Lab - Onion Cells

A picture of a microscope.


Learning Intentions

  • How to use a microscope.
  • How to prepare a slide to be viewed using a microscope.


Safety

  • Use caution when handling iodine, as it will stain your fingers.
  • Always carry the microscope with 2 hands, using the base and the arm.
  • Do not raise the stage so much that the objective lens touches the slide.


Materials

  • Microscope
  • Microscope glass slide
  • Microscope cover slip
  • Paper towel
  • Iodine solution
  • Onion
  • Tweezers
  • Camera

Procedure

  1. Collect your microscope, using the base and the arm to hold it securely.
  2. Using your fingers or the tweezers, remove a thin layer of membrane from the onion. Place the membrane on the middle of the microscopic glass slide. Make sure that there is only 1 layer, and that it is not wrinkled or folded over.
  3. Add a single drop of iodine onto the onion.
  4. Place the cover slip over the onion and press down gently. Try to avoid trapping air bubbles under the slide. Use the paper towel to soak up any excess iodine that comes out the sides of the cover slip.
  5. Rotate the revolving nosepiece on the microscope to the smallest objective lens.
  6. Place the slide on the stage, and use the positioning knobs to position the onion under the objective lens.
  7. Turn on the illuminator and look through the eyepiece.
  8. Adjust the coarse focus knob until you can see the cells of the onion. Use the fine focus knob to bring the picture into perfect focus.
  9. Take a picture of an onion cell.
  10. Rotate the revolving nosepiece to the medium objective lens. You will have to re-focus the image slightly.
  11. Take a picture of the onion cell.
  12. Lastly, view the onion through the largest objective lens. Be careful, as it is possible to raise the stage so high that the slide cracks against the objective lens.
  13. Take a picture of the onion cell.
  14. Extension: Mount the item you brought to class on a microscope slide, and take 3 pictures, as you did with the onion.
  15. Clean and/or dispose of the slides as instructed by your teacher.


Assessment

  • Includes all 3 pictures
    • For extending, include 3 pictures of the item you brought to class
  • Picture quality
    • Properly focused
    • Well lit
    • Easy to see the onion cell(s)

3 Pictures
 Picture Quality
1. Beginning Missing 2
 Hard to see
2. Developing Missing 1
 Small problems
3. Applying ✓ ✓
4. Extending Extra 3 pictures
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