Cells. Attnom Academy

S3.1.1 Chapter 1

Signs of life

How can I tell whether something is alive?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Anything that moves is alive.
You picked A. Read on. At the end, the chapter comes back to your guess.
B A plant is not alive, because it does not move about.
You picked B. Read on. At the end, the chapter comes back to your guess.
C A living thing does several jobs, such as grow, feed and make young.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

List the life processes shared by all living things
Use the life processes to decide if a seed or fire is alive

Think of a dog, a tree and a stone. The dog and the tree are alive. The stone is not. How do you know?

Scientists look for seven signs. Each one is a job that a living thing does. We call each job a life process. A living thing shows all seven during its life.

Draw it first

Draw a plant growing

1 Draw a seed lying in the soil.
2 Draw the same plant later. It is a young plant now, and taller.
3 Draw it later still. It has grown a lot. Growing is the first sign of life.

The seven signs are in the table. You can remember them with the first letters: M R S G R E N.

The seven life processes.

Animals move their whole body. Plants move too, but slowly. A plant turns its leaves to the light, and its roots grow down. Moving is only one sign. Wind moves a leaf, and that leaf is not alive because of it.

Sensitivity means noticing a change and reacting to it. You pull your hand back from a hot pan. A plant grows toward light.

Nutrition means taking in food. Animals eat. Plants make their own food from light, air and water.

Nutrition: a plant makes food. A goat and a person eat.

Respiration is not the same as breathing. Breathing brings air into your body. Respiration happens in every cell. It releases energy from food, using oxygen. Carbon dioxide and water are made too.

Excretion means getting rid of waste that the body makes. You breathe out carbon dioxide and you pass water and other waste. Reproduction means making young of the same kind.

See it three ways

With things

Run on the spot for a minute. You feel warm and you breathe faster. Your cells are releasing more energy from food.

As a picture

In words

Respiration uses food and oxygen. It releases energy. It makes carbon dioxide and water. It happens in every living cell, all day.

Now test two puzzling things. A candle flame moves, grows, and uses up air and wax. But it makes no young of its own. It cannot notice or react as a living thing does. Most of all, a fire is not made of cells. Every living thing is. So it is not alive.

A dry seed does not seem to move, grow or feed. But it is alive. It is resting. It respires very slowly. Give it water and warmth and it grows.

Some things need a careful look.

Try it

Wake a resting seed

You need: 3 dry bean seeds, or peas or maize, a cloth or paper, a clean jar, water, a ruler.

Be safe: Do not eat the seeds. Raw beans can make you ill. Wash your hands when you finish. If mould grows, do not open the jar near your face or sniff it. Throw it away and wash your hands.

Wet the cloth. Squeeze it so it is damp, not dripping. Put it in the jar.
Put the seeds between the cloth and the glass, where you can see them. Keep the jar somewhere warm.
Look every day for five days. Keep the cloth damp. Measure the root with the ruler and write it down.

What you will see: The seeds lay still for a day or two. Then a root came out and grew longer each day. The dry seed was alive and resting all the time.

If you cannot try it, this is what happens.

A wrong idea many people have

Anything that moves is alive.

Why it feels right: Animals move all the time, so moving seems to mean life.

The test that breaks it: A river flows, a cloud drifts and a car rolls. None of them is alive. A tree does not walk anywhere, and it is alive.

What is true: Moving is one of seven signs. A living thing shows all seven during its life.

Key words

life process: a job that every living thing does, such as growing.
respiration: releasing energy from food inside cells. It is not the same as breathing.
sensitivity: noticing a change and reacting to it.
excretion: getting rid of waste that the body makes.
nutrition: taking in food, or making it, as plants do.
reproduction: making young of the same kind.

Back to your guess

The answer is C: A living thing does several jobs, such as grow, feed and make young.

Moving is only one sign. A living thing shows seven signs in all. A plant shows every one of them, though often more slowly than an animal.

Keep it

Copy this into your exercise book.

Every living thing shows seven signs: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. A fire shows some, but it is not made of cells, so it is not alive. A dry seed is alive and resting.

Draw: Draw a seed, a young plant and a grown plant in a row, and write growth under them.

