Seeing Inside Cells

Initiate a class discussion prior to beginning the activity. Display these images of cells. Review and discuss cell structures already familiar to students.

Call attention specifically to

1. cell membrane
2. cytoplasm
3. nucleus
4. DNA

For each item (a–d), ask students to turn to each other to discuss

• its properties (shape, stretchiness)
• its environment (location)
• its function (the job it does)

Have students offer ideas and record some responses on this chart projected or drawn on chart paper or a white board.

What Cell Parts Are and Do

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This modeling activity requires quite a bit of setup. While you can have your students set up their envelopes for a few minutes at the start of the activity, and to reset when they are done, it may actually be less time consuming to do it outside of classroom time.

The numbers are minimums. It may take a lot less time to buy many extra supplies and overestimate numbers. Have the students reset between classes.

At this point in the activity, have the students review their supplies to make sure theirs match or exceed these minimum numbers. Have a stash of replacements at the head of the classroom where students can replenish their supplies if their numbers fall short.

Setting up containers

What does a chemical reaction look like in this model?

Demonstrate a chemical reaction in the model. A chemical reaction is a reconfiguration of the materials. See how in the animation two sets of two binder clips attached to a paper clip break apart and rearrange into a new configuration? This is a chemical reaction within this model world.

With only a handful of office materials on the desk (not even the cell membrane), model how something that the cell needs can be created by assembling it with component parts. (Ultimately, this skill will be used to show how the cell produces what it needs by combining the necessary materials.)

Start with a material you want students to make (include the name of it and which office materials are necessary to make it) and show them how to assemble it.

The materials represent, loosely, different kinds of proteins and other materials that make up the cell. The binder clips represent pieces of DNA. The silver and gold paper clips represent the lipids of different membranes. The other paper clips represent the materials that make up organelles. But not all the organelles are represented.

Model the "instruction center" for your cell. Ask students to look at the contents of Container 1 (with eight or more binder clips in four colors, at least two of each).

Ask:

What do you notice about the binder clips?

Students should notice there are four different colors.

Ask students to link four binder clips together. Ask one group to share their sequence of colors. A sequence might be, for example, blue, yellow, red, black.

Suggest (falsely) that this is the only sequence possible. Encourage students to interrupt to correct you. Ask for proof. Students can point out that there are other sequences. Tell them this four-clip sequence represents DNA.

Model the "instruction center" for your cell

Link four binder clips together. Place one strand of four binder clips inside the cell nucleus to represent DNA, like this.

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What order are your colors in?

Put the DNA in a nucleus. Ask students to look at the contents of Container 2. Ask:

How is the DNA protected from the other parts inside in the cell?

Intended answer:

The nucleus is separated by a nuclear membrane.

Ask students to use about half of the small paper clips to make a model of the nuclear membrane.

How is the DNA protected from the other parts inside in the cell?

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Put the DNA in a nucleus

Use about half of the regular sized paper clips to make a model of the nuclear membrane, like this.

Add a cell membrane. Ask students to look at the contents of Container 3.

Ask:

How is the inside of the cell protected from the other parts outside the cell?

Intended answer:

The cytoplasm and organelles are protected by a cellular membrane.

Ask students to use about half of the jumbo paper clips to make a model of a membrane. (They should have something resembling concentric circles around the DNA binder clips.)

Ask:

Are all the models exactly the same shape?

Intended answer: No.

Are all real cell membranes the same shape?

Intended answer: No.

Is it possible to change your membrane shape?

Intended answer: Yes. Do it!

Can you do another shape?

Intended answer: Yes. Do it!

Can a real cell do this?

Intended answer: Yes. Discuss.

How is the inside of the cell protected from the other parts outside the cell?

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Add a cell membrane

Use about half of the jumbo paper clips to make a model of a membrane, like this.

Are all the models exactly the same shape?

Add organelles. Ask students to look at the contents of Container 4. Ask students to link together several small colored paper clips. Ask them to link together one group of three colored paper clips, a group of two, and also set aside a single colored paper clip. Any color combinations will work.

