Assignment # _______
What does a scallion look like under the microscope?
SAFETY TIP - DANGER - *
1. Razor blades are sharp. Use care! 2. Do not use the sun as a light source, use only indirect classroom lighting. 3. Iodine is poisonous if swallowed. 4. Wash your hands carefully when you finish
Making a temporary wet mount of scallion skin. Clean your slide and coverslip with a piece of lens paper. 1. Place a drop of water on the center of the slide. 2. To obtain your specimen, grasp the outermost leaf of the scallion and pull down toward the roots until it breaks free from the rest of the scallion. 3. Remove a piece of the very thin transparent tissue which looks like skin from the rest of the leaf using your forceps (tweezers) and razor blade. 4. If you can't find this kind of scallion skin on your first attempt with the first leaf, try again with the same or another leaf. 5. No luck? Try the remaining leaves. If you are still unsuccessful, scrape the inside surface of the white portion of the stem that came off with a leaf. To do this properly, place the leaf outerside-down on a slide and gently scrape the inner surface with one corner of the razor blade until you free a small section of scallion "skin" of its inner tissues. You will be successful if you use a sharp blade, preferably one that has not been used before. 6. While the specimen is still on the slide, use your blade to cut out a small section, about one centimeter (I cm) square, of the thin skin. This is your specimen. It is only one cell layer thick! Being so thin makes it ideal for microscopic examination. 7. Place the specimen in the center of a drop of water on the slide. Use your toothpick and forceps to smooth out the specimen in the drop, if necessary. Folds or wrinkles will make it very difficult to see the cells precisely.
8. Cover the specimen with a coverslip. A good way to do this is to hold the coverslip between your forefinger (first finger) and thumb at about a 45-degree angle. Place the lower edge of the coverslip at one edge of the water drop. Now slowly drop the coverslip on top of the specimen. Use the dry end of the toothpick to tap out any tiny air bubbles you see. Congratulations — you have made a wet mount! 9. Wipe the bottom of the slide with lens paper to remove any water there. You are now ready to examine the specimen under your microscope.
A. Using the Low Power Objective
You almost always follow a specific routine when using the microscope. Start with low power and, after viewing the specimen, move on to the next higher power. One exception to this rule is when you are using the immersion oil lens. We'll discuss correct use of this lens at another time. For now, let's get back to basics and learn the steps to follow to obtain good focus with the low power objective. Clean the optical parts (glass) of the microscope with lens paper. 1. While looking at the objectives, move the low power objective into line with the body tube. You will hear or feel a click when the objective is in the correct position. 2. If your microscope is equipped with a light, turn it on. If your microscope has a mirror, get as much light as you can by using the flat side of the mirror for light from outdoors. Use the curved side of the mirror for artificial light. *CAUTION* Never reflect direct sunlight into your mirror because this might cause serious damage to your eye! 3. Adjust the mirror while looking through the eyepiece until the circle of light is at its brightest. Be sure that the diaphragm is fully open as you start and that the substage condenser (if your microscope has one) is up as far as it will go, that is, positioned against the bottom of the stage. 4. Center the specimen on the stage until it is directly under the low power objective. 5. Remove your eye from the eyepiece. While you are looking at the slide on the stage, turn the coarse adjustment until the low power objective is as close to the slide as possible without forcing it. Some microscopes are equipped with a "stop" which prevents the objective from hitting the slide. If this is the case, lower the objective as far as it will go. 6. Look through the eyepiece. Experienced microscopists keep both eyes open as they peer through their microscopes. Try to keep both eyes open. If you find this too difficult, try covering the eye that's not over the eyepiece with the palm of your hand, while keeping that eye open. 7. While looking through the eyepiece, slowly raise the body tube using the coarse adjustment knob until the specimen is in clear focus. 8. Adjust the light, using the diaphragm and substage condenser (if your microscope is equipped with one) until the skin is clearly visible. Don't waste your time looking at very dark circles; these are air bubbles. 9. Get the skin in perfect focus using the fine adjustment knob.
10. Draw the scallion skin as it appears through the microscope. Remember that the drawing should be accurate and show the proper size of the object you are viewing.
