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GCSE Biology · GCSEChapter 2 · Cell structure and microscopy
GCSE · Chapter 2

Cell structure and microscopy

By the end of this chapter you will be able to

  • Name the parts of animal and plant cells and say what each part does
  • Explain the difference between eukaryotic and prokaryotic cells
  • Use a light microscope on a prepared slide, from the lowest power first, and draw one cell
  • Calculate magnification, image size and actual size, and convert units
  • [H] Compare a light microscope and an electron microscope, using magnification and resolution

You should already know

  • Living things are made of cells (KS3 Biology)
  • Converting between metres, centimetres, millimetres and standard form (Chapter 1)

Where this leads

  • Specialised cells and the cell cycle (Chapter 4)
  • Diffusion across the cell membrane (Chapter 3)
  • Respiration in the mitochondria (Chapter 10)

GCSE Biology. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe the structures of animal, plant, bacterial and yeast cells, and their functions, Distinguish eukaryotic cells from prokaryotic cells, Use a light microscope correctly, and draw a labelled cell, Calculate magnification, image size and actual size, using standard form for very small lengths, [H] Describe electron microscopes and explain resolution.

Exercise P  Do you remember?

1
FWhat is 40 multiplied by 10? Write the number.
2
FHow many millimetres are there in 2 cm? Write the number.
3
FWrite the word for the part of a plant that makes food by photosynthesis, using a green pigment. Write the word.
4
FWhich is larger, a centimetre or a millimetre? Write the word.
5
FWhat is 1000 divided by 100? Write the number.
2.1

Animal and plant cells

GCSE
Describe the structures of animal and plant cells and their functions
Video lesson: Animal and plant cellsComing soon
Context. Hana looks at a leaf under a microscope and sees a box-shaped cell with a thick outer wall. Her friend says, “Our cheek cells do not have walls. Why do leaf cells have them?” To answer, Hana needs to know which parts each cell has, and what each part does.

Every living cell has a cell membrane and cytoplasm. A cell with a membrane-bound nucleus is called a eukaryotic cell. Animal and plant cells are both eukaryotic. The nucleus holds the genetic material, DNA, and controls the cell’s activities.

Plant cells have three extra structures that animal cells do not have: a cell wall made of cellulose, chloroplasts and a large permanent vacuole.

cell membranecytoplasmmitochondrionnucleusribosome
Labelled animal cell (not to scale)
cell wallcell membranechloroplastnucleuspermanent vacuolemitochondrion
Labelled plant cell (not to scale)
PartJobAnimalPlantBacterium
nucleuscontains the genetic material (DNA) and controls the cellyesyesno (DNA loop only)
cell membranecontrols what enters and leaves the cellyesyesyes
cytoplasma gel where most chemical reactions happenyesyesyes
mitochondriawhere aerobic respiration happensyesyesno
ribosomeswhere proteins are madeyesyesyes (smaller)
cell wallsupports and strengthens the cellnoyes (cellulose)yes (not cellulose)
chloroplastswhere photosynthesis happensnoyesno
permanent vacuoleholds cell sap and keeps the cell firmnoyesno
Parts of the cell, their jobs, and where they are found

Key ideas

  • Nucleus: contains DNA and controls the cell.
  • Cell membrane: controls what enters and leaves the cell.
  • Mitochondria: where aerobic respiration releases energy.
  • Ribosomes: where proteins are made.
  • Plant cells add a cell wall, chloroplasts and a permanent vacuole.
Worked example 2.1.1
Name the part of a plant cell that makes food by photosynthesis. Write the word.

Method. Find the part that contains chlorophyll.

  1. Look. Photosynthesis needs light and chlorophyll.
    light and chlorophyll
  2. Name it. Chloroplasts contain chlorophyll, so they make food by photosynthesis.
    chloroplasts
  3. Write the answer.
    chloroplasts

Check. Animal cells have no chloroplasts. ✓

Worked example 2.1.2
Name the structure that supports and strengthens a plant cell. Write the words.

Method. Find the outer layer of a plant cell.