S3.1.2 Chapter 2

Seeing small things

How can we see things that are too small for our eyes?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A We cannot. Tiny things are too small to be seen at all.
You picked A. Read on. At the end, the chapter comes back to your guess.
B A curved piece of clear material, such as a drop of water, can make small things look bigger.
You picked B. Read on. At the end, the chapter comes back to your guess.
C We only need to hold the tiny thing closer to our eyes.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Make a magnifier from a drop of water on clear plastic
Explain how a microscope lets us see cells

Every living thing is made of tiny parts called cells. A cell is the smallest part of a living thing that can do the jobs of life.

Most cells are far too small to see with your eyes alone. You need something to make them look bigger. A lens can do that.

A lens is a piece of clear material with a curved surface. It bends the light that goes through it. A lens that is thicker in the middle makes things look bigger.

Draw it first

Draw a drop of water that makes things bigger

1 Draw a flat sheet of clear plastic over a line of print.
2 Draw a drop of water on the sheet. It is curved on top and flat underneath.
3 Draw an eye above the drop. Draw the line of light from the print, through the drop, to the eye.

A drop of water is thicker in the middle than at its edge. So it works as a small lens. Look at print through the drop and it looks bigger.

A drop makes things look about two or three times bigger. To see a cell you need much more. For that we use a microscope.

A microscope has two lenses. The lens near your eye is the eyepiece. The lens near the thing you look at is the objective lens. Each one makes the thing look bigger.

A microscope. Light goes up through the slide and into the lenses.

You put the thing you want to see on a slide. A slide is a thin piece of glass or plastic. The light shines up through it. Only a very thin slice lets the light through.

The lenses multiply. Say the eyepiece makes things look 10 times bigger, and the objective lens 40 times bigger. Together they give 10 × 40 = 400 times. This is the magnification.

See it three ways

With things

Ana has a microscope. The eyepiece is marked 10× and the objective lens is marked 40×.

As a picture

In words

Each lens makes the thing look bigger. The two numbers are multiplied to give the total magnification.

In symbols

10 × 40 = 400

About how wide some small things are. Cells differ, so these are rough sizes.

Look through a microscope at a thin skin from an onion, and you see rows of cells. Each one has a straight edge and a dark spot inside. The spot is the nucleus.

Plant cells, as a microscope shows them. The picture is not to scale.

A thin scrape from the inside of a cheek shows animal cells. They have no hard straight edge. They are soft and rounded, and each has a nucleus.

Animal cells, as a microscope shows them. The picture is not to scale.

Try it

Make a water drop magnifier

You need: a piece of clear plastic, such as from a clean bag or bottle, water, a stick or a straw, a page with small print, a leaf or a feather.

Be safe: Never look at the Sun through the drop. Work indoors or in the shade. Ask a grown up to cut the plastic with scissors.

Lay the clear plastic flat on top of a line of small print.
Dip the stick in water. Let one drop fall on the plastic over the print.
Look straight down at the drop. Then move your head a little nearer and a little farther. Find where the print looks clearest.
Try a leaf, or a feather, under the plastic in the same way.

What you will see: Through the drop, the print looks bigger. The drop works best when it is small and round. A flat puddle does not magnify.

If you cannot try it, this is what happens.

A wrong idea many people have

To see a tiny thing better, hold it closer to your eye.

Why it feels right: A near thing looks bigger, so closer seems better.

The test that breaks it: Hold this page right up to your nose. The words are blurred. Your eye cannot focus on a thing that is too close. Hold a drop lens between, and the words are clear and bigger.

What is true: Your eye cannot focus on a very near thing. A lens makes a small thing look bigger and keeps it clear.

Key words

cell: a tiny part that living things are made of.
lens: a curved piece of clear material that bends light.
microscope: a tool with two lenses that shows very small things.
slide: a thin piece of glass or plastic that holds the thing you look at.
magnification: how many times bigger a thing looks.

Back to your guess

The answer is B: A curved piece of clear material, such as a drop of water, can make small things look bigger.

A curved lens bends light and makes things look bigger. Holding a thing too close to your eye only makes it blurred.

Keep it

Copy this into your exercise book.

