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KS3 Science · Year 7Chapter 2 · Cells and microscopes
Year 7 · Chapter 2

Cells and microscopes

By the end of this chapter you will be able to

  • Say that all living things are made of cells, and that some are single-celled
  • Name the structures of an animal cell and say what each one does
  • Name the extra structures of a plant cell and say what each one does
  • Name the parts of a light microscope and use it safely
  • Calculate magnification, image size or actual size, using the same units

You should already know

  • Living things and their parts, from primary school science
  • Fair tests and the use of a ruler and a table (Chapter 1 of this book)

Where this leads

  • Specialised cells, tissues and organs, which build on cell structure (Chapter 3)

National Curriculum for England, Key Stage 3 science, Year 7. Content: B1 all living things are made of cells; the structures of plant and animal cells and their functions, Use of light microscopes; magnification = image size / actual size.

Exercise P  Do you remember?

1
FWhich part of a plant takes in water from the soil? Write the word.
2
FWhat is the name of the process in which living things release energy from food? Write the word.
3
FHow many millimetres are in 1 centimetre? Write the number.
4
FName the gas that animals take in from the air when they breathe. Write the word.
5
FWhat is the name for the process by which green plants make food using light? Write the word.
2.1

All living things are made of cells

Year 7
B1: all living things are made of cells; some are single-celled and some are many-celled
Video lesson: All living things are made of cellsComing soon
Story. Sam looks at a drop of pond water under a microscope and sees tiny moving shapes. “Are these alive?” he asks. His teacher says, “Each living thing is made of cells. Some living things are a single cell, and others are made of millions.”

A cell is the smallest unit of a living thing that can carry out all the processes of life. All living things are made of cells.

An organism made of one cell is single-celled. Amoebae and yeast are single-celled. An organism made of many cells is multicellular, and humans, oak trees and fish are all multicellular.

Most cells are too small to see with the naked eye, so we use a microscope to see them.

Key ideas

  • All living things are made of cells.
  • A single-celled organism is one cell that does all the processes of life.
  • A multicellular organism is made of many cells that work together.
  • Most cells can only be seen with a microscope.
Worked example 2.1.1
Is an amoeba single-celled or multicellular? Write the word.

Method. Decide how many cells the organism has.

  1. Look at the description. An amoeba is a tiny living thing that is one cell.
    one cell
  2. Name the group. An organism made of one cell is single-celled.
    single-celled
  3. Write the answer.
    single-celled

Check. A human has many cells, so is multicellular. ✓

Worked example 2.1.2
A human is made of many cells. Is a human single-celled or multicellular? Write the word.

Method. Decide how many cells the organism has.

  1. Look at the description. A human is made of millions of cells.
    many cells
  2. Name the group. An organism made of many cells is multicellular.
    multicellular
  3. Write the answer.
    multicellular

Check. The amoeba is single-celled by contrast. ✓

Worked example 2.1.3
What is the smallest unit of a living thing? Write one word.

Method. Recall the basic unit.

  1. Recall. The cell is the basic unit of all living things.
    cell
  2. Check. A cell can carry out all the processes of life, so it is the smallest unit.
    processes of life
  3. Write the answer.
    cell

Check. Cells are the building blocks of living things. ✓

Watch out

Watch out: a dead organism was once made of cells, but it is not alive. Being made of cells is not the same as being alive now.

Exam tip

Exam tip. For single-celled, say “one cell”. For multicellular, say “many cells”.

Try it (with your teacher, or a grown-up at home). With a teacher, look at a prepared slide of onion skin or a cell model under a microscope. Count how many cells you can see in one field of view. Sketch the shape of two cells and label one thing you notice.