  1. Look. A plant cell has an outer layer outside the cell membrane.
    outer layer
  2. Name it. This outer layer is the cell wall. It is made of cellulose and gives support.
    cell wall
  3. Write the answer.
    cell wall

Check. Animal cells have no cell wall, so they rely on other supports. ✓

Worked example 2.1.3
Name the part that stores cell sap and keeps the cell firm. Write the words.

Method. Find the large fluid-filled space.

  1. Look. A plant cell has a large space filled with cell sap.
    cell sap
  2. Name it. This space is the permanent vacuole.
    permanent vacuole
  3. Write the answer.
    permanent vacuole

Check. The vacuole keeps the cell firm, which helps the leaf stand up. ✓

Watch out

Watch out: the cell wall is not the cell membrane. The cell wall is outside the membrane, and it is fully permeable, while the membrane controls what goes in and out.

Exam tip

Exam tip. When you name a part, write the full name. “Mitochondria” is correct, but “the bit that makes energy” will not earn the mark.

Try it (with your teacher). With your teacher, look at a prepared slide of plant cells and one of animal cells. Make a list of the parts you can see in each, and say which ones are missing from the animal cells.

Exercise 2A  Animal and plant cells

1
FWhich part of a cell contains the genetic material? Write the word.
2
FWhich part controls what enters and leaves a cell? Write the words.
3
FWhich part is the site of aerobic respiration? Write the word.
4
FWhich organelle makes food by photosynthesis? Write the word.
5
FWhich structure supports and strengthens a plant cell? Write the words.
6
FWhich organelle makes proteins? Write the word.
7
FName the liquid-filled space in a plant cell that holds cell sap. Write the words.
8
FWhich gel-like substance is where most chemical reactions take place in a cell? Write the word.
9
FDoes an animal cell have a cell wall? Write yes or no.
10
FWhich structure is found in both animal and plant cells? Write a, b or c. (a) chloroplast (b) cell wall (c) mitochondria
11
PSA student draws a leaf cell without any chloroplasts. Write one reason this drawing could be wrong. Accept any sentence that says leaf cells contain chloroplasts for photosynthesis.
12
PSName two structures that are found in plant cells but not in animal cells. Accept any two of: cell wall, chloroplasts, permanent vacuole.
13
RMuscle cells contain many mitochondria. Explain why, in one sentence. Accept any sentence that says muscle contraction needs energy from respiration, which happens in the mitochondria.
2.2

Bacterial and yeast cells; eukaryotes and prokaryotes

GCSE
Describe bacterial and yeast cells, Distinguish eukaryotic cells from prokaryotic cells
Video lesson: Bacterial and yeast cells; eukaryotes and prokaryotesComing soon
Context. Ali looks at a drawing of a bacterium next to a drawing of a yeast cell. “The bacterium has no clear nucleus,” he says. “So is it a different kind of cell from the yeast?” The answer depends on what is inside each cell.

Bacteria are prokaryotes. Their genetic material is a single loop of DNA that floats in the cytoplasm, not inside a nucleus. Many bacteria also contain small rings of DNA called plasmids. Bacteria have a cell wall, a cell membrane, cytoplasm and ribosomes, but they have no mitochondria.

Yeast is a single-celled fungus. It is a eukaryote, so it has a nucleus with a membrane. Yeast cells have a cell wall, a cell membrane, cytoplasm, a nucleus, mitochondria, ribosomes and a vacuole.

cell wallcell membranecircular DNAplasmidribosomesflagellum
Labelled bacterial cell (not to scale)
cell wallcell membranenucleusvacuolemitochondrion
Labelled yeast cell (not to scale)

Key ideas

  • Prokaryotes (such as bacteria) have no nucleus; their DNA is a loop in the cytoplasm.
  • Eukaryotes (such as animal, plant and yeast cells) have a nucleus.
  • Bacteria have plasmids; some bacteria have a flagellum to move.
  • Yeast is a fungus, a single-celled eukaryote.
Worked example 2.2.1
Say whether a bacterium is a prokaryote or a eukaryote. Write the word.

Method. Check whether the cell has a nucleus.