A drop of water on clear plastic is a lens. It makes print look bigger. A microscope has two lenses, and their magnifications multiply. Never look at the Sun through a lens.

Draw: Draw a drop of water on plastic over print. Add an eye above and a line of light to it.

S3.1.3 Chapter 3

Animal and plant cells

What is inside a cell, and is a plant cell the same as an animal cell?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A A cell has parts inside, and a plant cell has some parts that an animal cell does not.
You picked A. Read on. At the end, the chapter comes back to your guess.
B A cell is a flat bag with nothing inside it.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Plant cells and animal cells are exactly the same.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Draw and label the nucleus, cytoplasm, membrane, cell wall and chloroplast
Compare plant and animal cells in a table

In the last chapter you saw cells as boxes and ovals, each with a dark spot. Now look inside one. A cell has parts, like a house has rooms.

We start with an animal cell. People, dogs and birds are made of animal cells. We draw one in three steps.

Draw it first

Draw an animal cell

1 Draw a soft oval. This outline is the cell membrane. It is a thin skin around the cell.
2 Fill the inside with dots. The dots are the cytoplasm. It is a watery jelly where the cell does its work.
3 Draw a round nucleus inside, with a dark spot. The nucleus is the part that controls the cell.

Almost every plant and animal cell has a membrane, cytoplasm and a nucleus. A plant cell has more.

A plant cell has a stiff cell wall outside the membrane. It has small green parts inside, and each one is a chloroplast. It has a large vacuole, which is a bag of watery liquid.

A plant cell. The wall is the hatched band, drawn outside the membrane.

Look at the order from the outside. First comes the cell wall. Next comes the membrane. Then the cytoplasm fills the inside, with the vacuole, the nucleus and the chloroplasts in it.

The two kinds of cell, side by side.

See it three ways

With things

Imagine an animal cell. Now add a stiff box round it, add a large bag of water, and add little green parts. You have a plant cell.

As a picture

In words

Both kinds of cell have a nucleus, cytoplasm and a membrane. Of these two kinds, only the plant cell has a cell wall and chloroplasts. An animal cell never has them. A plant cell has a much larger vacuole.

The table says "some" for chloroplasts. Not every plant cell has them. Look at a leaf cell and a root cell.

A leaf cell has chloroplasts. A root cell does not.

Try it

Build a model cell

You need: a small cardboard box, a clear plastic bag, water, a dry bean or a pebble.

Be safe: Plastic bags can stop a small child from breathing. Keep them away from young children and from your face. Do not eat the bean.

Put the bean in the plastic bag. Add a little water, enough to cover it. Tie the bag shut.
Hold the bag in your hands. It is floppy, like an animal cell.
Now put the bag in the box and shut the lid. The bag keeps its shape, like a plant cell.
Match each part of the model with a part of a cell.

What you will see: The bag alone is floppy. The bag in the box keeps its shape. The stiff box does the job of a cell wall.

If you cannot try it, this is what happens.

A wrong idea many people have

Every plant cell has chloroplasts.

Why it feels right: Plants are green, so it seems that all their cells are green.

The test that breaks it: Look at a root pulled from the soil. It is pale, not green. Root cells have no chloroplasts, and the root is still part of a plant.

What is true: Only cells in green parts of a plant, such as leaves, have chloroplasts.

Key words

cell membrane: the thin skin around a cell.
cytoplasm: the watery jelly inside a cell, where it does its work.
nucleus: the round part that controls the cell.
cell wall: a stiff layer outside the membrane of a plant cell.
chloroplast: a small part in green plant cells.
vacuole: a bag of watery liquid in a cell.

Back to your guess

The answer is A: A cell has parts inside, and a plant cell has some parts that an animal cell does not.

A cell has parts, each with a job. A plant cell has three extra things: a cell wall, a large vacuole and, in green parts, chloroplasts.

Keep it

Copy this into your exercise book.

Plant and animal cells have a membrane, cytoplasm and, nearly always, a nucleus. A plant cell also has a cell wall, a large vacuole and, in green parts, chloroplasts.

Draw: Draw an animal cell and a plant cell next to each other. Label every part.