10. Would you describe the structure of scallion skin as smooth and continuous, or made of pieces joined together like a jigsaw puzzle?
11. Read the article about Robert Hooke. What did Hooke call the structures you see in the scallion skin?
B. Focusing Under High Power
You are now ready to go to high power. On most microscopes, this will be a 40x objective. Be sure that what you want to examine is in the center of the low power field, because when you switch to high power, the field of view will be smaller than the field you see under low power. If what you want to look at under high power is near the edge of the low power field of view, it will not appear under high power. Your microscope is probably parfocal. That means that the high power objective, when swung in line with the body tube, is almost in focus if the specimen was in focus using the low power objective. So your first step is to get the best possible image using the low power objective when the part of the specimen you want to see is in the center of the field of view. 1. Now swing the high power objective into line with the body tube. You may have to increase the amount of light entering the high power objective by changing the setting of the diaphragm and the condenser. 2. Remember, even though you see cells in two dimensions, the cells really have three: They have depth as well as length and width. You must focus up and down slightly with the fine adjustment knob so you can examine all depths of the specimen. 3. If you lower the high power objective too far or are using an incorrect coverslip, the objective lens may press on the coverslip and force water out from under the coverslip. That water may get on the objective. If that does happen, you should clean the objective as soon as possible with lens paper; otherwise, it will affect your ability to see images properly in the future. 4. If, at any time, you lose the focus with the high power, go back to low power and begin again. With the high power objective, you will see less of the specimen, but in much greater detail. 5. Draw a scallion skin cell as you see it under high power in the space below. Make your drawing as detailed as possible!
Scallion Skin Cell Under High Power
1. Describe some of the ways in which the individual scallion cells you observed differed. 2. What types of objects or structures did you observe inside scallion cells? 3. You looked a skin from the bulb portion of a scallion, what do you think you might see if you looked at a section of scallion leaf?
Assignment # _______
How can cell structures be better observed under a microscope? *
In the 1670s, a Dutch naturalist—Anton van Leeuwenhoek—constructed a simple microscope that could magnify things more than 250 times (250 x). Today's electron microscopes are capable of magnifications that can exceed 200,000 x. Both types of microscopes, however, rely upon using specimen samples that are very thin. In this laboratory lesson, we will be using a standard compound light microscope. Thinner specimens allow more light to pass through them to the lenses of the microscope; the specimen will appear brighter. On the other hand, as more light passes through the specimen, internal structure detail is less clear. In this exercise, you will explore several techniques that will allow you to view, in greater detail, cell structures in specimens that are very thin. SAFETY
1. Your teacher will review the proper use of a microscope with a light source. 2. Handle forceps and dissecting needles with care. 3. Avoid getting chemical stains on your hand, as they are difficult to remove.
compound light microscope forceps cover slips water dissecting needle slices of onion microscope slides eye dropper methylene blue or Lugol's solution lens paper
1. Use a piece of lens paper to carefully clean the microscope lenses, mirrors, microscope slides, and cover slips. 2. Add a drop of water to a clean microscope slide. Use the forceps to obtain a thin piece of scallion skin. Place the cover slip over the specimen by using the dissecting needle to support one side of the cover slip, and gently lowering it onto the specimen.) 3. View the specimen under low power. Make a drawing in the space provided below, labeling as many structures as you can see. 4. View the specimen under high power. Make a drawing in the space provided below. Label as many structures as you can see.
From Biology Laboratory Manual, Office of the Superintendent of Queens High Schools and the WRIT Bridge Project of the Queens Borough Institute for Writing and Critical Thinking.
5. Place a drop of methylene blue or Lugol's solution on one edge of the cover slip. Draw the stain through the specimen by touching the opposite edge of the cover slip with a piece of lens tissue or paper towel. 6. Observe the specimen under low power and then under high power. Illustrate, label, and color what you see, using the spaces provided below: Low power stained High power stained
Compare the stained and unstained specimens by redrawing each side by side. Carefully label each structure. unstained stained
1. Compare and contrast the onion cell, viewed unstained and stained. 2. Repeat the same technique using different specimens and different stains.
