Chapter 1 of 8

All 8 chapters
  1. 1Cell structure in detail, readunderstood
  2. 2Microscopes and size, readunderstood
  3. 3Diffusion, osmosis and active transport, readunderstood
  4. 4Cell division, readunderstood
  5. 5Enzymes, readunderstood
  6. 6Photosynthesis, readunderstood
  7. 7Testing photosynthesis, readunderstood
  8. 8Respiration, readunderstood

S4.1.1 Chapter 1

Cell structure in detail

A leaf and my skin are both made of cells. How can their cells be so different?

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

  • Describe the jobs of mitochondria, ribosomes and the nucleus
  • Compare plant, animal and bacterial cells

Your body is made of cells. So is a leaf, and so is the yeast in bread dough. Some living things are a single cell. You are made of tens of millions of millions of them.

In Level 3 you met the nucleus, the cytoplasm, the cell membrane, the cell wall and the chloroplast. This chapter adds three more parts. Then it puts three kinds of cell side by side: animal, plant and bacterial.

Draw it first

Build an animal cell, then a plant cell

  1. Three pictures, one after another. First an animal cell with the labels cell membrane, cytoplasm and nucleus, and a key that says the black dots are ribosomes. Then the same animal cell with the labels cell membrane, cytoplasm, nucleus, mitochondria and ribosomes. Last a plant cell with the labels cell wall, cell membrane, cytoplasm, nucleus, chloroplasts, permanent vacuole, mitochondria and ribosomes.cell membranecytoplasmnucleusribosome

    1 An animal cell has a cell membrane round it, cytoplasm inside, and a nucleus.

  2. Three pictures, one after another. First an animal cell with the labels cell membrane, cytoplasm and nucleus, and a key that says the black dots are ribosomes. Then the same animal cell with the labels cell membrane, cytoplasm, nucleus, mitochondria and ribosomes. Last a plant cell with the labels cell wall, cell membrane, cytoplasm, nucleus, chloroplasts, permanent vacuole, mitochondria and ribosomes.mitochondriacell membranecytoplasmnucleusribosomes

    2 It also has mitochondria and ribosomes. The black dots are ribosomes.

  3. Three pictures, one after another. First an animal cell with the labels cell membrane, cytoplasm and nucleus, and a key that says the black dots are ribosomes. Then the same animal cell with the labels cell membrane, cytoplasm, nucleus, mitochondria and ribosomes. Last a plant cell with the labels cell wall, cell membrane, cytoplasm, nucleus, chloroplasts, permanent vacuole, mitochondria and ribosomes.cytoplasmpermanentvacuolenucleusribosomescell membranemitochondriachloroplastscell wall

    3 A plant cell has all of these and three more: a cell wall, chloroplasts and a permanent vacuole.

The nucleus holds the genetic material, which is DNA. It controls what the cell does. Most animal and plant cells have one. A few lose it as they grow, such as the red blood cells of humans and other mammals.

Mitochondria are where aerobic respiration happens. This releases energy from glucose, using oxygen. Cells that use a lot of energy, such as muscle cells, have many mitochondria. Energy is not made here. It is released from food.

Ribosomes make proteins. They join small units called amino acids in the order the genetic material says. A cell uses proteins to build itself, and some proteins work as enzymes.

A table with the columns Part and Job. Nucleus: holds DNA. Mitochondria: respiration. Ribosomes: make proteins. Cytoplasm: reactions.PartJobnucleusholds DNAmitochondriarespirationribosomesmake proteinscytoplasmreactions
Respiration in the mitochondria releases energy from glucose. Most reactions of the cell happen in the cytoplasm. The cell membrane controls what goes in and out.

Now the extra parts of a plant cell. The cell wall is made of cellulose. It is stiff and holds the cell in shape. The permanent vacuole is a large space filled with a watery liquid called cell sap. It helps keep the cell firm.

Chloroplasts contain a green substance called chlorophyll. They absorb light for photosynthesis. Not every plant cell has them. A root cell is underground, in the dark, and has none.