Exercise 2A  All living things are made of cells

1
FWhat are living things made of? Write one word.
2
FName a single-celled organism. Write one name.
3
FIs a human multicellular? Write yes or no.
4
FWhat is the name of the smallest unit of a living thing? Write one word.
5
FAre most cells visible to the naked eye? Write yes or no.
6
FWhich is multicellular: an amoeba or an oak tree? Write the words.
7
FHow many cells does an amoeba have? Write the number.
8
PSIs a bacterium a living thing that is made of cells? Write yes or no.
9
PSA pupil says a dead leaf is alive because it is made of cells. Is the pupil right? Write yes or no.
10
PSWhat tool is used to see cells? Write one word.
11
PSWorking scientifically: describe how you would check that a sample is made of cells, using a prepared slide. Accept any method that uses a microscope and looks for small units.
12
RExplain why cells are called the building blocks of living things. Write one sentence.
13
RChallenge: explain how a single cell can be a whole living organism. Write one sentence.
2.2

Animal cell structures and their functions

Year 7
B1: the structures of an animal cell (cell membrane, cytoplasm, nucleus, mitochondria) and their functions
Video lesson: Animal cell structures and their functionsComing soon
Story. Priya is a nurse. She knows that muscle cells need a lot of energy to move. “Which parts of a cell make the energy?” she asks. Her teacher says, “Look at the structures inside the cell. Each one has a job.”

An animal cell has several structures, and each one has a function. The cell membrane is the outer boundary; it controls what goes in and out. The cytoplasm is a jelly-like substance where most chemical reactions take place. The nucleus controls the activities of the cell. The mitochondria are where aerobic respiration happens, and this releases energy.

nucleuscell membranemitochondriacytoplasm
A labelled animal cell.

The table gives the function of each structure of an animal cell.

PartWhat it does
cell membranecontrols what goes into and out of the cell
cytoplasma jelly where most of the chemical reactions happen
nucleuscontrols the activities of the cell and holds the genetic material
mitochondriawhere aerobic respiration happens, which releases energy

Key ideas

  • The cell membrane controls what goes in and out of the cell.
  • The cytoplasm is where most chemical reactions take place.
  • The nucleus controls the activities of the cell.
  • Mitochondria are where aerobic respiration happens, and release energy.
Worked example 2.2.1
Which structure controls what goes into and out of an animal cell? Write the words.

Method. Find the structure that is the boundary.

  1. Look at the functions. The structure that controls what enters and leaves is the boundary of the cell.
    boundary
  2. Name it. This is the cell membrane.
    cell membrane
  3. Write the answer.
    cell membrane

Check. The cell membrane is found in animal and plant cells. ✓

Worked example 2.2.2
Which structure is the site of aerobic respiration, where energy is released? Write the word.

Method. Find the structure that releases energy.

  1. Look at the functions. Aerobic respiration releases energy inside the cell.
    releases energy
  2. Name it. Mitochondria are where this happens.
    mitochondria
  3. Write the answer.
    mitochondria

Check. Mitochondria are found in many cells. ✓

Worked example 2.2.3
Which structure controls the activities of the cell? Write the word.

Method. Find the structure that controls the cell.

  1. Look at the functions. The structure that controls the activities of the cell is the control centre.
    control centre
  2. Name it. The control centre is the nucleus.
    nucleus
  3. Write the answer.
    nucleus

Check. The nucleus holds the genetic material. ✓

Watch out

Watch out: the cytoplasm is not the same as the nucleus. The cytoplasm is the jelly where reactions happen; the nucleus controls the cell.

Exam tip

Exam tip. Give the function, not just the name: “the nucleus controls the activities of the cell”.

Try it (with your teacher, or a grown-up at home). With a teacher, look at a prepared slide of animal cells, such as blood or cheek cells. Find the nucleus and sketch one cell. Label the nucleus and the cell membrane. Do not use any stain or heat, and wash your hands after the practical.

Exercise 2B  Animal cell structures and their functions

1
FWhich structure controls what enters and leaves a cell? Write the words.
2
FWhich structure controls the activities of the cell? Write the word.
3
FIn which structure do most of the chemical reactions in a cell take place? Write the word.
4
FWhich structure is the site of aerobic respiration? Write the word.
5
FIs the cell membrane found in an animal cell? Write yes or no.
6
FWrite the name of the process that releases energy in the mitochondria, using oxygen. Write the words.
7
PSA cell has more mitochondria than another cell. Which cell can release more energy by respiration? Write the words.
8
PSMuscle cells need a lot of energy to contract. Which structure should they have many of? Write the word.
9
PSWrite the function of the nucleus. Write one sentence.
10
PSWhich two structures in an animal cell control the cell? Write the two words.
11
PSWorking scientifically: a student draws an animal cell with a cell wall. Is the drawing correct? Write yes or no.
12
RExplain why a cell membrane is needed. Write one sentence.
13
RChallenge: explain how the nucleus and the mitochondria work together in a cell. Write one sentence.
2.3

