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Procedure

  1. Visualizing Onion and Human Cells
    • In this experiment you will prepare different types of cells from a plant and animal, onion and human respectively, and then visualize them under a microscope. HYPOTHESES: The experimental hypothesis is that the cells will appear different in overall shape and membrane structure, but there will be some shared similar structures, like nuclei, in both cells. The null hypothesis of the experiment is that there will be no difference in structure between the two cell types.
    • First, to prepare an animal cell slide, start by pouring 30 mL of distilled water into a beaker.
    • Then, use a glass dropper to place 2 - 3 drops of the water onto the center of a clean microscope slide.
    • Next, take a clean toothpick and scrape the inside of your cheek to gather cells. CAUTION: Do not scrape your cheeks too aggressively. The scraping should not cause discomfort or bleeding.
    • Put the end of the toothpick with the cheek cells onto the wetted area of the slide and mix the water and cheek cells together.
    • Next, add two drops of methylene blue staining solution to the wetted area of the slide and mix it in with a clean toothpick.
    • Let the mixture sit at room temperature for 3 minutes.
    • After 3 minutes have passed, use a glass dropper to add a drop of glycerol to the mixture.
    • Carefully, place a cover slip over the mix on the slide by first standing one end of the cover slip vertically and aligning it along to one side of the mixture. Then, carefully lower the other end of the cover slip down until the mixture is completely covered.
    • Finally, remove any excess liquid with blotting paper.
    • Next, prepare a slide of plant cells by placing five drops of distilled water into a clean watch glass.
    • Then, with forceps, take a thin strip of onion and place it into the water.
    • Apply 5 drops of safranin solution to another watch glass.
    • Now using a pair of forceps transfer the piece of onion from the distilled water into the safranin.
    • Allow the onion to soak for 30 seconds.
    • Then, again with forceps, move the onion back into the distilled water.
    • Now, place 3 drops of glycerol onto the center of a glass microscope slide.
    • Using the forceps again, transfer the onion into the glycerol on the slide.
    • Gently place a cover slip over the onion on the slide and blot away any excess liquid with a piece of blotting paper.
    • Next, you will visualize the slides you have prepared using a compound microscope. First, turn on the scope and set the objective lens to the lowest level of magnification. NOTE: A compound microscope typically has two levels of magnification.
    • Place the cheek tissue slide onto the stage of the microscope and bring the cells into focus by adjusting the stage and using the coarse adjustment knob.
    • When the cells are in focus, observe the individual cells and draw what you see. Repeat these observations at medium and high magnifications adjusting the focus with the fine adjustment knob. IMPORTANT: Do NOT use the coarse adjustment knob at high magnifications.
    • Next, rotate the nose piece so that the oil lens is almost in place.
    • Place a drop of immersion oil on top of the cover slip of the cheek cell slide.
    • Use the oil lens to view the cheek cells and record your observations.
    • Repeat these steps to view the onion plant cell, moving again from low to medium, high, and oil lenses (steps 20 – 24).
    • Record all of your observations and note the differences you see between animal and plant cells.
    • When you are finished, place the cheek cell slide and toothpicks into a diluted bleach solution. Dispose of the cover slips into a glass waste container.
    • Finally, turn off the microscope and clean the lenses with lens cleaning paper.
  2. Results
    • Calculate the total magnification for each of the viewing levels. Remember to account for both the eyepiece magnification and the objective lens magnification. NOTE: Eye piece: 10X magnification, Objective lens: 4X, 10X, and 40X magnification
    • Then, identify the structural differences between plant and animal cells using the observations made under the microscope.
    • Label the nuclei in both cell types and then highlight or note any differences that you see between them.

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