A plant cell with a scale bar of 10 µm. The labels are cell wall, cell membrane, cytoplasm, nucleus, chloroplasts, permanent vacuole, mitochondria and ribosomes.cytoplasmpermanentvacuolenucleusribosomescell membranemitochondriachloroplastscell wall10 µm
A leaf cell. The cell is about 40 µm wide. The bar shows 10 µm. Ribosomes are drawn much bigger than they are. Each is about 25 nm across, far too small to see at this scale.

Bacteria are cells too, but they are built differently. A bacterial cell has no nucleus. Its DNA is a single loop that lies free in the cytoplasm. It often has small extra rings of DNA called plasmids.

A bacterium has a cell wall, but the wall is not made of cellulose. It has ribosomes, but they are smaller than the ones in plant and animal cells. It has no mitochondria. Some bacteria swim with a tail called a flagellum.

A bacterial cell with a scale bar of 1 µm. The labels are cell wall, cell membrane, cytoplasm, loop of DNA, plasmids, ribosomes and flagellum.cell wallcellmembranecytoplasmflagellumplasmidsribosomesloop of DNA1 µm
A bacterium. It is about 2 µm wide, and the bar shows 1 µm. There is no nucleus. Ribosomes are drawn much bigger than they are. Each is about 20 nm across, far too small to see at this scale.
Two tables that compare three kinds of cell. The first has the columns Part, Animal and Plant. Nucleus: yes, yes. Mitochondria: yes, yes. Membrane: yes, yes. Ribosomes: yes, yes. Cell wall: no, yes. Chloroplasts: no, some. Vacuole: no, yes. DNA loop: no, no. The second has the columns Part and Bacterium. Nucleus: no. Mitochondria: no. Membrane: yes. Ribosomes: smaller. Cell wall: yes. Chloroplasts: no. Vacuole: no. DNA loop: yes.PartAnimalPlantnucleusyesyesmitochondriayesyesmembraneyesyesribosomesyesyescell wallnoyeschloroplastsnosomevacuolenoyesDNA loopnonoPartBacteriumnucleusnomitochondrianomembraneyesribosomessmallercell wallyeschloroplastsnovacuolenoDNA loopyes
The membrane is the cell membrane. The vacuole is the permanent vacuole. The DNA loop is the genetic material of a bacterium. Plant cells have chloroplasts only where there is light, so the table says some. A plant wall is cellulose. A bacterial wall is not.

See it three ways

As a picture

A bar chart of the width of three example cells. The axis is labelled width in µm, with a scale from 0 to 50 in steps of 10. The bar for animal reaches 20. The bar for plant reaches 40. The bar for bacterium reaches 2.01020304050animalplantbacteriumwidth in µm

In words

Real cells vary a lot. Animal cells are about 10 to 30 µm wide. Plant cells are about 10 to 100 µm. Bacteria are about 1 to 5 µm. The bars show one example of each. Take a plant cell 40 µm wide and a bacterium 2 µm wide. About 20 bacteria fit side by side across the plant cell.

In symbols

40 ÷ 2 = 20

Worked example

A bacterium is 2 µm wide. A plant cell is 40 µm wide. How many bacteria fit side by side across the plant cell?

  1. Both widths are in µm, so they can be divided.
  2. Number across = width of the plant cell ÷ width of a bacterium.
  3. 40 ÷ 2 = 20.

Answer: 20 bacteria

Try it

Build a model cell

You need: a clear plastic bag, water, a stone, two beans, a spoonful of small seeds or grains of rice, a cardboard box that the bag fits in, paper and a pencil for labels.

Be safe: Keep small seeds and stones away from young children. Do not eat or taste anything in the model. Do the water part over a tray or sink.

  1. Half fill the bag with water. This is the cytoplasm. Hold it over a tray or a sink.
  2. Put the stone in the bag. It stands for the nucleus. Add the two beans for mitochondria and drop in the seeds for ribosomes.
  3. Tie the bag closed. The bag is the cell membrane.
  4. Write a label for each part on paper and set it beside the model.
  5. Now push the bag into the box. The box is a cell wall. The bag fills the box and the box holds it in shape. Take the bag out and the shape changes.