  1. Look. A bacterium has no nucleus with a membrane.
    no nucleus
  2. Name the group. A cell without a nucleus is a prokaryote.
    prokaryote
  3. Write the answer.
    prokaryote

Check. Yeast is a eukaryote because it has a nucleus. ✓

Worked example 2.2.2
Name the small rings of DNA that many bacteria contain. Write the word.

Method. Find the extra DNA in the bacterium.

  1. Look. Some bacteria carry small extra rings of DNA in the cytoplasm.
    small extra rings
  2. Name them. These small rings are called plasmids.
    plasmids
  3. Write the answer.
    plasmids

Check. The main DNA is one large loop. ✓

Worked example 2.2.3
Name one structure that a yeast cell has, but a bacterial cell does not. Write the word.

Method. Compare the two diagrams.

  1. Compare. Yeast has a nucleus and mitochondria; bacteria have neither.
    nucleus and mitochondria
  2. Choose one. Choosing one of these answers the question.
    one of these
  3. Write the answer.
    nucleus

Check. Accept mitochondria too. ✓

Watch out

Watch out: a bacterium does not have a nucleus, but it still has DNA. Do not write that a bacterium has “no DNA”.

Exam tip

Exam tip. A yeast cell is a eukaryote, not a plant or an animal cell. Yeast is a fungus, which is a different group of living things.

Try it (with your teacher). With your teacher, look at a prepared slide of yeast cells under a microscope, or at a labelled photograph. Make a list of the parts you can see. Do not handle live microorganisms.

Exercise 2B  Bacterial and yeast cells; eukaryotes and prokaryotes

1
FAre bacteria eukaryotes or prokaryotes? Write the word.
2
FDoes a bacterial cell have a nucleus? Write yes or no.
3
FName the loop of DNA found in the cytoplasm of a bacterium. Write the words.
4
FName the small extra rings of DNA that some bacteria contain. Write the word.
5
FDo bacteria have mitochondria? Write yes or no.
6
FIs yeast a eukaryote? Write yes or no.
7
FYeast belongs to which group of living things? Write the word.
8
FName one structure found in both a yeast cell and a bacterial cell. Accept any one: cell membrane, cytoplasm, cell wall, or ribosomes.
9
PSWhich kind of cell has a nucleus: eukaryotes or prokaryotes? Write the word.
10
PSAli says bacteria cannot control their activities because they have no nucleus. Write one sentence explaining why this is not quite correct. Accept any sentence that says the DNA loop in the cytoplasm carries the instructions that control the cell.
11
PSName one difference between a yeast cell and a bacterium. Accept any one: yeast has a nucleus and bacteria do not; yeast has mitochondria and bacteria do not.
12
RChallenge: explain in one sentence why a scientist would call a yeast cell a eukaryote, but would not call a bacterium one. Accept any sentence that says yeast has a nucleus with a membrane, and bacteria do not.
2.3

Using a light microscope (required practical)

GCSE
Use a light microscope to observe, draw and label cells, Work in a safe, orderly way
Video lesson: Using a light microscope (required practical)Coming soon
Context. Hana has a prepared slide of onion cells. Her teacher reminds her: “Start with the lowest power, use the coarse focus first, and then the fine focus. Only draw what you can see clearly.”

A light microscope uses lenses to make an image of a thin specimen. The total magnification is the eyepiece lens magnification multiplied by the objective lens magnification. For example, an eyepiece lens of x10 with an objective lens of x40 gives a total magnification of x400.

The steps are: place the slide on the stage, switch on the light, start with the lowest-power objective lens, use the coarse focus to bring the specimen close, then the fine focus to make the image sharp. Then draw one cell, using a sharp pencil, with a clear outline, no shading, and label lines drawn with a ruler.

StepWhat to doWhy
1Place the prepared slide on the stage and clip itit stays still
2Start on the lowest power objective lensthe specimen is easy to find
3Use the coarse focus, then the fine focusthe lens does not hit the slide
4Draw one clear cell with a sharp pencil and label ita clear record of what you see
Steps for using a light microscope on a prepared slide

Key ideas

  • Always start with the lowest-power objective lens.
  • Use coarse focus first, then fine focus.
  • Total magnification = eyepiece lens x objective lens.
  • Draw with a sharp pencil; label lines are drawn with a ruler.
Worked example 2.3.1
A microscope has an eyepiece lens of x10 and an objective lens of x40. Find the total magnification.

