Wet Mounts – Onion Skin Cells

Objective:

Make a wet mount of an onion skin cell and estimate its size.

Materials:

microscope

glass slide

cover slipiodine stain

onion

Procedure:

  1. Measure and record the field of view diameter on low power here: ______
  2. Peel a translucent piece of tissue from the onion. (The smaller the piece the better.) Translucent means that you can see light through the specimen, but it is not transparent.
  3. One partner - make a wet mount of a sample of onion skin. Do not stain the specimen.

Other partner - make a wet mount of a sample of onion skin but stain the specimen with 2 drops of iodine. Cover slides with coverslips carefully using proper wet mount technique.

  1. Observe the unstained specimen first at low, medium and then high power. Adjust the diaphragm to obtain maximum contrast.
  2. Describe the appearance of unstained specimen at HIGH power. Include a description of colours, and structures that could be seen:
  1. Now examine the onion skin that has been stained with iodine at high power.
  2. Estimate the number of cells that could fit across the field of view width-wise on high power: ______

Calculate the width of one onion cell as seen at High power. Show your work.

  1. Draw two complete adjacent (end to end) stained cells at low power.
  2. Observe the stained onion cells under medium, and then high power. Make a drawing of one onion cell at high power, labeling all of its parts as you observe them. (At minimum, you should observe the nucleus, cell wall, and cytoplasm). Note: each structure only has to be labeled once even though two cells will be drawn.

Observations:

The labeled drawings should be completed on the back of this handout. These neat drawings MUST be completed using a pencil. Follow the rules for biological drawings. If you want to show detail, stiple but do not shade.

For each diagram, calculate the total magnification. Refer to the marking scheme on the back of the handout.

Questions (complete on a separate piece of paper):15 marks

  1. Why do we stain specimens? Support your answer using the observations made when examining the onion cells with and without the iodine stain. Be specific by referring to the substances and organelles affected by the stain(s). (4 marks)
  2. Stains increase contrast because certain structures absorb the stain more readily than others. If a stain was absorbed equally by all structures, why would this would not be a useful stain? (1 mark)
  3. If stains are not available, how can the visibility (contrast) of cell structures be increased? (2 marks)
  4. Why must the specimen you observe be very thin? (1 mark)
  5. Name three (3)organelles that are found in plant cells that are not found in animal cells. (2 marks)
  6. A chloroplast, normally found in plant cells, is not present in onion skin cells.
  7. What is the function of this organelle and describe? (2 marks)
  8. Why are there no chloroplasts in onion skin cells? (1 mark)
  9. How could you tell by the general shape of the onion skin cell that it was a plant cell? (2 marks)

Microscope DrawingsName:

Low Power: Magnification ______

High Power: Magnification ______

Marking Scheme for Diagrams:

Possible Mark / Your Score
Title – top of diagram, name of specimen, underlined, printed / 1
Name – top right, complete (full name), printed / 1
Labels – nucleus, cell wall, cytoplasm, correct spelling
Straight lines, no arrowheads, labels in even column on right side, printed / 5
Diagram – Neat, accurate, clear lines, all erasures are complete, no shading, stipling used for detail, only pencil (including underlining) / 2
Magnification correct, with an “X” written after it. / 1
Total / 10