Seeing Inside Cells

How big are cells?

Initiate a class discussion prior to beginning the activity. How small a thing will we observe today?

Paraphrase:

Is a plant cell bigger or smaller than an animal cell? Some plant cells are smaller, but the average plant cell is larger than an animal cell.

What's the smallest thing you can see with the naked eye? What's too big to see with a microscope?

What is the largest cell you can see without a microscope?

What might be smaller than a cell?*

How many of each of the following do you think would fit on top of a penny? *

• red blood cells?
• bacteria?
• Paramecium?
• skin cells?

Fun fact: If a cell were the size of the earth, an atom would be about as big as a 50-story building.

Note: The starred questions developed by SCALE / SFUSD

Prepare an onion skin slide

Each onion quarter will sepa­rate neatly into layers.

Hold one of the layers with both hands, so that it curves toward you.

Push your thumbs away from you and pull the two edges of the onion layer together.

Starting at one of the torn edges, pull the transparent, paper-thin layer of epidermis, or outer skin, from the surface. It comes off in a sheet.

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Make a wet mount of the epidermis as follows:

1. Use the razor blade to cut a piece of epidermis about 1 cm square.
2. Place this piece in a drop of water on a clean slide. Smooth out wrinkles in the epidermis with a dissecting needle.
3. Put on the cover glass. Be sure the water comes to its edges, but not beyond. Add water at the edge with a medicine dropper, or remove excess water with paper towel, as necessary.

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Students can use this template to record their observations of the cells in the lesson.

Students should describe what they see in the microscope with words and by making a labeled diagram.

At the end of the observations, you will be reviewing the diagrams students drew. You may want to state some of the expectations you have for their diagrams before they get started. As you evaluate their diagrams, you may choose to note diagrams that

• are drawn neatly with clear lines
• use color to enhance understanding, not decoratively (if colored pencils are used)
• note some parts of the cell
• appropriately omit extraneous details
• are accurately drawn and labeled
• are labeled neatly without overlaps
• include titles and magnification

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Look at onion skin cells

Look at the cells of the onion epidermis under low power.

What is their general shape?

If you did not know their origin, how could you tell they are plant cells?

Observe the cells under high power. What evidence is there that these are living cells?

Find a cell nucleus

Remove the slide from the microscope stage.

Place a drop of the stain (coloring) at one edge of the cover glass.

Draw it under the cover glass by touching a piece of paper towel to the opposite edge.

(CAUTION: Stains can cause permanent damage to clothes and laboratory surfaces. Handle carefully.)

Under low power look for a nucleus that is stained brown or blue.

What is its shape?

Switch to high power and look at the nucleus again.

Where is the nucleus located in the cell?

How can you tell that the cell has depth?

Look at the area between the nucleus and the edges of the cell. You will find a fine granular (grainlike) substance that has been stained lightly.

Does the substance fill the entire cell? Describe what it looks like. How might this sub­stance be different in cells that have not been stained?

Look at chloroplasts

Take one plant from the bowl. Break off one of the younger leaves near the tip.

Place the leaf with its underside up, in a drop of water on a clean slide. Put on a cover glass.

Look at this leaf under low power. Some cells seem to be packed with small green bodies. These bodies are called chloroplasts.

Do you think that these elodea cells have the same function as those of the onion you looked at? Why or why not?

The part of the onion from which you took cells is usually found below the ground. The elodea plant is found where sunlight strikes the plant.

What does this suggest about the function of chloroplasts?

Where are the chloroplasts located in the cell?

What is the shape of a chloroplast?

Describe the movement of chloroplasts.

Chloroplasts have no means of moving about. How might their movement be explained?

Other structures are present in cells of elodea. But all of these except the cell wall, are hidden by chloroplasts. With patience and careful observation of many cells, however, you may be able to find a nucleus and other parts of the cell.

What methods might you use to see these structures better?

Compare and contrast the two kinds of living cells you have seen. How are they alike? How do they differ?

Look at animal cells—your own!

Place a very small drop of tap water on a clean side. Rinse your mouth out with water. With the broad end of a toothpick, gently scrape the inside of your cheek. Deposit a little of the scraping into the drop of water by rolling the toothpick in it.

Stir the scraping into the drop of water with a toothpick until all of the drop is cloudy. Add a small drop of stain to the scraping. Place a cover glass on the slide. Observe with low power. You may find that some of the cells are folded over or piled on top of one another. Some may be broken. Find one or two that you can see clearly. Center these for viewing.

How does the edge of this kind of cell compare with the plant cells you observed?