Method. Multiply the eyepiece lens by the objective lens.

  1. Read the lenses. The eyepiece is x10 and the objective is x40.
    x10 and x40
  2. Multiply. Total magnification = 10 x 40.
    10 x 40 = 400
  3. Write the answer.
    400

Check. The total magnification is x400. ✓

Worked example 2.3.2
The eyepiece lens is x10. The objective lens is x100. Find the total magnification.

Method. Multiply the two lens values.

  1. Read the lenses. The eyepiece is x10 and the objective is x100.
    x10 and x100
  2. Multiply. Total magnification = 10 x 100.
    10 x 100 = 1000
  3. Write the answer.
    1000

Check. 1000 divided by 100 is 10. ✓

Worked example 2.3.3
Hana starts on the x4 objective lens. Which focus does she use first: coarse or fine? Write the word.

Method. Recall the order of focusing.

  1. Recall. The coarse focus moves the lens quickly to bring the specimen into view.
    moves quickly
  2. Decide. So the coarse focus is used first, and the fine focus makes the image sharp.
    coarse focus first
  3. Write the answer.
    coarse

Check. Starting on low power makes the specimen easier to find. ✓

Watch out

Watch out: never lower the objective lens onto the slide by using the coarse focus at high power. Use the coarse focus first on low power only.

Exam tip

Exam tip. A drawing should be large, show what you can see, and have labels with ruled lines. Never shade or colour the drawing.

Try it (with your teacher). With your teacher, use a light microscope on a prepared slide of onion skin cells. Start on the lowest power, focus, and draw one cell. Label the cell wall, cell membrane, nucleus and cytoplasm, using lines drawn with a ruler.

Exercise 2C  Using a light microscope

1
FWhich objective lens should you use first, the lowest or the highest power? Write the word.
2
FAn eyepiece lens of x10 and an objective lens of x4. Find the total magnification. Write the number.
3
FAn eyepiece lens of x10 and an objective lens of x40. Find the total magnification. Write the number.
4
FAn eyepiece lens of x10 and an objective lens of x100. Find the total magnification. Write the number.
5
FWhich focus is used first to bring the specimen into view: coarse or fine? Write the word.
6
FWhich focus makes the image sharp once the specimen is in view: coarse or fine? Write the word.
7
FWhich should be done first? Write a or b. (a) draw the cell (b) switch on the light and place the slide on the stage
8
FHow should the labels on a biological drawing be drawn? Write a or b. (a) with lines drawn using a ruler (b) written inside the cell
9
PSWhy should the coarse focus not be used at high power? Write one sentence. Accept any sentence that says the lens could hit the slide and break it or damage the specimen.
10
PSHana cannot see the nucleus clearly in her drawing. Write one reason she should check her focus. Accept any sentence that says the image may be out of focus, so the nucleus cannot be seen clearly.
11
PSHana draws one cell and says the cell wall is the thick line she can see. Name the part of the cell that is outside the cell membrane. Write the words.
12
RChallenge: Hana says her drawing shows the same size as the real cell because she drew it using the microscope. Explain why this is not correct, in one sentence. Accept any sentence that says the image is magnified, so it is larger than the actual cell.
2.4

Magnification, image size and actual size

GCSE
Calculate magnification, image size and actual size, Convert units and write very small lengths in standard form
Video lesson: Magnification, image size and actual sizeComing soon
Context. Hana measures an onion cell in a drawing made from her microscope image. The cell appears 48 mm wide in the picture. The magnification is x400. “How wide is the real cell?” she asks.

The magnification of an image is found by dividing the image size by the actual size. Both sizes must be in the same units. Rearrange the formula to find the image size or the actual size:

magnification = image size / actual size

Very small cells are measured in micrometres (um), where 1 um is 0.001 mm, or 1 x 10-6 m. Small lengths can also be written in standard form.