What you will see: The membrane holds the cell together but the bag sags. The box is stiff and keeps the shape. That is the job of the cell wall in a plant cell.

If you cannot try it, this is what happens.
A table that matches the model to the real cell. Columns: Model and Real part. Bag: cell membrane. Water: cytoplasm. Stone: nucleus. Beans: mitochondria. Seeds: ribosomes. Box: cell wall.ModelReal partbagcell membranewatercytoplasmstonenucleusbeansmitochondriaseedsribosomesboxcell wall

A wrong idea many people have

Bacteria have no nucleus, so they have no DNA.

Why it feels right: In plant and animal cells the DNA is in the nucleus. No nucleus seems to mean no DNA.

The test that breaks it: A bacterium has to make proteins and copy itself, as all cells do. It needs the instructions in DNA to do that.

What is true: A bacterium has DNA. It is a loop in the cytoplasm, with plasmids beside it, not inside a nucleus.

Key words

nucleus
the part that holds the genetic material and controls the cell.
mitochondria
the parts where aerobic respiration releases energy from glucose.
ribosomes
tiny parts that make proteins.
cell wall
the stiff outer layer of a plant or bacterial cell.
chloroplasts
the parts of a plant cell that absorb light for photosynthesis.
permanent vacuole
a large space of cell sap in a plant cell.
plasmids
small rings of DNA in a bacterial cell.
flagellum
a tail that some bacteria use to swim.

Back to your guess

The answer is A: Plant cells have a cell wall and chloroplasts, and animal cells do not.

Plant and animal cells both have a nucleus, mitochondria and ribosomes. A plant cell also has a cell wall, chloroplasts and a permanent vacuole.

Check yourself

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

  1. Which part of a cell makes proteins?

    Tap your answer.

    1. Not this one. The nucleus holds the instructions for proteins, but it does not make them. Ribosomes read the instructions and build the proteins. Go to S3.1.4. Try another.

    2. Yes, that is right. Ribosomes join amino acids into proteins, following the genetic material.

    3. Not this one. Mitochondria release energy from glucose. They do not make proteins. Try another.

    See the answer

    The ribosomes

    Ribosomes join amino acids into proteins, following the genetic material.

    • If you chose A: The nucleus holds the instructions for proteins, but it does not make them. Ribosomes read the instructions and build the proteins. Go to S3.1.4.
    • If you chose C: Mitochondria release energy from glucose. They do not make proteins.

  2. Where in a cell is energy released from glucose by aerobic respiration?

    Tap your answer.

    1. Yes, that is right. Aerobic respiration takes place in the mitochondria. It releases energy from glucose using oxygen.

    2. Not this one. The nucleus controls the cell, but it is not where energy is released. Try another.

    3. Not this one. Chloroplasts absorb light for photosynthesis, which stores energy in glucose. Aerobic respiration is in the mitochondria. Try another.

    See the answer

    In the mitochondria

    Aerobic respiration takes place in the mitochondria. It releases energy from glucose using oxygen.

    • If you chose B: The nucleus controls the cell, but it is not where energy is released.
    • If you chose C: Chloroplasts absorb light for photosynthesis, which stores energy in glucose. Aerobic respiration is in the mitochondria.

  3. Which of these is found in a leaf cell but not in an animal cell?

    Tap your answer.

    1. Not this one. Plant cells and animal cells both have mitochondria. Plants respire too. Try another.

    2. Not this one. All cells have ribosomes, because all cells make proteins. Try another.

    3. Yes, that is right. Chloroplasts are found in plant cells and algae, and not in animal cells. A leaf cell has them to absorb light.

    4. Not this one. Both plant and animal cells have a nucleus. Go to S3.1.3. Try another.

    See the answer

    Chloroplasts

    Chloroplasts are found in plant cells and algae, and not in animal cells. A leaf cell has them to absorb light.

