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Section 5 : Cells

Section 5 : Cells

Theme: Dealing with challenging ideas in science

Learning Outcomes

By the end of this section, you will have:

  • used pair discussion and probing questions to review and develop students’ understanding of the basic structure of plant and animal cells;
  • used mathematical activities that help students gain an idea of the relative size of cells;
  • helped students make and evaluate their own models of cells.

Introduction

Your students will have been taught about cells in primary school. However, they are likely to have a number of misconceptions about what cells are really like. Developing an understanding of the size of cells is difficult. The fact that cells can only be seen with the aid of a microscope adds to this difficulty. Research has shown that some students confuse ideas about cells and molecules, including their relative sizes. Although three-dimensional diagrams of cells may be shown in textbooks, photographs of cells as seen under the microscope are always two dimensional. It is difficult for students to imagine the 3D structure. Other common incorrect ideas that students may hold about cells include thinking that plant cells are surrounded by cell walls instead of cell membranes, rather than by both a membrane and a wall.

1. Focus on language to support understanding

Researchers have established a clear link between language and learning. When students discuss ideas with peers, they have time to draw on their memory of what they have done before, share ideas with their partner and clarify their thoughts by having to explain them to others. It also helps them to get used to the scientific words, which might not be familiar to them. You get the chance to listen to what they are saying and look at what they are writing, so that you are aware of their misconceptions when you plan your questions at the end. You are far more likely to address their misconceptions in this way. Too often when we use questions in a whole class discussion, we assume that because one student can give us a correct answer, the class as a whole understands the topic well.

Activity 1 will take more time than simply explaining cell structure to the whole class and asking them to copy labelled diagrams and notes. However, it will help the students to understand.

Case study 1: Creating a word wall

Mrs Keraro worked in a secondary school in Moshi, Tanzania. She was concerned that her 13-year-old students found scientific words difficult to pronounce and remember. She created a ‘word wall’ in the classroom. Every time they started a new topic she wrote the key words on card from an old cereal packet and stuck them on the wall. Whenever she had 5 or 10 minutes to spare in a lesson, she would play a game with her class. One person pointed to a word and someone else had to say it and explain the meaning. Alternatively, she divided the class into teams. She would say the meaning and one person from each team had to run to the wall and point to the word. She encouraged her students to make up different games. At the end of the year, their understanding of scientific words had improved a great deal.

She did this with the cells topic; she put up the technical words like ‘chloroplast’ and ‘membrane’, but also the easy words like ‘cell’ and ‘cell wall’. This is because she thought her students might think they knew what a ‘cell’ was – a small room where a prisoner is kept! Lots of scientific words have different meanings in real life and she knew that this often confused her students. She also put up two large photographs of cells as seen using a light microscope. She asked the students to look carefully at the pictures and to talk about them in their pairs. During their discussions, she asked them to write down three interesting observations about the object in each photo. She also asked them to think of two questions which they would like to ask about each of these objects.

Activity 1: Working in pairs to discuss cells

Before the lesson, draw diagrams of generalised animal and plant cells on the board, without labels. Ask each student to copy the diagrams. Also, on the board write the names of the main structures (see Resource 1). Tell pupils to work in pairs or threes to label the diagrams and annotate them with the functions of each part. No one is allowed to write in the label or the function until they all agree. Talking about the answers will help them to learn. While they are working, move round the room. Visit the back of the room first. When you discuss the labels, your initial questions will mainly focus on recall, but try to follow these up with a more demanding question. You could check their understanding at the start of the next lesson by using the true/false exercise (Resource 2). Again, let your students work in pairs and discuss the answers.

2. How big are cells?

It is very difficult for us to get a real idea of very small and very large sizes. So, when we are thinking about things like molecules, cells or the solar system it can be helpful to compare their size with things we are familiar with. In Case study 2, the teacher was fortunate enough to have a good, working microscope and was able to give concrete experience of one of the measurements on the worksheet. When the students do the calculations in Activity 2 they will consider the dimensions of a cell in a number of ways. The activity will help them to develop an understanding of cells, as the building blocks of living things, rather than as diagrams in a book. It will also give them practice of numeracy skills in science and give you an understanding of their ability in maths. This may affect your planning when teaching other science topics with a mathematical content.