FindUseExample
Magnificationimage size / actual size40 mm / 0.2 mm = 200
Image sizemagnification x actual size200 x 0.2 mm = 40 mm
Actual sizeimage size / magnification40 mm / 200 = 0.2 mm
Rearranging magnification = image size / actual size
UnitEqual to
1 mm1000 micrometres (um)
1 micrometre (um)0.001 mm
1 micrometre (um)0.000001 m
1 micrometre (um)1 x 10^-6 m
Units used for cell sizes

Key ideas

  • magnification = image size / actual size.
  • Image size = magnification x actual size.
  • Actual size = image size / magnification.
  • 1 mm = 1000 micrometres. 1 micrometre = 1 x 10-6 m.
Worked example 2.4.1
A cell is drawn 40 mm wide, and the image is at x200. Find the actual width of the cell in mm.

Method. Use actual size = image size / magnification.

  1. Write the formula. Actual size = image size / magnification.
    actual size = 40 / 200
  2. Calculate. Divide 40 by 200.
    40 / 200 = 0.2
  3. Write the answer.
    0.2 mm

Check. 0.2 x 200 = 40 mm. ✓

Worked example 2.4.2
A bacterium is 0.2 mm long at x100 magnification. Find the image length in mm.

Method. Use image size = magnification x actual size.

  1. Write the formula. Image size = magnification x actual size.
    100 x 0.2
  2. Calculate. Multiply 100 by 0.2.
    100 x 0.2 = 20
  3. Write the answer.
    20 mm

Check. 20 divided by 100 is 0.2 mm. ✓

Worked example 2.4.3
Convert 0.06 mm into micrometres. Write the number of micrometres.

Method. One millimetre is 1000 micrometres, so multiply by 1000.

  1. Choose the unit. The answer is wanted in micrometres, so the mm value must be multiplied by 1000.
    mm to micrometres
  2. Multiply. Multiply 0.06 by 1000.
    0.06 x 1000 = 60
  3. Check. Each micrometre is smaller, so there are more of them: 60 is a bigger number than 0.06.
    60 > 0.06
  4. Write the answer.
    60 micrometres

Check. 60 divided by 1000 is 0.06 mm. ✓

Watch out

Watch out: check the units before you divide. 40 mm divided by 200 gives 0.2 mm, not 0.2 micrometres.

Exam tip

Exam tip. Write the unit with every answer. A magnification has no unit; a size always has one.

Try it (with your teacher). With your teacher, measure the width of one cell in a drawing made from a prepared slide, in mm. Use the magnification written on the slide to work out the actual width in micrometres.

Exercise 2D  Magnification, image size and actual size

1
FWrite the formula for magnification. Write the words.
2
FAn image is 40 mm wide and the actual size is 0.2 mm. Find the magnification. Write the number.
3
FA cell is 0.5 mm wide and the magnification is x100. Find the image width in mm. Write the number.
4
FAn image is 60 mm wide at x200. Find the actual width in mm. Write the number.
5
FConvert 0.3 mm into micrometres. Write the number.
6
FAn image is 24 mm wide at x400. Find the actual width in mm. Write the number.
7
FWrite 0.00002 m in standard form. Write the answer using x 10^ for the power.
8
FA cell is 5 micrometres across. Write this length in metres in standard form. Write the answer using x 10^ for the power.
9
PSA bacterium is 2 micrometres long and is drawn at x1000. Find its image length in mm. Write the number.
10
PSA cell is 80 mm wide in a drawing at x400. Find the actual width in micrometres. Write the number.
11
PSA cell is 9 mm wide in an image at x300. Find the actual width in micrometres. Write the number.
12
RChallenge: the image of a cell is 60 mm, the magnification is x300, and the actual size is 0.2 mm. Explain whether these numbers are consistent. Accept any answer that finds 60 / 300 = 0.2 mm, so the numbers are consistent.
2.5

Electron microscopes and resolution [H]

GCSE
[H] Compare light and electron microscopes, [H] Explain resolution
Video lesson: Electron microscopes and resolution [H]Coming soon
Context. Ali looks at a bacterium in a school textbook. The picture was taken with an electron microscope. “It is much clearer than the light microscope pictures we used,” he says. “Why is that?”

[H] Higher tier only. This section is higher-tier content. It is not needed for the foundation tier, but the ideas link to the magnification work above.

