Cell Structure And Function


Cell Structure And Function
Recording Plant Cell Structure
Cell structure and function
All living organisms are made of cells, although some microscopic organisms only
have one cell in their complete body. Cells are the basic building blocks of life and,
while they can have a wide variety of different shapes and sizes, they all share a
number of common features. Biologists use chemicals called “stains” to color
structures in cells to make them stand out. In this experiment, you will use a stain
to make the structures easier to see and to learn the basic rules of biological
illustration. You will look at two types of plant cells (onion epithelium and daffodil
pollen grains) to see what they have in common and how they are specialized or
differentiated to do their particular task in the plant body. You will have to use a
microscope to see the cells and, even then, some structures are difficult to see. In
the first part of the experiment, you will look at strips of epidermal cells from an
onion. Epidermal cells are flattened cells that line the surfaces of organisms. The
epidermal cells that you will look at through the microscope line the inner surface
of the parts of the onion bulb that will grow into leaves.
Time required
90 minutes
plant samples: an onion and a
scalpel, craft knife, or razor blade
iodine solution (iodine dissolved in
dilute potassium iodide solution)
small amount of water in a beaker
sucrose solution (dissolve 41 g of
sucrose in 100 ml of deionized
150 ml mineral salt solution
(supplied by your teacher)
5 ml pipette
3 eyedroppers
test tube
glass rod
2 microscope slides and 4 cover slips
microscope with low and high
power lenses
paper towels
sheets of unlined paper
(81/2 × 11 inches)
sharp pencil
Note to teachers:
1. Dissolve the following amounts in
1 liter of deionized water:
4.17 g calcium nitrate
2.0 g boric acid
1.01 g potassium nitrate
2.17 g magnesium sulfate
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2. Dilute 10 ml of this solution with
90 ml of deionized water to
provide 100 ml of mineral salt
solution of the correct strength for
the experiment.
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Safety note
Iodine solution is harmful when swallowed. Wash your hands after handling the
solution and stained slides. Be careful when using the sharp knife to cut the
onion. Cover slips are thin squares of glass, which break easily to give very
sharp edges. Be careful when handling them, as they can give you nasty cuts. If
you do receive a cut, check carefully for slivers of glass left in the wound before
applying a bandaid.
Part A: Onion epidermal strips
1. Cut a slice of onion with the scalpel (or other sharp cutting edge) and remove
a thin sheet of epidermal cells with tweezers as shown in diagram 1 below.
(You will need to pull the onion apart to get at the thin “skin” lining the
inside of the layers the onion is made from.)
fleshy leaf of onion
thin layer of cells
can be pulled from
the inner surface
of the ‘leaf’
inner surface
where layer of
cells can be
Onion cell strip
2. Use the eyedropper to place a drop of water on a microscope slide. Cut a
small square of the epidermal strip and place it in the drop of water. Place a
cover slip gently on top, by laying the cover slip down at one side just
touching the water drop, and then letting the slip fall into place.
3. Look at the slide under low power on your microscope. Draw what you can
see on a sheet of plain paper. Advice on drawing biological specimens is given
in diagram 3 on page 1.01–4.
4. Remove the slide from the microscope and, using another eyedropper, add a
small drop of iodine solution at one edge of the cover slip.
5. Place a small square of paper towel at the other end of the cover slip (see
diagram 2 on the next page). This will draw liquid along the space between
the cover slip and the slide, and iodine solution will be drawn through the
onion epidermal strip.
6. Return the slide to the microscope and look at it under low power. Draw what
you can see.
7. Carefully slide the high power lens into place on your microscope. Use the fine
focusing knob to adjust the focus if necessary. Draw what you can see. Label
your drawing.
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or transmittal is copyright protected by the publisher.
cover slip
paper towel
Steps in preparing stained slide
Part B: Daffodil pollen grains
1. Tap some pollen grains from a daffodil onto a microscope slide. You should be
able to see these as a sticky yellow powder.