Case study 2: Looking at onion cells

Mr Baguma had one microscope to use with his class. He also had 40 glass microscope slides. He did not have cover slips for the slides, but he used a second slide instead of a cover slip when preparing slides with his class. He divided the class into groups of four. Mr Baguma showed the microscope to the whole class and pointed out the main parts and what they do. He demonstrated how to prepare a slide of onion cells to view using the microscope and explained how to use a ruler with the microscope to estimate the size of the cells (Resource 3). He then asked each group to make a slide of onion cells. The groups took it in turns to come up to the front bench to look at their slide using the microscope. While they were waiting to use the microscope, Mr Baguma set some questions and calculations for the class to work on to help them appreciate just how small cells really are (Resource 4). He realised that some of the students were finding the questions difficult, which was a problem as he needed to help with the microscope. So he encouraged the students to help each other. The rule was that they could only write down the answer if they understood where it had come from. Jophus is very good at maths and really enjoyed helping his friends. After each pair had measured their onion cells, they were allowed to write the measurements in a table Mr Baguma had drawn on the board. At the end of the lesson, they could see that there is variation in cell size, but that the variation falls within certain limits.

Activity 2:Thinking about the size of cells

Remind students that you can only see cells with a microscope. Discuss why this is so. Probe their understanding of magnification and use analogies such as buildings made of stones or bricks. If you are far away you can only see the building, but as you get closer you see the bricks or stones. Compare cells to atoms and molecules which are much too small even to see under a normal microscope. Ask pupils to guess how big cells really are. Explain that most cells are between 0.01 mm and 0.1 mm in size. Do they know anything else that is so small? Can they imagine this size? Ask them to carry out all or some of the calculations in Resource 4 . If there are students who find maths difficult, you could ask them to work in pairs. When you check the answers, discuss the extent to which these exercises helped their understanding and ask them to write their own questions.

3. Building Models of cells

One way of helping your students to visualise things like cells (or viruses or molecules) is to let them build models. Resource 6 explains some of the advantages of using models in science. A resourceful science teacher will collect materials such as cardboard packets, plastic, packaging materials, wood and clay so that when they wish to build models, they have materials the students can use. You could also ask your students to collect materials and keep them in a cardboard box in your classroom. When students see cells in diagrams or on microscope slides, it is quite difficult for them to imagine the cells in 3D. You should encourage them to think about materials that will best represent their ideas of what a cell is like. Getting them to plan and deliver a presentation about their model means that they will have to clarify their own thoughts and explain them to others. Our understanding of abstract concepts is closely linked to our ability to use language to order our thoughts about them. While there are advantages in asking students to present to the whole class, this can take a lot of time and many of the benefits are just as great if they do the presentation to a partner.

Case study 3: Making and assessing models

Mrs Muthui had been teaching for 2 years. When she was at college her tutor had encouraged her to use models with her students. Last year her students made models of cells, but Mrs Muthui did not think it had worked very well. The students did not really understand what she was looking for. So this year, she did it differently. She showed her students some of the ones that she had saved from last year. She asked them which one they thought was the best and to explain why. Together, they made a list of marking criteria for the models. She then gave the class two weeks to make a model, working in groups of two or three, and was delighted to find them in the classroom before and after school, working on their ideas. She organised a display and asked her students to mark each others’ models. She invited the head of department and the headteacher to see the display. Everyone was talking about it and some of the other teachers came to see as well. Mrs Muthui was delighted. The models were much more creative and imaginative than last year and she realised that sharing the marking criteria with the students had helped them to understand what was expected of them. She began to do this more often and gradually found that the students took more responsibility for their own learning.

You can see the criteria in Resource 5 – but don’t just use those, make up some of your own.