1. Why are some stains more effective in outlining structures than others? 2. What might occur if you used too little stain? Too much stain? 3. Some scientists claim that staining a specimen may change or alter its internal structures. What do you think? 4. Microscopic animals such as the paramecium are often hard to see under a microscope. What might happen to these organisms if we used a stain to make them appear in greater detail and with greater contrast? Design an experiment to prove your theory. 5. Obtain samples of commercially prepared microscope slides. Some have many different colors. Explain. 6. Imagine that you are a scientist, and that you have been asked to develop a new biological stain. What properties would this stain have? 7. An alternate technique used to stain specimens involves placing the drop of stain directly on the specimen, and then applying the cover slip. Try this technique. Which method do you feel is best; why?
Assignment # _______
How do animal cells compare with plant cells?
All living things are made of cells. Cells are the basic units of structure and function in living things. These statements are part of the CELL THEORY observed by botanist Schleiden and zoologist Schwann in 1839. There are similarities in plant and animal cells. All cells are surrounded by the thin PLASMA MEMBRANE and contain a jelly-like material called CYTOPLASM. Within the cytoplasm are cellular structures called ORGANELLES. The organelles include the NUCLEUS, or control center of the cell, the MITOCHONDRIA, RIBOSOMES, ENDOPLASMIC RETICULUM and GOLGI BODIES. There are also some differences between plant and animal cells. Plant cells contain organelles called CHLOROPLASTS which contain the green pigment and are needed to make food. Plant cells also are surrounded by a thick CELL WALL made of a non-living substance called CELLULOSE. In this lab investigation, you will observe animal cells by using prepared slides of human skin cells and plant cells from elodea, a freshwater plant.
microscope cover slips forceps prepared slides of human skin glass slides Lugol's iodine solution elodea (in a tank or large beaker) paper towels
A. Plant cell from an elodea leaf 1. Examine a sprig of elodea. With your forceps take a young leaf near its growing tip. Place it on a slide, add a drop of water and apply a cover slip. Observe the leaf under low power. 2. Observe the arrangement of cells in the leaf you are examining and draw your section of the leaf as it appears under low power. 3. Switch to high power. Carefully focus and draw a high-power view of several elodea cells. Label the cell parts. 4. Look for moving chloroplasts close to the cell wall. This movement is the result of cyclosis, the streaming movement of the cytoplasm of the cell. 5. Stain your elodea leaf with Lugol's solution on the microscope stage in the following way: Place a drop of Lugol's at one edge of the cover slip; holding a small strip of paper toweling against the opposite edge of the cover slip will draw off the water and draw the Lugol's solution under the cover slip. Observe whether nuclei and nucleoli have become visible.
Observations and Data:
1 Elodea Cells: Draw the cells under low and high power. Label each cell organelle that is visible. Low power High power
2. Describe the movements of the chloroplasts.
3. How can you tell that the elodea leaf has several layers?
4. What cell structures are made visible by staining with Lugol's?
Epithelial cells from human skin 1. Place a prepared slide of human epithelial cells on the stage of your microscope. 2. Focus under low power. Then carefully switch to high power and record your observations and label the cell parts in the spaces provided on the next page. 3. Draw two human epithelial cells as seen under high power. Label each cell organelle that is visible.
Epithelial cells from human skin
Epithelial cells from the cheek 1. Place a drop of water on a clean slide 2. Gently scrape the inside of your cheek with the end of a toothpick. 3. Place the end of the toothpick in the drop of water and rotate it so as to dislodge any epithelial cells which were scraped from the inside of the cheek. 4. Apply a cover slip and stain the cheek cells with methylene blue. 5. Observe the arrangement of cells on the cheek scrapings and draw two views, low power and high power, in the spaces provided on the next page.
Low power High power
6. List the structures present in both the elodea and human cells.
7. How do elodea and human epithelial cells differ in size and structure?
ENRICHMENT: 1. Plant cells found in roots lack chloroplasts. How would these plant cells obtain food?
2. Sometimes cyclosis is not visible in elodea leaf cells. Design an experiment to see how light affects cyclosis. Design an experiment to observe the effect of temperature on cyclosis.