Chapter 8 of 8

All 8 chapters
  1. 1Signs of life, readunderstood
  2. 2Seeing small things, readunderstood
  3. 3Animal and plant cells, readunderstood
  4. 4What each part does, readunderstood
  5. 5Cells with special jobs, readunderstood
  6. 6Cells, tissues and organs, readunderstood
  7. 7Diffusion, readunderstood
  8. 8Living things with one cell, readunderstood

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

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • 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. A yeast cell drawn in three steps. In the first, a small bud grows on the upper right of the cell. In the second, the bud is bigger and has a small nucleus of its own. In the third, the bud has broken away and is a new cell, still with its own nucleus. The yeast cell has a nucleus inside. Labels: nucleus, bud, new cell.nucleusbud

    1 Draw a yeast cell with a nucleus. Draw a small bud growing on its side.

  2. A yeast cell drawn in three steps. In the first, a small bud grows on the upper right of the cell. In the second, the bud is bigger and has a small nucleus of its own. In the third, the bud has broken away and is a new cell, still with its own nucleus. The yeast cell has a nucleus inside. Labels: nucleus, bud, new cell.nucleusbud

    2 Draw the same cell later. The bud has grown bigger, and a copy of the nucleus is inside it.

  3. A yeast cell drawn in three steps. In the first, a small bud grows on the upper right of the cell. In the second, the bud is bigger and has a small nucleus of its own. In the third, the bud has broken away and is a new cell, still with its own nucleus. The yeast cell has a nucleus inside. Labels: nucleus, bud, new cell.nucleusnew cell

    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.

Two bacteria. On the left is a rod shaped bacterium, labelled rod, with the words no nucleus. On the right is a round bacterium, labelled round. Neither has a nucleus. Labels: rod, no nucleus, round.rodno nucleusround
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.

A table of how wide some tiny things are. A pinhead is 1 mm. A yeast cell is about 0.005 mm. A bacterium is about 0.002 mm. The cells are far too small to see without a microscope. Labels: thing, width, pinhead, yeast cell, bacterium, 1 mm, 0.005 mm, 0.002 mm.thingwidthpinhead1 mmyeast cell0.005 mmbacterium0.002 mm
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

A yeast cell labelled one cell, beside a person and a tree labelled many cells. Labels: one cell, many cells, many cells.one cellmany cellsmany cells

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.

A table of microbes that help us and microbes that can harm us. Helping: bread rises, yoghurt, compost. Harming: tooth decay, food poisoning, cholera. Labels: helps us, can harm us, bread rises, yoghurt, compost, tooth decay, food poisoning, cholera.helps uscan harm usbread risestooth decayyoghurtfood poisoningcompostcholera
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.

Three boxes joined by arrows. The first says yeast and sugar. The second says carbon dioxide gas. The third says dough rises. It shows how yeast makes dough rise. Labels: yeast and sugar, carbon dioxide gas, dough rises.yeast and sugarcarbon dioxide gasdough rises
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.

  1. 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.
  2. 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.
  3. 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 line graph of the height of the dough in a glass, in centimetres, against the time in minutes. The dough is 3 cm high at 0 minutes and rises to 6 cm at 90 minutes. Labels: minutes, height in cm, 0, 30, 60, 90, 0, 2, 4, 6, 8.024680306090minutesheight in cm

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.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which of these is a living thing made of one cell?

    Tap your answer.

    1. Not this one. A mushroom is made of many cells. Yeast is also a fungus, but it is only one cell. Go to S3.1.8. Try another.

    2. Yes, that is right. A yeast cell is a whole living thing. It feeds, respires and makes new cells by budding.

    3. Not this one. A virus is not a cell at all. It is much smaller than a bacterium. Go to S3.1.8. Try another.

    See the answer

    Yeast

    A yeast cell is a whole living thing. It feeds, respires and makes new cells by budding.

    • If you chose A: A mushroom is made of many cells. Yeast is also a fungus, but it is only one cell. Go to S3.1.8.
    • If you chose C: A virus is not a cell at all. It is much smaller than a bacterium. Go to S3.1.8.

  2. What is different about a bacterium, compared with a yeast cell?

    Tap your answer.