Plant cell structures

Year 7
B1: plant cells have a cell wall, chloroplasts and a permanent vacuole, in addition to the structures of animal cells
Video lesson: Plant cell structuresComing soon
Story. Ben looks at a leaf and a root under a microscope. “The leaf cells are green, but the root cells are not,” he says. His teacher says, “Plant cells have some extra structures. Some of them are only in parts of the plant that are green.”

A plant cell has all the structures of an animal cell, and three more. The cell wall is outside the cell membrane. It is made of cellulose, and it strengthens and supports the cell. The chloroplasts contain chlorophyll, which absorbs light for photosynthesis. The permanent vacuole is a large space filled with cell sap, which helps keep the cell firm.

nucleusvacuolecell wall (outside)plant cell
A labelled plant cell with a cell wall, chloroplasts and a large vacuole.

The table gives the three extra structures of a plant cell.

PartWhat it does
cell wallmade of cellulose; strengthens and supports the cell
chloroplastscontain chlorophyll and carry out photosynthesis
permanent vacuolefilled with cell sap; helps keep the cell firm

Key ideas

  • A plant cell has a cell wall, chloroplasts and a permanent vacuole.
  • The cell wall is made of cellulose and strengthens the cell.
  • Chloroplasts contain chlorophyll and carry out photosynthesis.
  • Not every plant cell has chloroplasts; root cells do not.
Worked example 2.3.1
Which structure in a plant cell is made of cellulose and gives support? Write the words.

Method. Find the structure made of cellulose.

  1. Read the description. The cell wall is made of cellulose and supports the cell.
    cellulose
  2. Name it. So the structure is the cell wall.
    cell wall
  3. Write the answer.
    the cell wall

Check. Animal cells do not have a cell wall. ✓

Worked example 2.3.2
Which structure contains chlorophyll and carries out photosynthesis? Write the word.

Method. Find the structure that absorbs light.

  1. Read the description. Chlorophyll absorbs light, and photosynthesis happens in this structure.
    absorbs light
  2. Name it. So the structure is the chloroplast.
    chloroplast
  3. Write the answer.
    chloroplast

Check. Chloroplasts are green because of chlorophyll. ✓

Worked example 2.3.3
How many extra structures does a plant cell have compared with an animal cell? Write the number.

Method. Count the extra structures in the table.

  1. Count. The table lists the cell wall, the chloroplasts and the permanent vacuole.
    three
  2. Write the number. So a plant cell has three extra structures.
    3
  3. Write the answer.
    3

Check. Three extra structures are listed. ✓

Watch out

Watch out: the cell wall is not the same as the cell membrane. The cell wall is outside the membrane, and it is only in plant cells.

Exam tip

Exam tip. Name a structure that is in plant cells but not in animal cells: the cell wall, the chloroplasts or the permanent vacuole.

Try it (with your teacher, or a grown-up at home). With a teacher, look at a prepared slide of plant cells, such as onion skin. Find the cell wall. Look at a leaf cell and a root cell, if slides are available, and say which one has more chloroplasts. Do not stain or heat the slide.

