BIOLOGY EXPERIMENTS ON FILETM CELL STRUCTURE AND FUNCTION • 1.01–1

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Recording Plant Cell StructureTopic

Cell structure and function

IntroductionAll living organisms are made of cells, although some microscopic organisms onlyhave 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 anumber of common features. Biologists use chemicals called “stains” to colorstructures in cells to make them stand out. In this experiment, you will use a stainto make the structures easier to see and to learn the basic rules of biologicalillustration. You will look at two types of plant cells (onion epithelium and daffodilpollen grains) to see what they have in common and how they are specialized ordifferentiated to do their particular task in the plant body. You will have to use amicroscope to see the cells and, even then, some structures are difficult to see. Inthe first part of the experiment, you will look at strips of epidermal cells from anonion. Epidermal cells are flattened cells that line the surfaces of organisms. Theepidermal cells that you will look at through the microscope line the inner surfaceof the parts of the onion bulb that will grow into leaves.

Time required 90 minutes

Materials

plant samples: an onion and a daffodil

tweezersscalpel, craft knife, or razor blade iodine solution (iodine dissolved in

dilute potassium iodide solution)small amount of water in a beakersucrose solution (dissolve 41 g of

sucrose in 100 ml of deionized water)

150 ml mineral salt solution (supplied by your teacher)

5 ml pipette3 eyedropperstest tubeglass rod2 microscope slides and 4 cover slipsmicroscope with low and high

power lensespaper towelssheets of unlined paper

(81/2 × 11 inches) sharp pencilstopwatch

Note to teachers:

1. Dissolve the following amounts in1 liter of deionized water:

4.17 g calcium nitrate2.0 g boric acid1.01 g potassium nitrate2.17 g magnesium sulfate

2. Dilute 10 ml of this solution with90 ml of deionized water toprovide 100 ml of mineral saltsolution of the correct strength forthe experiment.

BIOLOGY EXPERIMENTS ON FILETM1.01–2 • CELL STRUCTURE AND FUNCTION

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ProcedurePart A: Onion epidermal strips1. 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 theinside of the layers the onion is made from.)

Onion cell strip

2. Use the eyedropper to place a drop of water on a microscope slide. Cut asmall square of the epidermal strip and place it in the drop of water. Place acover slip gently on top, by laying the cover slip down at one side justtouching 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 cansee on a sheet of plain paper. Advice on drawing biological specimens is givenin diagram 3 on page 1.01–4.

4. Remove the slide from the microscope and, using another eyedropper, add asmall 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 (seediagram 2 on the next page). This will draw liquid along the space betweenthe cover slip and the slide, and iodine solution will be drawn through theonion epidermal strip.

6. Return the slide to the microscope and look at it under low power. Draw whatyou can see.

7. Carefully slide the high power lens into place on your microscope. Use the finefocusing knob to adjust the focus if necessary. Draw what you can see. Labelyour drawing.

Safety noteIodine solution is harmful when swallowed. Wash your hands after handling thesolution and stained slides. Be careful when using the sharp knife to cut theonion. Cover slips are thin squares of glass, which break easily to give verysharp edges. Be careful when handling them, as they can give you nasty cuts. Ifyou do receive a cut, check carefully for slivers of glass left in the wound beforeapplying a bandaid.

1

inner surfacewhere layer ofcells can befound

outersurface

fleshy leaf of onion

thin layer of cellscan be pulled fromthe inner surfaceof the ‘leaf’

BIOLOGY EXPERIMENTS ON FILETM CELL STRUCTURE AND FUNCTION • 1.01–3

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Steps in preparing stained slide

Part B: Daffodil pollen grains1. 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 grainwill produce one of these structures, which are called pollen tubes.You mayneed to add more liquid if the slide starts to dry out. You can do this easily byplacing a drop of solution at the edge of the cover slip – it will be drawn inunder the slip by capillary action. The pollen grains should germinate withinabout 30 minutes. Draw what you can see.

Analysis1. 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.

Want to know more?Click here to see what we found.

2

A B C

eyedropper

sample cover slipstain

paper towel

BIOLOGY EXPERIMENTS ON FILETM1.01–4 • CELL STRUCTURE AND FUNCTION

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Drawing biological specimens

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Make sure your drawing islarge enough, and addmeasurements so thatusers can tell how bigthe original was.

Draw clean single lines –do not sketch or shadeor color

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Label everything clearly in pencil.Give every drawing a title.Labels should be clear of thediagram, not written over it.

Use a sharp pencilnot colored crayonor marker

TITLE

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BIOLOGY EXPERIMENTS ON FILETM8.01 • OUR FINDINGS

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Cell Structure And Function1.01 Recording Plant Cell Structure

1. You should have been able to see clearly the cell wall, the central vacuole, andpossibly the nucleus. Onion epidermis does not have chloroplasts.

2. The iodine solution stained starch grains in the cytoplasm blue-black, thusmaking 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 becausebiological material is so variable. Typically, you should see a result within fiveto ten minutes.

4. The wall of the pollen tube was thinner and more flexible than the highlysculpted wall of the pollen grain. Pollen grain walls are waterproof and have aprotective function, whereas the walls of the germination tube only need tocarry 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.

6. Onion epidermal cells cover surfaces in the onion, so they are flattened and fittogether neatly to form a kind of pavement of thin cells. The pores in thepollen grain cell walls allow the pollen tube to grow out; the onion epidermalcells do not have these because individual parts of the cell do not growoutwards as in pollen grains. The single nucleus in the onion epidermal cell isstandard for most plant cells; the two nuclei in the pollen grains are associatedwith 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 experimentto 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 totheir functions in the body. This is probably best done with preparedphotographs of the cells rather than attempting to view fresh materials under a microscope.

Feature Onion epidermal cells Pollen grains

Cell wall Thin and smooth with Very thick and sculpted (complexno noticeable pore. patterned outer surface) with a number

of noticeable pores.

Nuclei One Two

Vacuole Large; easily visible. Absent or small

Shape Flat; longer than it is broad. Spherical

Growth Does not grow noticeably. Germinates to produce a pollen tube.