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Cell Organization, Structure, and Function

Biology I Honors- Pine View

Materials (per team of 2):

lab aprons, safety glasses, microscope slide, coverslip, forceps, microscope, dropping pipet, paper towel, Lugol's iodine solution, salt solution(15%), onion, Elodea leaf, prepared slide of human cheek cells.

Prelab

Be prepared to answer these items verbally when your teacher comes to your lab group.

  1. A student uses a light microscope with a total magnification of 1500x, but the image is too blurry to be seen clearly. Why might this occur?
  2. Why is it important to locate and center a specimen on low power before attempting to focus it at high power?
  3. A student focuses on a specimen at low power and carefully centers it before changing to high power. At high power, however, he doesn't see the part of the organism he was interested in. What might be the problem?
  4. What is the diameter of the low power field of view of your microscope? High power?

 Procedure

  1. Separate one layer from a piece of onion. Hold it so the inside of the onion faces you. Snap the piece backwards. Using forceps, peel of a small section of the transparent layer of cells along the outer curve of the onion and lay it flat on a slide.
  2. Add a drop of Lugol's iodine and a coverslip.
  3. Examine the slide under both low and high power. Your view should be similar to Figure 1 below.  Sketch a few cells as they actually appear under high power. (Remember to use a small beaker to draw a circle on your paper to represent your field of view. Make your sketch inside this circle). Sketch a few cells as they would appear if you could see three dimensions. Identify and label the cell wall, nucleus, nucleolus, and cytoplasm. Estimate and record the cell size.
  4. Add a drop of salt solution to one edge of the coverslip. Touch a small piece of paper towel to the opposite edge. The paper towel will act as a wick, pulling the salt solution under the coverslip. Watch until the cytoplasm appears to pull away from the cell walls. The plasma membrane is the boundary between the cytoplasm and the cell wall. Sketch the cell as it appears now.
  5. Prepare a wet mount of an Elodea leaf and observe it under both low and high power.  Your view should be similar to Figure 2 below.  Sketch a few cells as they appear under high power. Add a drop of Lugol's iodine using the same technique you used to add salt to the onion cell. If there are any structures visible now that weren't visible without the Lugol's, add them to your sketch. Label your sketch with all cell parts visible and estimate the cell size.
  6. To view cheek cells, gently scrape the inside lining of your cheek with a clean toothpick. DO NOT GOUGE THE INSIDE OF YOUR CHEEK! (We will observe blood cells in a future lab!! ).  Gently tap the toothpick onto the center of a glass slide. Some of the cheek cells should fall onto the slide.  Add a drop of methylene blue stain (specific for animals) and cover with a cover slip.  Observe the cheek cells under both low and high power of your microscope. Draw a diagram of one cheek cell, using a pencil, and label its parts. (At minimum you should observe the cell membrane, nucleus, and cytoplasm. Your view should be similar to Figure 3 below. Find an area where the cells are separate and distinct, and sketch a few under high power. Label all visible cell parts and estimate and record cell size.
  7. Clean, dry, and put away slides and coverslips. Properly put away your microscope. Clean up the slide preparation area. Thoroughly wash and dry your hands. Answer the analysis items.

 

              

   Figure 1. Onion epidermis,400x.                Figure 2.  Elodea cells, 400x.                       Figure 3.  Human cheek cells, 400x.

 

Analysis

  1. What cell part(s) did you find in your samples of plant cells that you did not find around or in the cheek cells?
  2. Why are the plasma membranes of plant cells so difficult to see?
  3. How did Lugol's iodine solution help you in this activity? Why do biologists use stains to study cells?
  4. Inspired by what she has learned in biology class, a student decides to make a closer study of what she eats. She uses a razor blade to make a very thin section from a raw potato and mounts it in a drop of water. To her disappointment, she can barely make out the cells under the microscope. What might she do to improve her results?
  5. Write the correct name and function of the cell organelle indicated by numbers in the diagrams below.