Exercise 2C  Plant cell structures

1
FName the structure that is made of cellulose and is found in plant cells. Write the words.
2
FWhich structure contains chlorophyll? Write the word.
3
FWhich process happens in the chloroplasts? Write the word.
4
FDoes an animal cell have a cell wall? Write yes or no.
5
FWhat is the name of the large space in a plant cell that is filled with cell sap? Write the two words.
6
FName the two structures in a plant cell that are not in an animal cell, apart from the cell wall. Write the two words.
7
PSHow many extra structures does the table list for a plant cell? Write the number.
8
PSWhy do leaf cells contain many chloroplasts? Write one sentence.
9
PSIs it true that every plant cell has chloroplasts? Write yes or no.
10
PSA plant cell has no chloroplasts. Can it still be a plant cell? Write yes or no.
11
PSWorking scientifically: a pupil predicts that a root cell has more chloroplasts than a leaf cell. Is the prediction likely to be correct? Write yes or no.
12
RExplain why plant cells need a cell wall. Write one sentence.
13
RChallenge: explain why a plant cell needs both chloroplasts and mitochondria. Write one sentence.
2.4

The light microscope

Year 7
Use of light microscopes: name the parts, focus from low to high power, and use the microscope safely
Video lesson: The light microscopeComing soon
Story. Aisha is shown how to use a microscope for the first time. “Always start on the lowest power,” her teacher says. “Then focus slowly, so the lens does not touch the slide.”

A light microscope uses lenses to magnify small objects. You look through the eyepiece lens at the top. The objective lenses are nearer the specimen. The specimen is placed on the stage, and light comes from a light source underneath.

Always start on the lowest power. Use the coarse focus knob to bring the specimen into view, then the fine focus knob for a sharp picture. Never look at the Sun through a microscope, and carry the microscope with both hands.

Feyepiece lensobjective lensstagelight sourcefocus knob
A light microscope with its main parts labelled.

Key ideas

  • The eyepiece lens is the one you look through.
  • The objective lens is nearest the specimen.
  • Start on low power, use the coarse focus first, then the fine focus.
  • Never look at the Sun through a microscope.
Worked example 2.4.1
Which part of the microscope do you look through? Write the words.

Method. Find the lens at the top.

  1. Look at the diagram. The lens at the top of the microscope is the one you look through.
    at the top
  2. Name it. This is the eyepiece lens.
    eyepiece lens
  3. Write the answer.
    eyepiece lens

Check. The objective lens is the one nearer the slide. ✓

Worked example 2.4.2
Which knob should you use first to bring the specimen into view? Write the words.

Method. Think about the order of focusing.

  1. Think about the order. The coarse focus moves the stage quickly, so it finds the specimen fastest.
    quickly
  2. Decide. So the coarse focus is used first.
    coarse focus
  3. Write the answer.
    coarse focus

Check. The fine focus gives the sharp picture after this. ✓

Worked example 2.4.3
Why should you start on the lowest power? Write one reason.

Method. Think about what the low power shows.

  1. Think about the view. Low power shows more of the specimen, so it is easier to find the thing you want.
    more of the specimen
  2. Decide. So you start on the lowest power to find and focus the specimen safely.
    easier to find
  3. Write the answer.
    Low power shows more of the specimen, so it is easier to find the thing you want.

Check. Starting low protects the slide and the lens. ✓

Watch out

Watch out: do not use the high-power objective lens until the specimen is in focus on low power. Never turn the coarse focus while you look down the eyepiece.

Exam tip

Exam tip. In a question about a microscope, name the part, then say what it does.

Try it (with your teacher, or a grown-up at home). With a teacher, use a light microscope to look at a prepared slide. Start on the lowest power, use the coarse focus, then the fine focus. Draw what you see and label the cell wall, if you can see one.

Exercise 2D  The light microscope

1
FWhich lens do you look through at the top of the microscope? Write the words.
2
FWhat is the name of the part where the slide is placed? Write the word.
3
FWhich part of the microscope gives the light? Write the words.
4
FWhich lens is closest to the specimen? Write the words.
5
FWhich focus knob should you use first? Write the words.
6
FShould you look at the Sun through a microscope? Write yes or no.
7
PSThe eyepiece is x10 and the objective lens is x4. What is the total magnification? Write the number.
8
PSThe eyepiece is x10 and the objective lens is x40. What is the total magnification? Write the number.
9
PSWhich objective lens is the high-power one, x4 or x40? Write the number.
10
PSWorking scientifically: a pupil uses the fine focus first, then the coarse focus. Is this the best order? Write yes or no.
11
RExplain why a microscope should be carried with both hands. Write one sentence.
12
RChallenge: explain why a pupil should use the coarse focus to find the specimen, then the fine focus to sharpen it. Write one sentence.
2.5

Magnification, image size and actual size

Year 7
Use of light microscopes: magnification = image size / actual size, using the same unit for both
Video lesson: Magnification, image size and actual sizeComing soon
Story. Omar draws a cell as he sees it through the microscope. He measures his drawing with a ruler and finds the cell is 8 mm long. “The microscope was set to x40,” he says. “So how big is the real cell?”