    • If you chose A: Plant cells and animal cells both have mitochondria. Plants respire too.
    • If you chose B: All cells have ribosomes, because all cells make proteins.
    • If you chose D: Both plant and animal cells have a nucleus. Go to S3.1.3.

  4. Ana looks at the cell in the picture. She says it has no nucleus. Is she right?

    A bacterial cell with the labels cell wall and flagellum, and a key that says the black dots are ribosomes.cell wallflagellumribosome
    Tap your answer.

    1. Not this one. Bacterial cells have no nucleus. They are built differently from plant and animal cells. Try another.

    2. Yes, that is right. A bacterium has no nucleus. Its DNA is a loop in the cytoplasm, sometimes with small plasmids.

    3. Not this one. It has DNA. The DNA is a loop in the cytoplasm, not inside a nucleus. Try another.

    See the answer

    Yes. Its DNA is a loop that lies free in the cytoplasm.

    A bacterium has no nucleus. Its DNA is a loop in the cytoplasm, sometimes with small plasmids.

    • If you chose A: Bacterial cells have no nucleus. They are built differently from plant and animal cells.
    • If you chose C: It has DNA. The DNA is a loop in the cytoplasm, not inside a nucleus.

  5. Joseph says root cells of a plant have chloroplasts, because they are plant cells. What is true?

    Tap your answer.

    1. Yes, that is right. Chloroplasts are for photosynthesis, which needs light. Root cells are underground, so they have none.

    2. Not this one. Chloroplasts need light. Cells in the root are underground and have none. Try another.

    3. Not this one. Root cells have no chloroplasts at all, since they get no light. Try another.

    See the answer

    He is wrong. Root cells are in the dark and have no chloroplasts.

    Chloroplasts are for photosynthesis, which needs light. Root cells are underground, so they have none.

    • If you chose B: Chloroplasts need light. Cells in the root are underground and have none.
    • If you chose C: Root cells have no chloroplasts at all, since they get no light.

  6. Why does a muscle cell have many mitochondria?

    Tap your answer.

    1. Not this one. Making protein is the job of ribosomes. Mitochondria release energy. Try another.

    2. Yes, that is right. Moving needs energy. Mitochondria release it, so a busy cell has many.

    3. Not this one. Animal cells cannot make food from light. Only cells with chloroplasts can. Go to S3.1.4. Try another.

    4. Not this one. The number of mitochondria is about energy, not about the nucleus. Try another.

    See the answer

    It needs to release a lot of energy to make the muscle move.

    Moving needs energy. Mitochondria release it, so a busy cell has many.

    • If you chose A: Making protein is the job of ribosomes. Mitochondria release energy.
    • If you chose C: Animal cells cannot make food from light. Only cells with chloroplasts can. Go to S3.1.4.
    • If you chose D: The number of mitochondria is about energy, not about the nucleus.

  7. A plant cell is 60 µm wide. A bacterium is 3 µm wide. How many times wider is the plant cell?

    See the answer

    20 times wider

    60 ÷ 3 = 20. Both widths are in µm.

  8. A row of bacteria, each 2 µm wide, lies side by side across an animal cell 30 µm wide. How many bacteria are in the row?

    See the answer

    15 bacteria

    30 ÷ 2 = 15.

  9. A cell has a cell wall, a nucleus and chloroplasts. Is it an animal, plant or bacterial cell?

    See the answer

    A plant cell

    Of these three, only a plant cell has chloroplasts. A nucleus rules out a bacterium, and a cell wall rules out an animal cell.

Keep it

Copy this into your exercise book.

Nucleus: holds DNA, controls the cell. Mitochondria: aerobic respiration, releasing energy. Ribosomes: make proteins. Plant cells add a wall, chloroplasts and a vacuole. Bacteria: no nucleus, a DNA loop.

Draw: An animal cell and a plant cell side by side, with each part named.

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 (1.0 MB). It opens on any phone.