An electron microscope uses a beam of electrons instead of light. It has a much higher magnification and a much better resolution than a light microscope. Resolution is the ability to tell apart two points that are close together. Electrons have a much shorter wavelength than light, so smaller structures can be seen.

The specimen for an electron microscope must be dead and placed in a vacuum, so living cells cannot be seen. A light microscope can show living cells.

TypeHow it worksMagnificationNotes
Light (optical)visible light passes through the specimenup to about x2000living cells can be seen
Electrona beam of electrons passes through or over the specimenmuch higher, far more than a light microscopeonly dead, fixed specimens; a vacuum is needed
Light and electron microscopes compared

Key ideas

  • [H] Electron microscopes have a higher magnification and better resolution than light microscopes.
  • [H] Resolution is the ability to tell apart two points that are close together.
  • [H] Electron microscopes need dead specimens in a vacuum.
  • [H] Ribosomes are too small to see with a light microscope but can be seen with an electron microscope.
Worked example 2.5.1
[H] An electron micrograph shows a bacterium 50 mm wide at x50000. Find the actual width in mm. Write the number.

Method. Use actual size = image size / magnification.

  1. Write the formula. Actual size = image size / magnification.
    50 / 50000
  2. Calculate. Divide 50 by 50000.
    50 / 50000 = 0.001
  3. Write the answer.
    0.001

Check. 0.001 mm = 1 micrometre. ✓

Worked example 2.5.2
[H] Which microscope has the better resolution? Write the words.

Method. Recall which microscope shows smaller separate points.

  1. Recall. The electron microscope can tell apart points much closer together.
    closer together
  2. Decide. So the electron microscope has the better resolution.
    electron microscope
  3. Write the answer.
    electron microscope

Check. A higher magnification alone does not give better resolution. ✓

Worked example 2.5.3
[H] Name one structure that can be seen with an electron microscope but not with a light microscope. Write the word.

Method. Compare the sizes of the structures.

  1. Compare. Ribosomes are too small for a light microscope to show clearly.
    too small
  2. Name it. Ribosomes can be seen with an electron microscope.
    ribosomes
  3. Write the answer.
    ribosomes

Check. Ribosomes are found in all the cells in this chapter. ✓

Watch out

Watch out: a high magnification does not always give a clear image. Resolution decides how much detail can be seen.

Exam tip

Exam tip. [H] When comparing microscopes, give one difference in magnification or resolution and one difference in the specimen or the cost.

Try it (with your teacher). With your teacher, use a light microscope and an electron micrograph picture of the same kind of cell. Write down two things that the electron micrograph shows more clearly.

Exercise 2E  [H] Electron microscopes and resolution

1
F[H] Which type of microscope uses a beam of electrons? Write the words.
2
F[H] Which microscope has the higher magnification? Write the words.
3
F[H] Which microscope can be used to see living cells? Write the words.
4
F[H] Give one reason why an electron microscope cannot show living cells. Accept any sentence that says the specimen must be dead and placed in a vacuum.
5
F[H] Name one structure that is too small to be seen clearly with a light microscope but can be seen with an electron microscope. Write the word.
6
F[H] Define resolution. Write the words.
7
F[H] Which microscope has the better resolution: light or electron? Write the word.
8
PS[H] Name one advantage of a light microscope over an electron microscope. Accept any one: it can show living specimens, it is cheaper, or it is simpler to use.
9
PS[H] An electron micrograph of a cell has a magnification of x50000. The image of a ribosome is 40 mm wide. Find the actual width in mm. Write the number.
10
PS[H] Explain why an electron microscope can show more detail than a light microscope. Accept any sentence that says electrons have a much shorter wavelength than light, so smaller points can be told apart.
11
R[H] Compare a light and an electron microscope in two sentences. Accept any answer that gives one difference in magnification or resolution, and one difference in the specimen, such as living or dead.
12
R[H] Explain why a high magnification alone does not give a good image. Accept any sentence that says the image also needs good resolution, because a blurred magnified image shows no more detail.