Activity 3: Making and presenting models

In teaching about cells, you will have introduced your students to cells that are adapted to a particular function, and you will have encouraged them to draw diagrams of the cells in their notebooks. Ask them to make a 3D model of one of the cells they have learned about. Give them materials such as cardboard, water, clay, wool, plastic drinks bottles, plastic bags or yoghurt pots, but also encourage them to use any other available materials.

When they have made their models, ask them to prepare a spoken presentation on the model. They should explain the structure of their cell and how it is adapted to its function. Encourage them to point out any aspects of the real cell which they could not show accurately on their model. They should all get the chance to work in pairs, giving their presentation to their partner. If you have time, you could choose the best models and ask those students to make a presentation to the whole class.

Resource 1: Background information on cells

Background information/subject knowledge for teacher

Diagrams of a plant and an animal cell

Diagram 1 : Typical plant cell in 3D, cut through to see inside.
Diagram 2 : Typical animal cell, shown in section.

Teachers’ resources

Names of cell structures for students to label diagrams of plant and animal cells:

  • A.Central cell vacuole
  • B.Cell membrane
  • C.Nucleus
  • D.Cytoplasm
  • E.Chloroplast
  • F.Vacuole (small)
  • G.Cellulose cell wall

Information about cell structures for students to use as annotations on diagrams of plant and animal cells:

(A structure may have more than one annotation. Some annotations apply to both plant and animal cells.)

  1. The outer layer of this cell is firm and rigid. It is made of cellulose. It gives the cell a definite shape.
  2. The outer layer of this cell is not very rigid. It causes the shape of the cell to be flexible.
  3. This is a large central area in the middle of the cell. It is filled with liquid. This helps to make the cell firm and gives support to the whole plant.
  4. This structure controls what goes into and what comes out of the cell.
  5. This structure controls what the cell is like and how it works.
  6. This is a jelly like fluid containing many granules. Activities such as releasing energy and making proteins happen here.
  7. These structures are used to make food by photosynthesis.
  8. These are small vacuoles found in cells. They may contain food particles, chemicals made by the cell or germs that are being destroyed by the cell.

Photographs of plant and animal cells as seen using a light microscope:

Cells from a moss leaf.
Cells from inside the human cheek.

Resource 2: True/false exercise on cells

Teacher resource for planning or adapting to use with pupils

Plant and animal cells: true–false exercise

The following statements are about cells.

Read each statement and then in your group decide, for each statement, whether it is true (T) or false (F). If you are not sure put ‘?’.

Write the letter or symbol to show your decision in the middle column. Use the last column to explain your reasons.