The magnification tells us how many times bigger an object looks. It is found using:

magnification = image size / actual size

The image size and the actual size must be in the same unit. Rearranging the formula gives actual size = image size / magnification, and image size = magnification x actual size.

There are 10 millimetres in 1 centimetre, and 1000 micrometres in 1 millimetre. The symbol for a micrometre is µm.

The table gives the image size, the magnification and the actual size for four cells.

Cell or partSize in the imageMagnificationActual size
onion cell8 mmx40200 micrometres
animal cell60 mmx400150 micrometres
nucleus4 mmx20020 micrometres
plant cell25 mmx100250 micrometres

Key ideas

  • magnification = image size / actual size
  • actual size = image size / magnification
  • image size = magnification x actual size
  • 1 mm = 1000 micrometres (µm); use the same unit for image and actual size.
Worked example 2.5.1
An onion cell is 8 mm long in the image, at x40. What is its actual size in millimetres? Write the number of millimetres.

Method. Use actual size = image size / magnification.

  1. Write the values. The image size is 8 mm and the magnification is x40.
    8 mm and x40
  2. Divide. Actual size = 8 divided by 40.
    8 / 40 = 0.2
  3. Write the answer.
    0.2 mm

Check. Check: 0.2 x 40 = 8, the image size. ✓

Worked example 2.5.2
The nucleus is 4 mm in the image, at x200. What is its actual size in micrometres? Write the number of micrometres.

Method. Find the actual size first, then change the unit.

  1. Divide. Actual size = 4 divided by 200.
    4 / 200 = 0.02
  2. Change the unit. There are 1000 micrometres in 1 millimetre, so 0.02 mm is 0.02 x 1000 micrometres.
    0.02 x 1000 = 20
  3. Write the answer.
    20 micrometres

Check. The answer is 20 micrometres, which is 0.02 mm. ✓

Worked example 2.5.3
A plant cell is 0.25 mm long, and the image is 25 mm long. What is the magnification? Write the number.

Method. Use magnification = image size / actual size.

  1. Use the same unit. Both sizes must be in the same unit, so 0.25 mm and 25 mm are used as they are.
    same unit
  2. Divide. Magnification = 25 divided by 0.25.
    25 / 0.25 = 100
  3. Write the answer. So the magnification is x100.
    x100
  4. Write the answer.
    x100

Check. Check: 0.25 x 100 = 25, the image size. ✓

Watch out

Watch out: the image size and the actual size must be in the same unit. Change 1 mm to 1000 micrometres before you divide, if one is in mm and one is in micrometres.

Exam tip

Exam tip. Write the formula first, then put in the numbers, and say which unit you have used.

Try it (with your teacher, or a grown-up at home). With a teacher, draw a cell from a prepared slide at the magnification on the microscope. Measure the image with a ruler in millimetres. Use magnification = image size / actual size to find the actual size, if the magnification is given on the microscope.

Exercise 2E  Magnification, image size and actual size

1
FIn magnification = image size / actual size, which term is on the bottom of the division? Write the words.
2
FHow many millimetres are in 1 centimetre? Write the number.
3
FHow many micrometres are in 1 millimetre? Write the number.
4
FAn animal cell is 6 mm in the image, and the magnification is x30. What is the actual size? Write the number of millimetres.
5
PSA cell is 8 mm wide in the image, and its actual size is 0.2 mm. What is the magnification? Write the number.
6
PSA nucleus is 2 mm in the image, at x100. What is its actual size in millimetres? Write the number.
7
PSA plant cell is 25 mm in the image, and its actual size is 0.25 mm. What is the magnification? Write the number.
8
PSAn onion cell is 0.2 mm long, and it is drawn at x40. How long is the drawing in millimetres? Write the number of millimetres.
9
PSA cell is 0.2 mm long. Write the length in micrometres.
10
PSWorking scientifically: the image of a cell is measured as 40 mm and the actual size is 0.2 mm. Which unit must be used in the formula? Write the words.
11
RChallenge: an image is 40 mm and the actual size is 0.2 mm. A pupil says the magnification is x20. Is the pupil right? Write yes or no, and then the correct magnification.

