Title: Using a Compound Light Microscope and Comparing Cells - Bacteria, Plant Cells and Animal Cells

Transcription

1 Title: Using a Compound Light Microscope and Comparing Cells - Bacteria, Plant Cells and Animal Cells Background: Many objects are too small to be seen by the eye alone. They can be seen, however, with the use of an instrument that magnifies, or visually enlarges the object. One such instrument, which is of great importance to biologists, and other scientists, is the compound light microscope. A compound light microscope consists of a light source or mirror that illuminates the object to be observed, an objective lens that magnifies the image of the object and projects it into the viewer s eye. Objects, or specimens, to be observed under a microscope are generally prepared in one of two ways. Prepared or permanent slides are made to last a long time that are usually purchased from biological supply houses. Temporary or wet mount slides are made to last only a short time- usually one laboratory period. The microscope is an expensive precision instrument that requires special care and handling. Ever since the first microscope was used, biologists have been interested in studying the cellular organization of all living things. After hundreds of years of observations by many biologists, the cell theory was developed. The cell theory states that the cell is the basic unit of living things, they contain structures called organelles that carry out life processes and they can be classified by the types of organelles they contain. In plant and animal cells, similarities and differences exist because of varied life functions. In this investigation, you will learn the parts of a compound light microscope, the functions of those parts, and the proper use and care of the microscope. You will also learn the technique of preparing wetmount slides and compare the structures of a bacterial cell, typical plant cell (Onion and Elodea) and a typical animal cell (human cheek cell). Procedure: Microscope rules 1. Always carry or move a microscope with two hands, one on the arm, and one on the bottom. 2. Always use the lowest power lens (the shortest lens) when you take a slide on and off the stage. 3. Always start with the lowest power lens (the shortest lens). Get the slide in focus there, first using the coarse focus knob (the large knob) to get it close, THEN using the fine focus knob (the small knob) to get it perfectly in focus. From there, you can switch to a higher power lens. 4. Always look from the SIDE of the microscope, not through the eyepiece, when switching lenses to avoid hitting the lens on the slide. 5. Only use the coarse focus knob (the large knob) when you are using the lowest power lens (the shortest lens). Using the coarse focus knob on a higher power can crack the lens! 6. When you are finished, turn off the light, put it back on the low power objective, lower the stage completely and cover the microscope. Bacteria cells 1. Obtain a prepared bacteria slide 2. Make sure the lowest power lens (the shortest lens) is in place over the stage and the microscope light is turned on. Place the slide onto the stage of the microscope. 3. Look through the eyepiece and turn the coarse focus knob (the largest knob) until an image comes into focus. Now use the fine focus knob (the smallest knob) to make the image as focused as possible. 4. Looking from the SIDE of the microscope, NOT through the eyepiece, rotate the lenses to the next highest powered lens (10x). If you need to, use the fine focus knob (the smallest knob) to get the image into focus. DO NOT USE THE LARGE KNOB!! 5. Again, looking from the SIDE of the microscope, rotate the lenses to the highest powered lens (40x). If

2 you need to, use the fine focus knob (the smallest knob) to get the image into focus. 6. Draw a picture of what you see. Label the picture bacteria cells 400x. 7. Switch to the lowest power lens and THEN remove the slide. Set it aside for now. Onion skin cells 8. Add 1 drops of iodine to the center of a glass slide. Be careful! Iodine can stain your clothes. 2 drops iodine 9. Take a small piece of onion. Use tweezers to peel off onion skin the skin from the underside (the rough, white side) of the onion. 2 drops iodine 10. Carefully lay the onion skin flat in the center of the glass slide slide on top of the iodine and add 1 drops of iodine to the top of the onion skin. 11. Stand a thin glass cover slip on its edge near the onion skin, next to the drop of iodine. 12. Slowly lower the other side of the cover slip until it covers the onion skin completely. If there are air cover slip bubbles, gently tap on the glass to chase them out. 13. Make sure the lowest power lens (the shortest lens) is in place over the stage and the microscope light is turned on. Place the slide onto the stage of the microscope. 14. Look through the eyepiece and turn the coarse focus knob (the largest knob) until an image comes into focus. It should look like a brick wall or like lizard skin. 15. Now use the fine focus knob (the smallest knob) to make the image as focused as possible. 16. Looking from the SIDE of the microscope, NOT through the eyepiece, rotate the lenses to the next highest powered lens (10x). If you need to, use the fine focus knob (the smallest knob) to get the image into focus. DO NOT USE THE LARGE KNOB!! You may see a small dot in the middle of each cell. 17. Again, looking from the SIDE of the microscope, rotate the lenses to the highest powered lens (40x). If you need to, use the fine focus knob (the smallest knob) to get the image into focus. You should see a dark blob in the middle of each cell. 18. Draw a picture of what you see. Label the picture Onion skin cells 400x. Label as many parts of the cell as you can see. 19. Switch to the lowest power lens and THEN remove the slide. Set it aside for now. Elodea leaf 1. Tear off one small leaf from the elodea plants floating in the fish tank. 2. Add one drop of tap water to the slide. 3. Stand a thin glass cover slip on its edge near the leaf, next to the drop of water. 4. Slowly lower the other side of the cover slip until it covers the leaf completely. Make sure there are no air bubbles. 5. Make sure the lowest power lens (the shortest lens) is in place over the stage. Place the slide onto the stage of the microscope. 6. Look through the eyepiece and turn the coarse focus knob (the largest knob) until an image comes into focus. It should look like small green bricks or like lizard skin. 7. Now use the fine focus knob (the smallest knob) to make the image as focused as possible. 8. Looking from the SIDE of the microscope, rotate the lenses to the 10x lens. If you need to, use the fine focus knob (the smallest knob) to get the image into focus. You should be able to see lots of small green dots in each cell.