2. Use the pipette to transfer 3 ml of the mineral salt solution and 1 ml of sucrose
solution to a clean test tube. Mix the two solutions with a glass rod.
3. Use a third eyedropper to add a drop of the mixture to the daffodil pollen.
Place a cover slip on top.
4. Look at the slide under low power on your microscope. Draw what you can
5. Look at the pollen grains every minute or so until they start to germinate. You
should be able to see a small tube growing out of a pollen grain. Each grain
will produce one of these structures, which are called pollen tubes.You may
need to add more liquid if the slide starts to dry out. You can do this easily by
placing a drop of solution at the edge of the cover slip – it will be drawn in
under the slip by capillary action. The pollen grains should germinate within
about 30 minutes. Draw what you can see.
1. List the structures you could see in the onion cells without the iodine stain.
2. What effect did the iodine have on the cells? List any differences you could see
with and without the stain applied.
3. How long did it take for the pollen grains to start growing?
4. Did the wall of the pollen tube look the same as the walls of the pollen grain
5. Summarize the main differences between the pollen grains from the daffodils
and the epidermal cells in the onion.
6. Explain how the differences in structure between the cells you describe in
question 5 are related to the different jobs the cells have to do.
Want to know more?
Click here to see what we found.
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or transmittal is copyright protected by the publisher.
Use a sharp pencil
not colored crayon
or marker
Draw clean single lines –
do not sketch or shade
or color
Label everything clearly in pencil.
Give every drawing a title.
Labels should be clear of the
diagram, not written over it.
Make sure your drawing is
large enough, and add
measurements so that
users can tell how big
the original was.
Drawing biological specimens
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or transmittal is copyright protected by the publisher.
Cell Structure And Function
1.01 Recording Plant Cell Structure
1. You should have been able to see clearly the cell wall, the central vacuole, and
possibly the nucleus. Onion epidermis does not have chloroplasts.
2. The iodine solution stained starch grains in the cytoplasm blue-black, thus
making them visible. The cell walls were also dark and easier to see.
3. The time will depend on the temperature and the freshness of the pollen –
there is no way to predict exactly when germination will occur because
biological material is so variable. Typically, you should see a result within five
to ten minutes.
4. The wall of the pollen tube was thinner and more flexible than the highly
sculpted wall of the pollen grain. Pollen grain walls are waterproof and have a
protective function, whereas the walls of the germination tube only need to
carry the nuclei from the pollen grain down to the female nuclei in the ovule.
5. Some of the differences we found are summarized in the table below.
Onion epidermal cells
Pollen grains
Cell wall
Thin and smooth with
no noticeable pore.
Very thick and sculpted (complex
patterned outer surface) with a number
of noticeable pores.
Large; easily visible.
Absent or small
Flat; longer than it is broad.
Does not grow noticeably.
Germinates to produce a pollen tube.
6. Onion epidermal cells cover surfaces in the onion, so they are flattened and fit
together neatly to form a kind of pavement of thin cells. The pores in the
pollen grain cell walls allow the pollen tube to grow out; the onion epidermal
cells do not have these because individual parts of the cell do not grow
outwards as in pollen grains. The single nucleus in the onion epidermal cell is
standard for most plant cells; the two nuclei in the pollen grains are associated
with their role in flowering plant reproduction.
You could explore the rate of growth of pollen tubes in different temperatures.
How do you think the pollen tube “knows” which way to grow: is it light,
gravity, or a chemical stimulus that guides it? You could set up an experiment
to see if the direction of the light affects the direction of growth of the tubes.
You can also review a range of other cell types and relate their structure to
their functions in the body. This is probably best done with prepared
photographs of the cells rather than attempting to view fresh materials under
a microscope.
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Published by Facts On File, Inc. All electronic storage, reproduction,
or transmittal is copyright protected by the publisher.