Chapter summary

SkillMethod
CellsAll living things are made of cells. Single-celled organisms are one cell; multicellular organisms have many.
Animal cellCell membrane controls what enters and leaves. Cytoplasm: chemical reactions. Nucleus: controls the cell. Mitochondria: respiration.
Plant cellPlant cells also have a cell wall (cellulose), chloroplasts (photosynthesis) and a permanent vacuole (cell sap).
MicroscopeStart on low power. Coarse focus first, then fine focus. Never look at the Sun.
Magnificationmagnification = image size / actual size, in the same unit. 1 mm = 1000 micrometres.

Can you…?

  • I can say that all living things are made of cells, and give examples of single-celled and multicellular organisms
  • I can name the structures of an animal cell and say what each one does
  • I can name the three extra structures of a plant cell and say what each one does
  • I can name the parts of a light microscope and use it safely
  • I can calculate magnification, image size or actual size, using the same units

Key terms

cell
the smallest unit of a living thing that can carry out the processes of life
single-celled
made of one cell, such as an amoeba or yeast
multicellular
made of many cells
cell membrane
the boundary of a cell; controls what goes in and out
cytoplasm
the jelly-like substance where most chemical reactions take place
nucleus
the structure that controls the activities of the cell
mitochondria
the structures where aerobic respiration happens, releasing energy
cell wall
a strong layer outside the membrane of plant cells, made of cellulose
chloroplast
a structure in plant cells that contains chlorophyll and carries out photosynthesis
permanent vacuole
a large space in a plant cell, filled with cell sap
magnification
how many times bigger an image is than the actual object
micrometre (µm)
one thousandth of a millimetre

Exercise 2Z  Check-up

1
What are all living things made of? Write one word.
2
Which structure controls what enters and leaves an animal cell? Write the words.
3
Which structure is the site of aerobic respiration? Write the word.
4
Name the structure that is made of cellulose and found only in plant cells. Write the words.
5
Which structure in a plant cell contains chlorophyll? Write the word.
6
Which lens do you look through at the top of a microscope? Write the words.
7
The eyepiece is x10 and the objective is x40. What is the total magnification? Write the number.
8
A nucleus is 4 mm in the image at x200. What is its actual size in micrometres? Write the number of micrometres.
9
Explain why plant cells need a cell wall. Write one sentence.
10
Why should you start on the lowest power when you use a microscope? Write one reason.

Total 16 marks. Allow about 16 minutes.

Exercise 2X  Practice test questions

1
FName the structure that controls the activities of a cell. Write the word.[1 mark]
2
FGive the function of the mitochondria in an animal cell. Write one sentence.[2 marks]
3
FName two structures found in a plant cell but not in an animal cell. Write the two names.[2 marks]
4
PSWrite the magnification of an image that is 30 mm long when the actual size is 0.3 mm. Write the number.[2 marks]
5
PSThe eyepiece lens is x10 and the objective lens is x4. Write the total magnification.[1 mark]
6
FWhich focus knob should be used first when using a microscope? Write the words.[1 mark]
7
PSA cell is 0.2 mm long. Write the length in micrometres. Write the number of micrometres.[1 mark]
8
RExplain why a microscope must not be used to look at the Sun. Write one sentence.[2 marks]
9
PSA plant cell is drawn 50 mm long at x100. Write its actual size in millimetres.[2 marks]
10
RWrite one reason why a plant cell has chloroplasts and an animal cell does not. Write one sentence.[2 marks]
← Chapter 1: Working scientifically (KS3)Chapter 3: Specialised cells, tissues and organs →

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