Making fossil fuels last longer

We can make fossil fuels last longer by using less energy. We could walk or cycle whenever we can, or use a bus instead of using

a car. Walking and cycling would make us fitter and healthier, and there would be less pollution if there were not as many cars on

the roads. We could also save energy by keeping our houses cooler and wearing more clothes.

Renewable energy resources

Renewable energy resources:

• include solar, wind, tidal, wave, biomass, geothermal and hydroelectricity

• do not produce harmful gases

• can be expensive

• will not run out.

Energy in food

Humans and other animals need energy to live. We get our energy from chemical energy stored in food. We need to choose our

food so that we get the right amount of energy. If we eat too much we could get fat and become unhealthy. If we do not eat

enough we will get thinner and may become ill.

The unit for measuring energy is the joule (J). There is a lot of energy stored in food, so we usually measure the energy in food

using kilojoules (kJ). 1kJ = 1000J.

Energy from the Sun

Most of the energy resources we use originally came from the Sun. Only geothermal energy, nuclear power and tidal power do not

depend on energy from the Sun.


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Energy continued

Nothing would happen without energy. Energy is needed to:

• keep our bodies working

• make machines work

• heat homes, schools and offices.

Energies in action

• heat energy

• light energy

• sound energy

• electrical energy

• kinetic (movement) energy.

Stored energy

Some energy has to be stored so that it is ready for use when we need it.

• Chemical energy is stored in food, fuels and cells.

• Gravitational potential energy is stored in high up things.

• Strain energy is stored in stretched or squashed things.

• Nuclear energy is stored inside atoms.

How is energy measured?

Energy is measured in joules (J) or kilojoules (kJ). A kilojoule is 1000 joules.

Energy changes

Energy needs to be changed to be useful.

in the cells in the wires in the bulb

and bulb

An energy flow diagram.

Many energy changes take place in everyday life. Often wasted energy is produced in the forms of heat or sound. Energy cannot

be made or destroyed, but can only be changed from one form to another. This is the law of conservation of energy.

Force and Motion

Speed

Speed tells us how fast something is going.

We can work out the mean (average) speed of something by using this formula:

mean speed = distance travelled ÷ time taken.

Speed can be measured in:

• metres per second (m/s)

• kilometres per hour (km/h)

• miles per hour (mph).


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We can show how things move on a distance–time graph. This graph shows Kieron walking to school.

Forces

Balanced forces are forces which are the same size but work in opposite directions. Unbalanced forces make things change

speed, change shape or change direction.

If forces are balanced:

• a stationary object stays stationary

• a moving object continues to move at the same speed.

If forces are unbalanced:

• a stationary object will start to move

• a moving object will change its speed or direction.

The motorbike is accelerating because the

forward force is greater than the backward

force.

The motorbike is going at a steady speed. The

forces are balanced.

A car or motorbike uses fuel to move at a steady speed because it needs a force from the engine to balance the forces of air

resistance and friction.

The amount of air resistance on something can be reduced by giving it a smooth, streamlined shape. The air resistance increases

as the speed increases, so cars use up more fuel per mile when they are travelling fast. Air resistance is caused by air particles

hitting the moving object. The particles transfer energy to the object, which is why objects moving through air can get hot.

Mass and weight

The mass of something is the amount of substance or ‘matter’ it contains. It is measured in kilograms (kg). Weight is the force of

gravity pulling on a mass. It is a force, so it is measured in newtons (N).

Gravity

Gravity is the force of attraction between two masses. The force of gravity is stronger if:

• the objects have large masses

• the objects are close together.

On Earth, the gravity pulls on every kilogram of mass with a force of 10 N. Gravity is not as strong on the Moon, because the

Moon has a much smaller mass than the Earth. If you went to the Moon your mass would not change, but your weight would be

less than on Earth because the Moon’s gravity is weaker.

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