S3.1.4 Chapter 4

What each part does

What job does each part of a cell do?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A The cell wall decides what goes in and out of the cell.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The nucleus controls the cell, and the membrane decides what goes in and out.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The cytoplasm is just empty space.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain the job of the nucleus, membrane and cell wall
Explain why only green plant cells have chloroplasts

A cell is like a tiny workshop. Each part has a job. When every part does its job, the cell stays alive.

Start at the edge. The cell membrane is a thin skin. It is like a door with a guard. It lets in food and water and lets out waste. It keeps out things the cell does not need.

Draw it first

Draw what the cell membrane does

1 Draw a cell and point to its membrane. It is the skin round the cell.
2 Draw an arrow going in through the membrane. Write food in.
3 Draw an arrow going out through the membrane. Write waste out.

Next, the nucleus. It holds the cell's instructions. These tell the cell how to grow and what to make. The nucleus is the control centre.

The nucleus sends instructions to the rest of the cell.

The cytoplasm is the jelly that fills the cell. Most of the cell's work happens in it.

Now the plant cell. Its cell wall is stiff. It gives the cell a firm shape. Water and food pass through the wall easily. The thin membrane inside it decides what enters the cell.

Water in the vacuole pushes on the wall. The wall holds firm.

A plant cell full of water pushes hard against its wall, like a ball full of air. The wall holds firm and so the plant stands up. A plant with too little water goes limp.

Last, the chloroplast. It catches light. It uses the light, with water and carbon dioxide gas, to make food for the plant. This is called photosynthesis.

A chloroplast uses light to make food.

A chloroplast can only work in the light. A leaf is in the light, so its cells have many. A root is under the soil, in the dark, so its cells have none.

See it three ways

With things

Look at a whole plant. The leaves are green and they are in the light. The roots are pale and they are in the dark.

As a picture

In words

Chloroplasts need light to make food. So only cells in the light, in the green parts of a plant, have them.

Try it

Wake a limp stalk

You need: 2 limp stalks or leaves of a green vegetable, a cup, water, a clock.

Be safe: Wash your hands when you finish. Do not eat the stalks.

Choose two stalks that are limp. Bend one gently and feel how floppy it is.
Stand one stalk in a cup of water. Leave the other in the open air, in the same room.
After two hours, bend each one gently. Which feels firmer?

What you will see: The stalk that stood in water is firmer. Its cells filled with water again and pushed on their walls.

If you cannot try it, this is what happens.

A wrong idea many people have

The cell wall decides what goes in and out of a plant cell.

Why it feels right: A wall sounds like a gate, and it is at the edge of the cell.

The test that breaks it: Water flows through the cell wall easily. The wall is made of fibres with small gaps in them. It is the thin membrane just inside that decides what enters.

What is true: The membrane decides what goes in and out. The cell wall holds the shape and lets water through.

Key words

nucleus: the part that controls the cell and holds its instructions.
cell membrane: the thin skin that decides what goes in and out of a cell.
cell wall: the stiff layer that gives a plant cell its shape.
chloroplast: the part that uses light to make food. It is found in the green parts of plants.
photosynthesis: making food from light, water and carbon dioxide.

Back to your guess

The answer is B: The nucleus controls the cell, and the membrane decides what goes in and out.

The nucleus is the control centre. The thin membrane lets in what the cell needs and keeps out what it does not. The cytoplasm is full of jelly where the work is done.

Keep it

Copy this into your exercise book.

The nucleus controls the cell. The membrane decides what goes in and out. The cell wall gives a plant cell its shape. Chloroplasts use light to make food, so only green parts have them.

Draw: Draw a plant cell. Label the nucleus, membrane, wall and chloroplast, and write each part's job.

S3.1.5 Chapter 5

Cells with special jobs

Why are some cells long and thin, and others round and flat?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Each kind of cell has a shape that helps it do its job.
You picked A. Read on. At the end, the chapter comes back to your guess.
B All the cells in a body look the same.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The shape of a cell is just chance. It has nothing to do with its job.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how nerve, red blood and root hair cells suit their jobs
Match a special cell to the job it does

Your body has many kinds of cell. A specialised cell has a shape that suits one job. Look at three of them.

