AP Biology
Name
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Cells and Microscopy
Investigating Cells
As you learned in 10th grade, there are two basic types of cells. Prokaryotic cells are very small
and lack a true nucleus and all other membrane-bound organelles. All members of the Kingdoms
Eubacteria and Archaea have prokaryotic cells. On the other hand, eukaryotic cells are larger, have a
true nucleus and a variety of membrane-bound organelles. All organisms in the Kingdoms Protista,
Fungi, Animalia and Plantae are composed of eukaryotic cells.
The purpose of today's laboratory exercise is to become acquainted with the structures of a
variety of prokaryotic and eukaryotic cells.
Estimating Cell Size
When using a microscope, it is helpful to know the size of your field of view. This will give you an idea of the
size of the specimen you observe. Measuring your field of view is simple. Place a clear ruler on the microscope
stage, lining a milimeter mark up with the left-hand side of the field of view. Estimate the number of milimeters
you are able to see to the nearest 10th.
 How big is your field of view in milimeters?
 How big is your field of view in micrometers (μm). Remember, 1 mm = 1,000 μm.
Eukaryotic Cells
Eukaryotic cells are surrounded by a plasma (cell) membrane, which controls the movement of
material into and out of the cell. In addition to a cell membrane, plant cells have a rigid outer cell wall.
As mentioned above, all eukaryotic cells have a membrane-bound nucleus and many other membranebound organelles. The nucleus contains chromatin, which consists of DNA and associated histone
proteins. We are able to see the nucleus under a light microscope because of its large size. In plants we
can view plastids, which are fairly large and often pigmented. Other organelles, like the mitochondria,
are just barely at the limits of resolution of the light microscope. Mitochondria can be seen, but special
techniques must be used. Most of the other cellular organelles are not visible with the ordinary light
microscope. An electron microscope is required to see these "invisible" organelles.
Plant Cells
Typically, plant cells have more
structures visible with the light
microscope than animal cells. The
exterior cell wall is probably the most
noticeable structure. It is made of the
complex carbohydrate, cellulose. The
cell membrane lies just inside the cell
wall. Suspended in the cytoplasm of
the cell are the plastids. Plastids are a
family of relatively large membranebound organelles, the appearance and
functions of which vary depending upon
the type of plant cell in which they are
found. For example, one type of plastid
is the chloroplast. Chloroplasts contain
chlorophyll, a green photosynthetic
pigment, and serve as sites of
photosynthesis. Chloroplasts are abundant in green leaves, as you will see when you study Elodea cells
below. Chromoplasts are plastids that contain pigments other than chlorophyll. You will observe red
chromoplasts in a bell pepper. Finally, amyloplasts are plastids that serve as sites of starch
accumulation. Because they are colorless, you will have to stain them when you observe them in potato
slices.
Plant cells also have a large central vacuole, which is filled with water and dissolved materials.
This vacuole tends to push the nucleus and cytoplasm against the cell wall.
Procedure for StudyingTypical Green Plant Cells
Elodea is a common pondweed used to study photosynthesis, cellular structure, and cytoplasmic
streaming. In addition to studying general plant cell structure, you should be able to easily observe the
green-colored chloroplasts. As you examine them, you will see them moving in the cell. This is called
cytoplasmic streaming or cyclosis.
1.
Remove a young leaf from the top of a sprig of Elodea.
2.
Place this leaf in a drop of water on a microscope slide and add a cover slip. Be careful
not to let the leaf dry out.
3.
Examine your leaf under 100X and 400X magnifications (using 10X and 40 X objective lenses).
 What shape are Elodea cells?
 Approximately how many chloroplasts are in each Elodea cell?
 What is the approximate size, in micrometers, of a single Elodea cell?
 Draw a region showing five or six Elodea cells at 100X (i.e. 100X magnification) in your lab
notebook.
 Draw and label one Elodea cell at 400X in your lab notebook.
Procedure for Studying Amyloplasts
You will be using a very thin slice of potato to study the starch-containing amyloplasts. Since
these plastids are colorless and difficult to see in ordinary preparations, you will be staining them with
iodine. Iodine turns purple when it comes in contact with starch.
1.
Use a razor blade to make a very thin section of potato. BE CAREFUL!