3 9. Again, looking from the SIDE of the microscope, rotate the lenses to the 40x lens. If you need to, use the fine focus knob (the smallest knob) to get the image into focus. The little green dots should get larger. 10. Draw a picture of what you see. Label the picture Elodea leaf cells 400x. Label as many parts of the cell as you can see. 11. Switch to the lowest power lens and THEN remove the slide. Set it aside for now. Human cheek cells 1. Add one drop of methylene blue to the middle of a clean slide. Be careful! Methylene blue will stain your clothes and skin. 2. Use the flat side of a toothpick to gently scratch the inside of your cheek. DO NOT GOUGE YOUR CHEEK - you don t need chunks of skin and definitely don t want to draw blood. 3. Gently touch the toothpick to the drop of dye on the slide. Some of your cheek cells should drift off into the dye. 4. Throw the toothpick away. 5. Stand a thin glass cover slip on its edge near the drop of dye. 6. Slowly lower the other side of the cover slip until it covers the dye completely. Make sure there are no air bubbles. 7. Make sure the lowest power lens (the shortest lens) is in place over the stage. Place the slide onto the stage of the microscope. 8. Look through the eyepiece and turn the coarse focus knob (the largest knob) until an image comes into focus. It should look like scattered blobs. Move the slide around until a nice cluster of blobs moves into the center of your image. 9. Use the fine focus knob (the smallest knob) to make the image as focused as possible. 10. Looking from the SIDE of the microscope, NOT through the eyepiece, rotate the lenses to the 10x lens. If you need to, use the fine focus knob (the smallest knob) to get the image into focus. 11. Again, looking from the SIDE of the microscope, rotate the lenses to the 40x lens. If you need to, use the fine focus knob (the smallest knob) to get the image into focus. 12. Draw a picture of what you see. Label the picture Human cheek cells 400x. Label as many parts of the cell as you can see. 13. Switch to the lowest power lens and THEN remove the slide. Questions: Answer must be in complete sentences 1. When you move the slide to the left, in what direction does the image appear to move? 2. When you move the slide up, in what direction does the image appear? 3. Explain why a specimen to be viewed under the microscope must be thin? 4. Why should a glass slide and a coverslip be held by their edges? 5. Why is it a good idea to place your microscope at least 10cm from the edge of the table? 6. Suppose you were observing an organism through the microscope and noticed that it moved toward the bottom of the slide and then it move to the right. What does this tell you about the actual movement of the organisms? 7. What is the shape of an Elodea cell? 8. What is the shape of the cheek cell? 9. How are bacteria (prokaryotic) cells different than the plant and animal (eukaryotic) cells that you looked at? 10. How are the plant and animal cells you looked at similar in structure? What organelles do they have in common?

4 11. How are the plant and animal cells you looked at different in structure? What organelles are different? 12. In general, the surface of a tree has a harder feel than the surface of a dog. What cell organelle can be used to explain this difference? 13. If you were given a slide containing living cells of an unknown organism, how would you identify the cells as either plant or animal cells? Conclusion: Write a 3 to 5 sentence paragraph (restate what this lab was about, why this lab was done, & what you found out about cells).

5 Data: 1. Draw a microscope and label all the parts. Microscope and Types of Cells Lab 2. Fill in the magnification of each objective and the eyepiece of your microscope. To determine the total magnification, multiply the magnification of each objective by the magnification of the eyepiece. Objective Magnification of objective Magnification of eyepiece Total magnification Low power Medium High Detailed drawing of bacteria cell Detailed drawing of bacteria cell

6 Detailed drawing of onion cell Detailed drawing of onion cell Detailed drawing of elodea cell Detailed drawing of elodea cell Detailed drawing of cheek cell Detailed drawing of cheek cell