Cheek/Onion Lab Name __________________________ Comparing Animal & Plant Cells Date _____________ 407 Instructor note: The fully text of this lab may have been significantly deleted or altered for the purposes of illustrating issues about lab safety in our course. Background One of the types of science that scientists do is called qualitative science. In this type of science scientists often spend their energy trying to describe a new phenomenon or a new type of organism. One of the goals of this science program is to have students OBSERVE and DESCRIBE cells in a variety of organisms using microscopes and hand lenses. In this lab, you will observe and describe cells in two different organisms: humans (animals) and onions (plants). Planning (b) MATERIALS: Onion, razor blade, forceps, dissecting scissors, toothpick, probe, glass slide, cover slip, water, Lugol solution (for staining onion cells), methylene blue (for staining cheek cells), water, light microscope, pencil, Lab Book. METHOD: Part I: HUMAN CHEEK CELLS A. Clean your glassware and microscope as usual. B. Place a drop of diluted methylene blue (or Lugol solution) on the center of the slide. C. Using the blunt or rounded end of the toothpick, scrape the inside of your cheek near the back of your mouth just above your lower teeth. Some of the cheek cells will stick to the toothpick. D. Place the end of the toothpick with the cells on it into the drop of stain. Twirl the toothpick slowly and gently for a few seconds, just long enough to dislodge the cells. E. Put on a cover slip as you did when making other wet mounts. F. Look for single cells or small groups of cells. The cells will appear as pentagons (similar to a rectangle but with five sides). The nucleus will be dark brown or blue depending on the type of stain you used. The cytoplasm will stain more lightly than the nucleus. The cell membrane will be darkly stained on the outside edge of the cell. Don’t waste your time looking at any small bits of food that might have been transferred from your mouth along with the cells. You can recognize food particles because they look like threads or rods. G. Make a ½ page drawing of the slide under high power and label it Figure 1. Be sure to capture a few single cheek cells AND a small group of cells that are attached to each other. Part II: ONION “SKIN” CELLS A. Clean your glassware and microscope as usual. B. Place a drop of Lugol solution on the center of the slide. Place a drop of water on the slide next to the drop of stain. C. Using the toothpick, mix the two drops together. D. To obtain your specimen, get a single piece of onion from your teacher. E. Remove a piece of very thin transparent tissue—which looks like skin—from the onion. Use the forceps to peel the tissue from the onion. Try to obtain as large a sheet of onion tissue as possible from the onion. F. Use your small dissecting scissors and/or razor blade to cut the tissue into a small square section, about a 1 cm. x 1 cm. square. G. Place the sample on the slide and use the toothpick, probe, 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. H. Cover the specimen with a cover slip. Use the toothpick to tap out any air bubbles you see. I. The cells will appear as thin rectangles (not perfect, they kind of have pointed ends). The nucleus will be dark brown. The cytoplasm will stain more lightly than the nucleus. The cell wall will be darkly stained on the outside edge of the cell. Notice how the onion cells are almost identical and how they fit together in a tight geometric pattern. J. Make a ½ page drawing of the slide under high power and label it Figure 2.
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