2.
Place the section in a drop of water on a microscope slide.
3.
Add a drop of iodine to the potato section.
4.
Add a cover slip and examine the section at 100X and 400X.
Describe the appearance of the amyloplasts.
Are any other cellular structures stained intensely?
What can you conclude about the location of starch in storage cells of the potato?
Approximate the size, in μm, of a single amyloplast.
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Procedure for Studying Chromoplasts
You will be using a very thin slice of red pepper to study the red color-containing chromoplasts.
1.
Use the razor blade to prepare a very thin section of a red pepper.
2.
Place the section in a drop of water on a microscope slide, add a cover slip, and study the
preparation under 100X and 400X powers.
3.
Examine the cellular structures and locate the plastids.
Describe the appearance of the chromoplasts.
Are they similar in size and shape to the other plastids you studied? Explain the similarities and
differences.
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Procedure for Studying
Non-Green Plant Cells and Mitochondria
Cells of the thin inner epidermis of an onion show many of the features found in non-green plant cells.
Additionally, these preparations are ideally suited for demonstrating the presence of mitochondria. In
both cases, a special stain (a coloring) will be added. Follow the illustrated instructions below and make
an onion epidermal cell preparation as shown in figures A through E.
Observing Mitochondria
1.
Prepare another onion epidermal tissue slide, but this time place the onion epidermis in a 5%
sucrose solution instead of water.
2.
Add 3 to 5 drops of Janus Green B (or any other mitochondrial stain) following the technique
illustrated in F and G above.
3.
Immediately, view your preparation at 400X. You should be able to see the mitochondria which
appear as very small rods and spheres at the periphery of the cell. They should be blue in color
(if you are using a stain other than Janus Green B, they may appear a different color).
4.
If you don't see anything, you may have to add a few more drops of stain to the edge of the cover
slip. You should do this on the side opposite to the side to which you first applied the stain.
Describe what you see.
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Animal Cells
Animal cells do not have rigid cell walls, plastids or large central vacuoles. Although they vary
tremendously in size and shape, all animal cells are eukaryotic as are all plant cells. As examples of
animal cells, we will look at human epidermal cells and kidney cells.
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Procedure for Studying
Human Epidermal Cells
1.
Following the directions outlined below,
examine cells obtained from the inner
epidermal lining of your cheek
2.
Observe at 100X and 400X. Describe what
you see.
3.
Add a couple of drops of 0.01% methylene
blue to the edge of the cover slip and draw it
through just like you did with the onion
epithelium slide.
4.
Observe at 100X and 400X.
What visible structures in the stained
preparation were invisible in the unstained
preparation?
How big is a cheek cell?
Draw a couple of representative cells at 400X
in your lab notebook.
What similarities and differences are there
between your cheek cells and the Elodea cells?
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Active Transport
Active transport is the energy-requiring movement of a substance across a barrier or a gradient. This
transport could not occur without the input of energy. In the biological world, energy is supplied
by the living cell. We can observe the principles of active transport by examining whether or not
the uptake of a substance will take place in living and/or nonliving cells. You will read about
active transport in chapter 7, but we will observe it today while we have our microscopes out.
This experiment will be conducted using yeast cells. Yeast are eukaryotic, single-celled organisms from
the Fungi kingdom. Like most unicellular organisms, they lack mouth and must take in food by
active transport through the cell membrane.
Neutral red is a dye that is actively transported into the living yeast cell. Neutral red is red in an acidic
solution and yellow in a basic solution. A sodium carbonate (Na2CO3) solution is basic.
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Procedure
Obtain two Erlenmeyer flasks, 2 g yeast, 50 mL carbonate solution, and 50 mL neutral red.
Place a drop of yeast solution on a slide and observe the yeast under a microscope at 40X.
Draw a few yeast cells in your lab notebook.
What is the approximate size of a yeast cell?
Into Flask #1 place half the yeast and half the sodium carbonate. Mix well and heat gently. Boil
the solution for 2-3 minutes. When the solution is cool, add about half the neutral red. The
solution should be yellow.
To flask #2 add the remaining Na2CO3 and neutral red. Gently swirl in the remaining yeast and
watch for a color change.
Record your observations.
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State a possible explanation for these observations.
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