Download Students will make a wet mount slide of onion cells and observe the

Adapted from Onion Rings, AIMS
Students will make a wet mount slide of onion cells and observe the cell walls. Then students will
stain a wet mount slide to observe the onion cell’s nucleus, cytoplasm, and cell membrane.
Materials:
Whole white onion
Cover slip
Tweezers
Microscope
Methylene Blue (1% solution)
Goggles
Slide
Dropper bottle with eye dropper
Newspapers to cover lab table
Teacher Preparation:
1. Cut each onion in half and soak in half-filled cups of water overnight. (This makes it easy to
separate the onion’s membrane.)
2. Have tweezers, eyedroppers, water, and methylene blue stain ready.
3. Put dropper bottles of stain in tip proof boxes at each workstation.
4. When students are ready to stain the onion slide, have them work on newspaper covered tables.
5. Make copies of the student directions for each lab group.
6. Make copies of the What You Should See paper for each group.
7. You may want to laminate the copies for future use.
Post Lab Discussion questions:
1. What was the general shape of the onion cell? (rectangular)
2. Describe what you saw without the stain.
3. What is the purpose of so many cells close together? (strength and protection)
4. Is an onion composed of one cell or many cells? (many cells)
5. Why is it easier to see the onion cells after they are stained with mythelene blue? (The stain
creates contrast between light and dark structures.)
6. All plant cells have cell walls. What is the function of cell walls? (Provides strength and
protection.)
7. Each cell has a control center. What is the control center called? (nucleus)
8. Inside the cell wall is a very thin line. It is the cell membrane. What is the purpose of the cell
membrane? (allows dissolved materials to enter and leave the cell)
9. Why are onion cells more like squares and rectangles than ovals? (The cells “fit together”
without gaps in the corners. This makes the cells stronger.)
Possible Extensions:
1. Have students make wet mount slides using red and green onions. Compare the cell shapes.
2. Prepare wet mount slides of other plant cells (i.e., carrots or geraniums).
SBISD Sixth Grade Science
‘08-‘09
Preparing a wet mount slide of onion skin.
1. Clean the slide and cover slip.
2. Break an onion slice in two. Carefully pull the slice apart.
3. Use tweezers to pull off a very thin piece of onion skin.
4. Place the skin in the center of the slide. (Keep it from folding.) Flatten it as much as possible.
5. Add a drop of water to the onion skin and cover with a cover slip.
6. Press the cover slip down carefully to remove any air bubbles.
7. Place the slide on the microscope stage. Set lens to low power, adjust the focus so the onion
slice is clear.
8. Draw four or five cells in the small circle on your lab sheet. Label the cell walls.
9. Switch to high power and try to identify the cell membrane, nucleus, and cytoplasm.
Staining an onion cell.
1. Lift the cover slip and add one to two drops of methylene blue to the slide.
2. Lower the cover slip and examine the cell on high power.
3. With the methylene blue you should be able to see structures of the cell you could not see
before. Draw the cell membrane, nucleus, and cytoplasm in the larger circle on your lab
sheet. Label your drawing of the cell structures.
4. After discussing the Post Lab Questions write about the lab and what you learned.
Preparing a wet mount slide of onion skin.
1. Clean the slide and cover slip.
2. Break an onion slice in two. Carefully pull the slice apart.
3. Use tweezers to pull off a very thin piece of onion skin.
4. Place the skin in the center of the slide. (Keep it from folding.) Flatten it as much as possible.
5. Add a drop of water to the onion skin and cover with a cover slip.
6. Press the cover slip down carefully to remove any air bubbles.
7. Place the slide on the microscope stage. Set lens to low power, adjust the focus so the onion
slice is clear.
8. Draw four or five cells in the small circle on your lab sheet. Label the cell walls.
9. Switch to high power and try to identify the cell membrane, nucleus, and cytoplasm.
Staining an onion cell.
1. Lift the cover slip and add one to two drops of methylene blue to the slide.
2. Lower the cover slip and examine the cell on high power.
3. With the methylene blue you should be able to see structures of the cell you could not see
before. Draw the cell membrane, nucleus, and cytoplasm in the larger circle on your lab
sheet. Label your drawing of the cell structures.
4. After discussing the Post Lab Questions write about the lab and what you learned.