Turn to the high-power objective. The nucleus of each cell should be clearly visible. You may have to adjust (usually reduce) the light.

Using the fine adjustment, focus up and down on one cell to deter­mine its shape.

How does the shape of this animal cell compare with the shape of the plant cells you observed?

How would you be able to tell that this cell was from an animal and not a plant?

How are the plant and animal cells you have seen alike?

Would you expect cells from other parts of your body to be exactly like the cells from inside your cheek? Why or why not?

An organism may have many small cells rather than a few large cells. Suggest one or more possible explanations for this.

Observe mitosis

Select a root of an onion that is 2 to 3 cm long. Place it on a clean slide: Cut off 1 to 2 cm of the root tip. Discard any remaining upper portion.

Place three drops of 1 N hydrochloric acid on the root tip.

Wait about 1 minute. While you are waiting, heat the slide by passing it back and forth over a flame (CAUTION: Hold the slide with forceps so you won’t burn your fingers. Do not allow the liquid on the slide to boil.) Heat the slide in this way two or three times. Carefully blot off the acid with a strip of paper towel.

Cover the root tip with two or three drops of toluidine blue O stain. Warm the slide for about one minute as you did before. Carefully blot off the excess stain.

Add a drop of fresh stain. Apply a cover glass. Place the slide in a folded paper towel on a hard surface. Press the cover glass with your thumb, using a steady, firm pressure. Be careful not to break the cover glass.

Clean the stain from the slide and cover glass. Examine under low power of your microscope. When you have located the cells, switch to high power. The cells may be stained so darkly that you cannot see the individual parts. If this happens, dilute the stain by placing 1 or 2 drops of water at one edge of the cover glass. Place a piece of paper towel at the opposite edge to pull the water through. Repeat this until you can see the stained portions of the cell.

Find these stages of mitosis:

1. The nuclear material forms long, slender threads that are stained. At this stage, the separate chromosomes cannot be identified. They form a loose ball of tangled and twisted threads.
2. The individual chromosomes are much shorter and thicker. At this stage, each chromosome has two strands, or chromatids. The strands are held together at the centromere.
3. The chromatids have separated and are now two separate groups of chromosomes.
4. The two nuclei are present in the cell with strands of chromo­somes still visible.

Make simple sketches of what you find. Describe the structures you see.

What problems did you have in making this study?

Study a prepared slide of dividing cells in the onion root tip. (The stain used for the slide is different from the one you used.)

Scan the entire length of the section of root tip, using low power. Where do you observe cells dividing? Where are the cells not dividing?

Locate some cells in division. Switch to high power and look for the following stages:

1. Early appearance of chromosomes in the nucleus. Find round structures that stain at this early stage. These are the nucleoli.
2. The chromosomes are easy to see and grouped in the middle of the cell.
3. Chromosomes have separated and moved toward the poles.
4. A new cell wall is formed between the new cells.

What has happened to the nucleoli?

What is the condition of the nuclear membrane?

Find the spindle. The end of the spindle fibers that are near the center of the cell are attached to the centromeres of the chromosomes. How many chromosomes can you see? What part do you think the spindle fibers play in moving the chromosomes?

Look for the beginning of the new cell wall. Where does it appear?

Cells divide rapidly in embryos (young organisms, at very early stages of development). These cells are good material in which to study mitosis. Carefully examine a prepared slide of either Ascaris (a worm) or whitefish embryos. Look for the following stages:

1. The chromosomes are long and threadlike.
2. The double chromosomes are attached to spindle fibers at the center of the cell. Look at the poles of the spindle and compare them with those of the plant cells you studied.
3. The chromosomes are separating and the cell is pinching in two.

How many individual chromosomes are in one cell? How do the poles of the spindle of an animal cell differ from those of a plant cell?

Compare the separating of chromosomes and dividing of these cells with what you saw in plant cells.

What structures do you see in the dividing animal cell that were present in the dividing plant cell?

In this activity, students have illustrated similar cells from different samples. Ask students to share their diagrams.

Ask:

What are some things that you see in common between the different images the class drew?

As you discuss the diagrams, look for things that your students did well in their cell diagrams. For example, note diagrams that accomplish items on the list.

• Diagram is drawn neatly with clear lines.
• Parts of the cell are noted.
• Diagram appropriately omits extraneous details.
• Diagram is accurately drawn and labeled.
• Diagram is labeled neatly without overlaps.
• Diagram includes titles and magnification.
• Color enhances understanding and is not used decoratively (if colored pencils are used).