First, a nerve cell. Its job is to carry messages from one part of the body to another. A nerve cell draws in three steps.

Draw it first

Draw a nerve cell

1 Draw a small round cell body with a nucleus inside it.
2 Draw a very long, thin fibre going out from it. This is the part that carries the message.
3 Draw short branches at both ends. They pass the message to the next cell.

A nerve cell can be about a metre long. Some run from your foot up to your back. Its long fibre carries a message a long way. The fastest messages travel about 100 metres every second.

Next, the red blood cell. It carries oxygen round the body in your blood. Its shape is a flat disc with a dent on each side.

A red blood cell. It has a dented middle and no nucleus.

The flat, dented shape gives a large surface, so oxygen can get in and out quickly. The cell is also small and bendy, so it can squeeze through the thinnest blood tubes. In people and other mammals, a red blood cell has no nucleus. That leaves more room to carry oxygen.

Last, the root hair cell. It grows in the skin of a root, and it takes in water from the soil. It has a long thin hair that reaches between the grains of soil.

A root hair cell. The hair is many times longer than the cell is wide.

The hair gives the cell a much larger surface area. Surface area means how much outside a thing has. More surface means more water can pass in, so the plant gets water faster. A root cell is in the dark soil, so it has no chloroplasts.

See it three ways

With things

Think of a telephone wire. It is long and thin so that it can carry a message far. A nerve cell is made for the same reason.

As a picture

In words

The shape of each cell suits its job. A long thin cell carries messages. A dented disc carries oxygen. A cell with a long hair takes in water. The three are not drawn to scale. A nerve cell can be far longer than the others.

Each cell has one job.

Try it

Pass a message down a line

You need: 4 or more people, a clock or a way to count seconds.

Stand in a line and hold hands. Everyone but the first person closes their eyes.
The first person squeezes the hand of the next. Each person passes the squeeze on as soon as they feel it.
The last person says "yes" aloud. Count how many seconds it takes.
Try again with a longer line. Does it take longer?

What you will see: The squeeze runs down the line, one person after another. A message runs along nerve cells in a similar way, but much faster.

If you cannot try it, this is what happens.

A wrong idea many people have

Every cell has a nucleus.

Why it feels right: Pictures of cells almost always show a nucleus.

The test that breaks it: Look at the red blood cell in the picture. It has a dented middle and no nucleus. Red blood cells in people lose their nucleus as they grow.

What is true: Most cells have a nucleus, but a red blood cell in a mammal has none.

Key words

specialised cell: a cell with a shape that suits one job.
nerve cell: a long thin cell that carries messages.
red blood cell: a dented disc that carries oxygen round the body.
root hair cell: a root cell with a long hair that takes in water.
surface area: how much outside a thing has.

Back to your guess

The answer is A: Each kind of cell has a shape that helps it do its job.

Many cells are built for one job. A long thin cell carries messages. A flat dented disc carries oxygen. A cell with a hair takes in water.

Keep it

Copy this into your exercise book.

A nerve cell is long and branched, to carry messages. A red blood cell is a dented disc, to carry oxygen. A root hair cell has a long hair, to take in water.

Draw: Draw a nerve cell, a red blood cell and a root hair cell. Write each cell's job beside it.

S3.1.6 Chapter 6

Cells, tissues and organs

How do tiny cells make a whole body, or a whole tree?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Cells join in groups, and the groups join to make the parts of the body.
You picked A. Read on. At the end, the chapter comes back to your guess.
B A big living thing is one very large cell.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The cells in a body all work alone and never work together.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Put cell, tissue, organ, organ system and organism in order of size
Give an example of each level in a plant and an animal

A dog is not one huge cell. It is made of an enormous number of tiny cells. They join together in steps, from small to large.

A tissue is a group of the same kind of cell, doing one job. Muscle cells together make muscle tissue.

An organ is made of different tissues that work together. The heart is an organ. It has muscle tissue to squeeze, and other tissues too.

An organ system is a group of organs that do one big job together. The heart and the blood tubes form the blood system.

An organism is a whole living thing, such as a dog or a tree. Draw the steps from cell to organism.