Statement True, false or unsure Comment – reasons for your choice
1 All cells have a nucleus.
2 There is a cell membrane around all cells.
3 All cells have a cell wall.
4 Chromosomes are found in the cytoplasm.
5 The cell membrane controls what the cell will look like and how it behaves.
6 The nucleus controls what passes into and out of a cell.
7 A nucleus is smaller than a chloroplast.
8 A chloroplast is larger than a mitochondrion.
9 All cells have a central cell vacuole filled with fluid.
10 The cell membrane is made of cellulose.
11 The nucleus is always found in the middle of a cell.
12 A nucleus is smaller than a molecule.
13 Some cells in your body are one metre long.
14 If you looked through a magnifying glass at a red blood cell, it would look like the dot at the end of this sentence.
15 A fully grown human is made of about one hundred million, million cells.
16 Fifty typical cells lying side by side would measure about 1 mm.
17 Cells are black and white. There is no colour in a cell.
18 If you could shrink and stand in a cell, everything would be silent and still.
Teachers’ notes: statements 1–9
Statement True, false or unsure Notes for teachers
All cells have a nucleus. True, with some qualifications. See notes. All plant and animal cells have a nucleus. In some cells, the nucleus may have disintegrated by the time the cell reaches maturity. Red blood cells have a nucleus when they are developing. However, when they are mature and doing their job of carrying oxygen round the body, the nucleus has broken down. The whole cell is full of the oxygen-carrying pigment called haemoglobin. Bacterial cells are usually described as having a nuclear area, rather than a true nucleus, because there is not a nuclear membrane round the nuclear area.
There is a cell membrane around all cells. True
All cells have a cell wall. False Plant cells are surrounded by a cell membrane and outside this is a fairly rigid cell wall. The cell wall gives the plant cell a more definite shape than an animal cell. Animal cells do not have a cell wall. They are surrounded by a cell membrane only. A cell membrane is much more flexible that a cell wall.
Chromosomes are found in the cytoplasm. False Chromosomes are found in the nucleus.
The cell membrane controls what the cell will look like and how it behaves. False The nucleus controls what the cell looks like and how it behaves.
The nucleus controls what passes into and out of a cell. False The cell membrane controls what passes into and out of a cell.
A nucleus is smaller than a chloroplast. False A nucleus is about three times as big as a chloroplast.
A chloroplast is larger than a mitochondrion. True A chloroplast is three or four times larger than a mitochondrion.
All cells have a central cell vacuole filled with fluid. False Plant cells have a central cell vacuole filled with fluid, called cell sap. Animal cells do not. Animal cells may contain one or several small vacuoles.
Teachers’ notes: statements 10–18
10 The cell membrane is made of cellulose. False Plant cell walls are made of cellulose. Cell membranes of both plant and animal cells are made of protein and lipid.
11 The nucleus is always found in the middle of a cell. False The nucleus may be found in the middle or around the edge of a cell.
12 A nucleus is smaller than a molecule. False A nucleus is much larger than a molecule. The nucleus contains chromosomes – 46 in human cells. Each chromosome is made of a DNA molecule.
13 Some cells in your body are as long as metre. True Nerve cells are made up of a compact cell body with long thread like fibres extending out from the cell body. Some of these fibres, called axons, may be up to one metre long. Nerve messages are carried along these fibres.
14 If you looked through a magnifying glass at a red blood cell, it would look like the dot at the end of this sentence. True
15 A fully grown human is made of about one hundred million million cells. True
16 Fifty typical cells lying side by side would measure about 1 mm. True
17 Cells are black and white. There is no colour in a cell. False Chloroplasts are found in most plant cells. Chloroplasts contain the green pigment chlorophyll. Cells in flower petals will contain coloured pigments either in special structures like chloroplasts in the cytoplasm or dissolved in the cell sap. Animal cells, e.g. in the skin and eye of human will also contain coloured pigments.
18 If you could shrink and stand in a cell, everything would be silent and still. False

Everything would certainly not be still. Cytoplasm is continually moving around. Molecules would be moving in and out of cells. In a muscle cell you might find yourself squeezed and released from time to time.

The swishing of the streaming cytoplasm is likely to set up vibrations. Sound is vibration. If you could manage to shrink to fit in a cell your ears could probably manage to detect these vibrations as sounds!

Resource 3: Working with onion cells

Teacher resource for planning or adapting to use with pupils

Preparing a slide of an onion cell and measuring a cell

You will need:

Microscope

Scissors

Microscope slide

Dropper pipette

Cover slip

Clear plastic ruler

Dilute iodine solution.

Preparing the onion slide

What to do:

  1. Slice an onion in two, lengthwise.
  2. Remove one of the thick leaf-like structures from inside.
  3. Pull away a piece of the thin papery lining of its inner surface.
  4. Using scissors, cut a small square of this lining, about 5 mm x 5 mm.
  5. Place this square on the centre of a slide.
  6. Add a drop of dilute iodine solution – make sure the solution spreads below as well as above the square of onion skin. The iodine acts as a stain to make the structures in the cell easier to see.
  7. Carefully lower a cover slip over the onion skin. Try to avoid trapping air bubbles.
  8. Place the slide on the microscope stage. Examine first using the low power. Focus carefully.
  9. Choose an area of the slide where the cells can be clearly seen. Switch to high power and refocus.
  10. Look for the structures shown in the photographs in Resource 1.