Paraphrase:

There are many more structures within a cell, but your newly made groups of one, two, and three colored paper clips will represent the rest of the “stuff” inside the cell, which we call organelles.

Feel free to address this in a more rigorous way if appropriate for your class.

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Model organelles between the two membranes

Link one group of three colored paper clips, a group of two and place these between the nucleus and the membrane, in the cytoplasm. Place a single colored paper clip in the nucleus, like this.

Add materials outside the cell. Ask students to look at the contents of Container 5.

Ask:

What are some examples of where various cells are located?

Intended answer: Cells are found within organs, floating in a pond, in a worm, etc.

Are cells ever completely alone with nothing around?

Intended answer: No.

Ask students to surround the outside of the cell with the miscellaneous items in Container 5.

Finally, remind students that they have remaining in Containers 1-4 additional materials not used to make our cell model. Ask students to attach all of the remaining items in Containers 1-4 to the miscellaneous items the students just arranged on the outside of the cell model in groups of two to five items. Non-matching items should be linked together, and every group should include some of the items not used in the final model. For example, a jumbo paper clip might be connected to a twist tie, or two colored clips and a binder clips might link together with a regal clip. Basically it should look like a bit of a mess with lots of mixed up structures outside the cell. This arrangement represents the cell’s environment.

Step back and look at the models together. Paraphrase:

The collection of paper clips and other office supplies on the surface in front of you is a model of a cell.

You will duplicate it by bringing similar materials in and out of the cell and taking apart and using pieces of those materials to build cell parts, just like real cells do.

Note that this is a two-dimensional model of what is actually a three-dimensional shape.

Where can you find cells? Are cells ever completely alone with nothing around?

Add materials outside the cell

Surround the outside of the cell with items from all the cups, linked in random ways, like this.

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In this part of the activity, we explore the functions of the paper clip cell model. For each simulation, start with a teacher demonstration, and then have students follow.

Discuss the function of the membrane. Turn the students’ attention back to the membrane. Ask them “What would happen if the membrane broke? Or wasn’t there?” Also ask them to brainstorm about “What would the cell do if it needed to take in more materials?” Entertain all ideas, but guide students to the understanding the cell membrane (amazingly) can let in certain things and let out certain things at different times depending of what the cell system needs. While the middle school standards do not go beyond this understanding, please feel free to offer additional information about the functions of cell membrane.

How does this cell work?

What would happen if the membrane broke? Or wasn’t there?

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Simulate absorbing materials by transporting across the membrane. To represent the cell taking in materials, ask students "There’s all this stuff out here. Food. Materials. How do we move it into the cell?" Students then select one group of items currently outside the cell and then have the students lift the portion membrane up and slide the item inside. It should be an item that looks like one of the elements of a membrane or something in the cytoplasm. Have students offer critique of this action as a model. (Cells are more sphere-like than circle-like, the process of the items moving through the membrane is much more complex, etc.)

What would the cell do if it needed to take in more materials?

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Simulate using energy and materials. Next, ask students to take apart the item newly brought into the cell. Ask them what they think this represents (breaking down, chemical change). Guide them toward understanding that this process is somewhat like fueling and that the cell can use these broken down components for energy and to make structures.

As you demonstrate these next steps of how a cell grows and reproduces, point out the actions you are taking and why. For example, point out the structure you are trying to recreate: for example, "I'm going to make this organelle using materials two pink paper clips and a purple paper clip."

Build one organelle together. Ask students to build x with materials y and z, and show the "fluid" process of materials moving into the cell, the cell creating what it needs from the component parts, and dispose of the unused waste.

Simulate disposing of waste. Ask the class, "There’s all this stuff in the cytoplasm, stuff our cells don’t need. How do we move it out of the cell?" The last process to simulate will be waste disposal. Now that the newly brought in item is broken down into pieces, some materials aren’t needed. Simulate the cell expelling the waste by lifting up a portion of the membrane and having the waste leave the cell and re-enter the outside environment.