    1. Not this one. Yeast has a nucleus. A bacterium does not, and that is one big difference. Go to S3.1.8. Try another.

    2. Not this one. Bacteria are smaller than yeast cells. The table of widths shows this. Go to S3.1.8. Try another.

    3. Yes, that is right. A bacterium is a single cell with no nucleus. It is also smaller than a yeast cell.

    See the answer

    It has no nucleus.

    A bacterium is a single cell with no nucleus. It is also smaller than a yeast cell.

    • If you chose A: Yeast has a nucleus. A bacterium does not, and that is one big difference. Go to S3.1.8.
    • If you chose B: Bacteria are smaller than yeast cells. The table of widths shows this. Go to S3.1.8.

  3. Why does bread dough rise when it has yeast in it?

    Tap your answer.

    1. Yes, that is right. Yeast feeds on sugar and makes carbon dioxide. The gas forms bubbles that stretch the dough.

    2. Not this one. Dough without yeast hardly rises. The gas that the yeast makes does the work. Go to S3.1.8. Try another.

    3. Not this one. The yeast cells are far too small to fill the dough. It is the gas bubbles that make it rise. Go to S3.1.8. Try another.

    See the answer

    The yeast makes carbon dioxide gas, and the bubbles push the dough up.

    Yeast feeds on sugar and makes carbon dioxide. The gas forms bubbles that stretch the dough.

    • If you chose B: Dough without yeast hardly rises. The gas that the yeast makes does the work. Go to S3.1.8.
    • If you chose C: The yeast cells are far too small to fill the dough. It is the gas bubbles that make it rise. Go to S3.1.8.

  4. Which sentence about microbes is true?

    Tap your answer.

    1. Not this one. Yeast and the bacteria in yoghurt are microbes, and they do not make us ill. Go to S3.1.8. Try another.

    2. Yes, that is right. Yeast, yoghurt bacteria and compost microbes help us. A few microbes cause diseases such as cholera.

    3. Not this one. Some microbes do cause disease, such as cholera. Washing and clean water keep them away. Go to S3.1.8. Try another.

    See the answer

    Some microbes cause disease and others are useful.

    Yeast, yoghurt bacteria and compost microbes help us. A few microbes cause diseases such as cholera.

    • If you chose A: Yeast and the bacteria in yoghurt are microbes, and they do not make us ill. Go to S3.1.8.
    • If you chose C: Some microbes do cause disease, such as cholera. Washing and clean water keep them away. Go to S3.1.8.

  5. Which is a good way to keep disease microbes away?

    Tap your answer.

    1. Yes, that is right. Clean hands, clean water and covered food keep disease microbes away.

    2. Not this one. Clear water can still hold microbes, such as the ones that cause cholera. Drink clean water, or boil it first. Go to S3.1.8. Try another.

    3. Not this one. Warmth helps microbes grow. Keep food covered and cooked well. Go to S3.1.8. Try another.

    See the answer

    Wash your hands with soap and clean water before you eat.

    Clean hands, clean water and covered food keep disease microbes away.

    • If you chose B: Clear water can still hold microbes, such as the ones that cause cholera. Drink clean water, or boil it first. Go to S3.1.8.
    • If you chose C: Warmth helps microbes grow. Keep food covered and cooked well. Go to S3.1.8.

  6. From an earlier chapter

    Which cell in your body has no nucleus?

    Tap your answer.

    1. Not this one. A nerve cell has a nucleus in its round body. Look at the picture of the nerve cell. Go to S3.1.5. Try another.

    2. Not this one. A cheek cell has a nucleus, as you saw down the microscope. Look at the picture of the animal cells. Go to S3.1.2. Try another.

    3. Yes, that is right. A red blood cell in a mammal has no nucleus. That leaves more room to carry oxygen.

    See the answer

    A red blood cell

    A red blood cell in a mammal has no nucleus. That leaves more room to carry oxygen.

    • If you chose A: A nerve cell has a nucleus in its round body. Look at the picture of the nerve cell. Go to S3.1.5.
    • If you chose B: A cheek cell has a nucleus, as you saw down the microscope. Look at the picture of the animal cells. Go to S3.1.2.

  7. Dough in a glass was 3 cm high at the start and 6 cm high after 90 minutes. How many centimetres did it rise?

    See the answer

    3 cm

    Take the first height from the last height. 6 − 3 = 3.

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.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (754 KB). It opens on any phone.