BIOLOGY EXPERIMENTS ON FILE TM CELL STRUCTURE AND FUNCTION • 1.01–1
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.
90 minutes
plant samples: an onion and a daffodil tweezers 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 water)
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
(8 1 /
2
× 11 inches) sharp pencil stopwatch
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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1.01–2 • CELL STRUCTURE AND FUNCTION BIOLOGY EXPERIMENTS ON FILE TM
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.)
1 fleshy leaf of onion outer surface thin layer of cells can be pulled from the inner surface of the ‘leaf’
Onion cell strip inner surface where layer of cells can be found
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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BIOLOGY EXPERIMENTS ON FILE TM
2 eyedropper
CELL STRUCTURE AND FUNCTION • 1.01–3 sample cover slip paper towel stain
A B C
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 see.
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 itself?
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.
Click here to see what we found.
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1.01–4 • CELL STRUCTURE AND FUNCTION
3
Use a sharp pencil not colored crayon or marker
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BIOLOGY EXPERIMENTS ON FILE TM
Draw clean single lines – do not sketch or shade or color
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Label everything clearly in pencil.
Give every drawing a title.
Labels should be clear of the diagram, not written over it.
TITLE
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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8.01
• OUR FINDINGS BIOLOGY EXPERIMENTS ON FILE TM
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.
Feature
Cell wall
Nuclei
Vacuole
Shape
Growth
Onion epidermal cells
Thin and smooth with no noticeable pore.
One
Large; easily visible.
Flat; longer than it is broad.
Does not grow noticeably.
Pollen grains
Very thick and sculpted (complex patterned outer surface) with a number of noticeable pores.
Two
Absent or small
Spherical
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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