Chapter summary

SkillMethod
Animal cellNucleus, cytoplasm, cell membrane, mitochondria, ribosomes.
Plant cellThe animal cell parts plus a cell wall, chloroplasts and a permanent vacuole.
BacteriumProkaryote: a DNA loop and plasmids in the cytoplasm; no nucleus or mitochondria.
YeastA single-celled fungus and a eukaryote with a nucleus and mitochondria.
Light microscopeStart on the lowest power; coarse focus, then fine focus; total magnification = eyepiece x objective.
MagnificationMagnification = image size / actual size; keep the units the same.
Units1 mm = 1000 micrometres; 1 micrometre = 1 x 10-6 m.
[H] Electron microscopeHigher magnification and better resolution; needs a dead specimen in a vacuum.

Can you…?

  • I can name the parts of animal, plant, bacterial and yeast cells and say what they do
  • I can say whether a cell is a eukaryote or a prokaryote
  • I can use a light microscope on a prepared slide, starting on the lowest power
  • I can calculate magnification, image size and actual size, and convert units
  • I can write very small lengths in standard form
  • [H] I can compare light and electron microscopes, and explain resolution

Key terms

cell membrane
the layer around the cell that controls what enters and leaves
cytoplasm
the gel-like substance in a cell where many reactions happen
nucleus
the part of a eukaryotic cell that holds DNA and controls the cell
cell wall
a strong outer layer of plant, bacterial and fungal cells that gives support
eukaryote
an organism whose cells have a nucleus with a membrane
prokaryote
an organism whose cells have no nucleus with a membrane, such as a bacterium
plasmid
a small ring of DNA found in some bacteria
magnification
how many times bigger an image is than the actual object
micrometre
one thousandth of a millimetre (1 x 10-6 m)
resolution
[H] the ability to tell apart two points that are close together

Exercise 2Z  Check-up

1
Name the part of an animal cell that contains DNA. Write the word.
2
Name the structure found in plant cells, but not in animal cells, that holds cell sap. Write the words.
3
Is a bacterial cell a prokaryote or a eukaryote? Write the word.
4
Name the small extra rings of DNA in some bacteria. Write the word.
5
A microscope has an eyepiece lens of x10 and an objective lens of x40. Find the total magnification. Write the number.
6
An image is 36 mm wide at x300. Find the actual width in mm. Write the number.
7
Convert 0.12 mm into micrometres. Write the number.
8
Write 0.0005 m in standard form. Write the answer using x 10^ for the power.
9
Name one structure that a yeast cell and a plant cell both have, and say what it does. Accept any one of: nucleus (holds DNA), mitochondria (aerobic respiration), cell membrane (controls movement), or cytoplasm (chemical reactions).
10
A pupil says a bacterium has no DNA because it has no nucleus. Explain why this is wrong, in one sentence. Accept any sentence that says the bacterium has a DNA loop in the cytoplasm.

Total 25 marks. Allow about 25 minutes.

Exercise 2X  Exam-style questions

1
FName the part of a cell that controls the cell’s activities. Write the word.[1 mark]
2
FName the organelle where aerobic respiration takes place. Write the word.[1 mark]
3
FName two structures that are found in plant cells but not in animal cells. Accept any two of: cell wall, chloroplasts, permanent vacuole.[2 marks]
4
PSA microscope image of a cell is 30 mm wide at x300. Find the actual width in mm. Write the number.[2 marks]
5
PSConvert 0.1 mm into micrometres. Write the number.[2 marks]
6
PSYeast is a eukaryote. Explain why, using one feature of a yeast cell. Accept any sentence that says yeast has a nucleus with a membrane.[2 marks]
7
FExplain how to use a light microscope to look at a prepared slide. Write three steps. Accept any answer with: start on the lowest power, use the coarse focus, then use the fine focus.[3 marks]
8
PSHana draws a cell that is 48 mm wide in her drawing, at a magnification of x400. Find the actual width of the cell in micrometres. Write the number.[3 marks]
9
PSDescribe two differences between a plant cell and an animal cell, and give one job of a chloroplast. Accept any answer with two differences from cell wall, chloroplasts and permanent vacuole, and a job such as photosynthesis.[4 marks]
10
PSA student measures a bacterium as 0.002 mm long. Write this in micrometres, then in metres in standard form. Accept 2 micrometres and 2 x 10^-6 m.[5 marks]
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