Draw it first

Draw the steps up from a cell

1 Write cell, then tissue. Draw a bar for each, with the tissue bar longer.
2 Add organ. Its bar is longer still.
3 Add organ system, with a longer bar again.
4 Add organism, the longest bar. Draw an arrow marked bigger beside them.

Here is a tissue. Every cell is the same kind and they fit together. Together they do one job.

A tissue is many cells of one kind.

Plants have the same levels. A leaf cell is a cell. The skin of a leaf is a tissue. The whole leaf is an organ. The stem and leaves together form the shoot system, which is an organ system. The roots form the root system. The whole tree is the organism.

The five levels in a plant.

An animal has them too. A muscle cell is a cell. Heart muscle is a tissue. The heart is an organ. The heart and blood tubes are the blood system. The whole dog is the organism.

The five levels in an animal.

See it three ways

With things

Tighten the muscle at the front of your upper arm and feel it go hard. That muscle is an organ.

As a picture

In words

A muscle is an organ. It is made of muscle tissue, which is made of muscle cells. It also has tissue that carries blood and tissue that carries messages.

Your skin is an organ too. It is the biggest organ you have. Here are some organs and what they do.

Four organs and their jobs.

Try it

Find an organ on yourself

You need: your own body, a quiet place.

Sit still for a minute. Put two fingers on the inside of your other wrist, below the thumb. Feel your pulse. It comes from your heart.
Put a hand on your chest. Breathe in slowly. Feel your chest rise. Your lungs are filling with air.
Run a fingertip along your arm. Your skin is an organ too.

What you will see: Each thing you felt came from an organ. Each organ is made of tissues, and each tissue is made of cells.

If you cannot try it, this is what happens.

A wrong idea many people have

An organ is just a very big cell.

Why it feels right: An organ is a bigger thing than a cell, and the word sounds like one object.

The test that breaks it: Look at the heart in the table of levels. It is made of many tissues. Each tissue is made of millions of cells. One cell cannot be a heart.

What is true: An organ is made of tissues, and tissues are made of cells.

Key words

tissue: a group of the same kind of cell, doing one job.
organ: different tissues that work together, such as the heart.
organ system: a group of organs that do one big job together.
organism: a whole living thing.

Back to your guess

The answer is A: Cells join in groups, and the groups join to make the parts of the body.

Cells join into tissues. Tissues join into organs. Organs work together in systems. Together the systems make the whole living thing.

Keep it

Copy this into your exercise book.

Cells make tissues. Tissues make organs. Organs form organ systems. Systems together make an organism. The order is cell, tissue, organ, organ system, organism.

Draw: Draw five bars, each longer than the last. Write the five levels beside them, smallest first.

S3.1.7 Chapter 7

Diffusion

Why can you smell cooking from across a room?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A The smell can only travel if a wind blows it.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Smell particles spread out by themselves, from where they are crowded.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Your nose pulls the smell towards it.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain diffusion as spreading from a crowded place to a less crowded one
Show diffusion with a drop of ink or tea in still water

Someone cooks in one corner of a room. After a little while, you can smell the food at the other side. Nobody carried the smell across. How did it get there?

Everything is made of tiny particles. In a gas the particles are far apart, and they are always moving. A smell is made of particles too.

Moving air carries a smell much of the way. But even in still air, the smell particles spread on their own. That spreading is what this chapter is about.

Draw it first

Draw a smell spreading

1 Draw a box of air. Put the smell, dark dots, close together in one corner.
2 Draw the same box a little later. The dark dots have moved out into the empty places.
3 Draw it much later. The dark dots are spread through the whole box.

The smell particles bump about, and they move in every direction. Near the pot there are many of them, so many bump into their neighbours and move away. Few come back. So the smell spreads out into the less crowded air.

This spreading is called diffusion. Particles spread from a place where they are crowded to a place where they are less crowded. They do it by their own movement, with no stirring.

In the end the particles are spread evenly. They do not stop moving. Just as many go one way as the other, so you see no more change.

Diffusion happens in liquids too, but it is much slower. Do not drop colour in from the top. Colour is heavier than water, so it sinks in streaks. That is not diffusion. Lay a little colour at the bottom of still water instead, and watch it.