Introduce reproduction. Ask students to offer various examples of reproduction—mammals have embryos develop within females, plants have seeds, and so on. Mention that cells also reproduce in a way that we can simulate using our clippy model in front of us.

Present the challenge. Review the components of the cell that they have already made: membrane, nucleus membrane, DNA, and the “other” structures.

Paraphrase:

Cells make copies of themselves. How would you make another copy of this cell model? You might just organize all the pieces outside the cell into all the structures you need in order to make a second cell. Unlike you, cells don't have hands and arms they can use to change the world outside of themselves. Instead, they consume the materials they need and grow in size until they are so big they must split into two to become more efficient again.

Is it possible for your cell model to make an exact duplicate of itself by letting in, changing, and letting out materials to the cell’s environment? You can't take everything you need into your cell all at once. Figure out how to make it happen!

Explore. Allow students time to discuss and test various approaches with their model. Circulate encouraging students to consider how a cell might need only part of the item in the environment and must transform it to use it. Also ask groups to consider what to do if the if it gets “too crowded” inside the cell? (They could enlarge the membrane and expel waste.) Students are allowed to exchange items with other groups, as long as the items are located outside the membrane.

Highlight and guide. Call out groups that have breakthroughs and ask them to share. Guide students to the point where their process includes: bringing in a variety of materials, breaking them apart, constructing copies of the existing cell parts by using some of the broken down parts, increasing the size on the membrane, expelling waste.

Some groups will really struggle to understand that some items will have to come into the cell – small silver clips for the nuclear membrane, and how some items will have to come out.

The hardest part of the process for students to intuit will probably be pinching together the membrane and correctly separating all the duplicate structures. If students get stuck, show a video clip of a real cell dividing and ask them if they can get any ideas from that.

Capture cell reproduction

Is it possible for your cell model to make an exact duplicate of itself by letting in, changing, and letting out materials in the cell’s environment?

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Does anything outside the cell look like the things inside the cell?

What should happen in the cell if it gets too crowded in the cell?

Have students reset the cell as they did in the step "Construct a cell model." Now, ask students to create a slide show that shows one way that a cell can grow and reproduce.

Each step should show the cell doing only one of four things, and students can use the captions within their presentation.

• The membrane brings in new materials.
• The cell breaks materials apart.
• The cell builds copies of cell parts or changes the membrane.
• The cell disposes of waste.
• A membrane changes.

The PDF below includes these captions as a printable.

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One way to describe this process to students is to tell them they should have the cells "pose" with a caption each time the students snap a photo. While posing, none of the parts should be touching each other and make sure the membrane(s) are unbroken.

Have each group document major steps in their cell’s reproduction process. Students arrange the office supplies to take at least six (and fewer than 15) photographs in order. Students may want to include small, numbered cards in the photos to keep track of the order. They can also hold up their fingers next to the picture to indicate order. Students arrange the photos and advance through them in order to display a quasi-animated slideshow using Google Slides, Microsoft Powerpoint, or a photo gallery feature on a tablet or computer.

Extension: More advanced students can make a full stop-motion animation rather than just a slideshow.

Make Slide Shows

Students share their slide shows.

An example set of images:

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This modeling activity requires quite a bit of setup. Consider spending a few minutes of class time asking students to reset the office supply sets.

Resetting supplies

When students finish capturing images of the process of "How a Cell Grows and Reproduces," it's time to share the results.

Present slide shows. Groups take turns presenting their slides to each other. Discuss similarities and differences among all the slideshows. Ask how the process is consistent across all groups. Also acknowledge that differences in the details are to be expected.

Remind your students of the complex behavior of cells that these models cannot capture. These are simplified representations. All models have shortcomings. Also emphasize that there are many cell types and plant and animal cells differ in important ways, etc.

Have students take notes on the presentations, noting differences and similarities between models.

Be prepared to share with others:

• key features of your procedure
• interesting things you noticed about the cell and how it reproduces
• things about the model that are probably not accurate or that are missing
• questions that you might have

As you compare your cell slide show to others:

How do these models represent the real life of a cell? How do they miss the mark?

Do all the models of reproduction work exactly the same way?