Measuring the onion cell

What to do:

  1. Place the ruler on the microscope stage under the low power objective lens.
  2. Move the ruler so the edge with the scale can be focused in the centre of the field of view of the microscope, as in Diagram 1 below.
Diagram 1: The field of view of a microscope.
  1. Use the scale to measure the field of view of your microscope.
  2. The diameter of the field of view in Diagram 1 is approximately 5 mm.
  3. You can use the measurement of the field of view in your microscope to estimate the size of objects viewed with the same objective lens.
  4. The cell viewed in Diagram 2 would be about 2 mm long if viewed with the microscope with the field of view shown above.
Diagram 2: Cell.
  1. Estimate the length and width of your onion cell using this method.

Using a microscope

The main parts of a light microscope are shown below

Diagram 3: Main parts of a light microscope.

Resource 4: Magnification exercise

Teacher resource for planning or adapting to use with pupils

Magnification exercise

1

One of the plant cells in the photograph really measures about 0.01 mm x 0.07 mm. Draw it in your book so its shape and proportions are nearly the same as in the picture.

Calculate magnification of the cell in this way:

Length of cell in drawing ÷ width of real cell = X ______

Mean magnification = X _________

Add this magnification to your drawing.

2

Draw an animal cell that is nearly round in shape.

Measure the diameter of your drawing.

Assume this cell is really 0.02 mm in diameter.

Calculate the magnification and show it on your drawing.

3

Draw a line, 10 mm long.

How many of the animal cells could fit on this line side to side?

How many of the plant cells could fit lengthwise on this line side to side?

4

A person’s stride length is about 1 metre.

If you were magnified by the same amount as your drawing of an animal cell, how far could you go in one stride?

5

Measure the length of a small animal such as an ant.

If its skin cells are 0.02 mm long. How many are there along the length of the animal?

6

Write your own question that helps you to understand the size of a cell.

Why does it help you to get a better understanding of this?

See if your neighbour or the rest of the class can work out the answer.

Resource 5: Assessing models

Teacher resource for planning or adapting to use with pupils

Criteria for assessing models of cells

  1. Is the model clearly labelled?

    It should be obvious what all the parts are and they should be labelled.

  2. Does it include all the relevant parts?

    All the key parts should be present.

  3. Are the parts in the right proportion in terms of size?

    For example, if a tennis ball is used for the nucleus, the chloroplasts could be marbles.

  4. Does the model reflect the nature of the cell? Have appropriate materials been used to represent the parts?

    For example, is the cell wall made of a rigid material? Is the cytoplasm squashy?

  5. Does the model reflect the 3D aspects of cells?

    Children find this aspect of cells difficult to imagine and one of the main reasons for making a model is to show the 3D nature of the cell.

Resource 6: Using models in science

Teacher resource to support teaching approaches

Using models in science

Using models or analogies is a very powerful way of helping children to understand scientific ideas. Used properly, models can also help to develop critical thinking. You can do this by helping children to evaluate the strengths and weaknesses of a model.

Some general principles to think about when planning lessons with models are:

  • introduce the model early in the teaching of the topic, then use the model consistently until it is replaced by a more sophisticated one
  • ensure students make links between the model and the real situation
  • ensure students recognise the differences between the model and what it is illustrating
  • encourage students to apply their understanding to explain new ideas
  • encourage students to identify strengths and weaknesses in any model
  • increase the sophistication of the model when necessary.

A useful approach when you are planning a sequence of lessons based on a model such as the particle model might be:

  1. Teach the original model explicitly – show which part relates to which, making sure students understand and picture it.
  2. Test the original model by applying it – students practise using the model to explain simple ideas. For example, explaining why gases can be compressed, liquids can’t be compressed, solids are hard, etc.
  3. Challenge the original model – by using it to explain more complicated things like melting, dissolving and evaporating.
  4. Develop a ‘better model’ – if necessary explore the development of a better model with the students or provide a more sophisticated one.

Once students have a good understanding of the particle model, this will help them to understand concepts such as why materials have different properties, osmosis, Brownian motion, density, elements, compounds and chemical change.