Colour spreads up through still water, from where it is crowded.

In still water, diffusion is slow. The edge of the colour may creep up only a centimetre or two in a day. A small shake or a warm spot makes currents that mix it faster. So keep the cup still to see diffusion alone.

See it three ways

With things

Lay a little colour at the bottom of a jar of still water. Look after an hour and after a day. The colour has spread a little further up each time.

As a picture

In words

Ink particles move among the water particles. They spread from where there are many ink particles to where there are few. This goes on until they are mixed evenly.

Your cells use diffusion. A cell uses up oxygen, so there is less oxygen inside than outside. Oxygen spreads in through the membrane. The cell makes carbon dioxide, so there is more inside than outside. It spreads out.

A cell takes in oxygen and lets out carbon dioxide by diffusion.

Here is the membrane, with oxygen on both sides. The arrow shows the way that more of the oxygen goes.

More oxygen is outside the cell, so more goes in than comes out.

Try it

Colour at the bottom of still water

You need: a clear glass or jar, water, ink or food colour, a thin straw or a hollow grass stem, a ruler, a place where the glass will not be knocked.

Be safe: Ink and food colour stain. Cover the table. Keep them away from your eyes and mouth. Do not drink the water. Wash your hands after.

Fill the glass with water. Leave it for ten minutes on a table, so the water is still.
Dip the straw a little way into the colour. Put a finger over the top end of the straw, to trap the colour inside.
Lower the straw slowly to the bottom of the glass. Lift your finger so the colour runs out. Lift the straw out gently. Do not stir.
Look at once. Measure with the ruler how high the colour reaches, in centimetres. Look again after an hour, and the next day.

What you will see: The colour stays low, because it is heavier than water. Its top edge slowly turns blurred and creeps upward. This is diffusion, and it is slow. It takes a day to move a centimetre or two.

If you cannot try it, this is what happens.

A wrong idea many people have

When the ink is spread evenly, the particles have stopped moving.

Why it feels right: Nothing seems to change any more, so it seems that nothing moves.

The test that breaks it: Particles never stop moving. In an even mixture, just as many ink particles move one way as the other. Lay a little more colour at the bottom and you see it spread again.

What is true: The particles keep moving. The mixture only looks still because there is no more net change.

Key words

particle: one of the tiny pieces that everything is made of.
diffusion: the spreading of particles from where they are crowded to where they are less crowded.

Back to your guess

The answer is B: Smell particles spread out by themselves, from where they are crowded.

Smell particles move all the time. They spread from the crowded place near the pot into the emptier air, until they reach you. This spreading is called diffusion.

Keep it

Copy this into your exercise book.

Diffusion is the spreading of particles from where they are crowded to where they are less crowded. It needs no stirring. In still water it is slow. Cells take in oxygen by diffusion.

Draw: Draw a box with dark dots crowded in one corner. Draw it again later with the dots spread out.

S3.1.8 Chapter 8

Living things with one cell

Can a whole living thing be just one cell?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Yes. Yeast and bacteria are living things made of a single cell.
You picked A. Read on. At the end, the chapter comes back to your guess.
B No. Every living thing is made of many cells.
You picked B. Read on. At the end, the chapter comes back to your guess.
C A single cell cannot be alive. It is only a part of something.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Name living things made of one cell, such as yeast and bacteria
Explain how some microbes cause disease and others are useful
Make bread dough rise with yeast and explain why it rises

A person is made of many cells, and so is a tree. But some living things are just one cell. That one cell does the jobs of life by itself.

A microbe is a living thing too small to see without a microscope. Many microbes are made of one cell. Yeast is one of them. It is a fungus, like a mushroom, but it is only one cell. It has a nucleus, and it feeds on sugar.

Draw it first

Draw a yeast cell making a new cell

1 Draw a yeast cell with a nucleus. Draw a small bud growing on its side.
2 Draw the same cell later. The bud has grown bigger, and a copy of the nucleus is inside it.
3 Draw it later still. The bud has broken away. It is a new yeast cell, with its own nucleus.

This is how yeast reproduces. One cell becomes two. Each of them can grow a bud. So the number grows fast when yeast has sugar and warmth.

Bacteria are the other well known one cell microbes. One of them is a bacterium. A bacterium has no nucleus. Bacteria come in shapes such as rods and balls.

Bacteria. Each is one cell, with no nucleus.

Bacteria live almost everywhere: in soil, in water, in the air, on your skin and inside you. They are even smaller than yeast cells.

About how wide they are. Yeast and bacteria are far too small to see.

See it three ways

With things

You are made of many cells, and so is a tree. A yeast cell is a whole living thing on its own.

As a picture

In words

A one cell living thing does every job in its single cell. A person has many cells that share the jobs, and some of the cells are special.

Microbes affect us in two ways. Some are useful. Yeast makes bread rise. Some bacteria turn milk into yoghurt. Bacteria and other microbes in a compost heap rot dead leaves so that they feed the soil.

A few microbes cause disease. Bacteria in dirty water cause cholera. Bacteria on teeth, with sugar, cause tooth decay. Bacteria in badly kept food cause food poisoning.

Some microbes help us. A few can harm us.

You can keep disease microbes away. Wash your hands with soap and clean water before you eat and after the toilet. Drink clean water. Keep food covered and cooked well.

A virus is not a cell. It is far smaller than a bacterium. Colds and flu are caused by viruses. Do not mix up a virus with a bacterium.

Now see yeast at work. Yeast feeds on sugar. It releases energy from the sugar and makes carbon dioxide gas. In dough, the gas makes bubbles. The bubbles push the dough up.

Why dough rises.

Try it

Make dough rise

You need: 2 big spoons of flour, 1 small spoon of sugar, 1 small spoon of dried yeast, a little warm water, a narrow clear glass or jar, a rubber band, a ruler.

Be safe: Use water that is warm to the hand, never hot. Hot water can burn you and it kills yeast. Do not eat the raw dough. Wash your hands. If you bake, a grown up must use the oven. Yeast is safe to use. Never try to grow other microbes at home, such as from soil, food or a swab.

Mix the flour, the sugar and the yeast in a cup. Add warm water a little at a time, until you have a thick, sticky dough.
Put the dough in the clear glass. Press it flat. Slide the rubber band onto the glass at the top of the dough. Measure the height of the dough in centimetres.
Keep the glass in a warm place. Measure the height every half hour for an hour and a half.

What you will see: The dough grows taller, and you can see small bubbles of gas in it. The yeast made carbon dioxide, and it pushed the dough up. Your heights will differ from the graph, which shows example readings.

If you cannot try it, this is what happens.

A wrong idea many people have

All microbes are germs that make us ill.

Why it feels right: We hear about microbes mostly when we are sick.

The test that breaks it: Yeast makes bread rise and harms nobody. Bacteria turn milk into yoghurt. Both are microbes. Most microbes are harmless or helpful.

What is true: Most microbes do no harm, many are useful, and only a few cause disease.

Key words

microbe: a living thing too small to see without a microscope.
yeast: a one cell living thing that feeds on sugar and makes carbon dioxide gas.
bacteria: tiny one cell living things with no nucleus.
disease: an illness, often caused by a microbe.
virus: a tiny thing that causes illness. It is not a cell.

Back to your guess

The answer is A: Yes. Yeast and bacteria are living things made of a single cell.

Some living things are one cell. That one cell does every job of life: it feeds, respires, grows and makes young.

Keep it

Copy this into your exercise book.

Yeast and bacteria are living things made of one cell. Yeast has a nucleus and bacteria have none. Some microbes help us and a few cause disease. A virus is not a cell. Yeast makes gas that makes dough rise.

Draw: Draw a yeast cell with a bud growing on it. Label the nucleus and the bud.

Draw a plant growing

Wake a resting seed

Draw a drop of water that makes things bigger

Make a water drop magnifier

Draw an animal cell

Build a model cell

Draw what the cell membrane does

Wake a limp stalk

Draw a nerve cell

Pass a message down a line

Draw the steps up from a cell

Find an organ on yourself

Draw a smell spreading

Colour at the bottom of still water

Draw a yeast cell making a new cell

Make dough rise

Key words, A to Z