Name: _____________________________
Block: _______
Onion Osmosis Lab Activity
Problem: How does salt influence osmosis in relation to an onion cell?
Materials: (per group): red onion epidermis, forceps, dropper, distilled water, sodium chloride (salt) solution,
paper towels, microscope, slide, and cover slip
Important information: Plant cells are surrounded by a cell membrane and a tough cell wall. The cell
membrane will shrink and grow depending on how much water is in the cell. The cell wall will not move, and
because of this it determines how much water a cell can actually take in. The cell wall keeps the cells from
bursting from too much water.
PROCEDURE:
1. Peel off a very, very thin piece of red onion (have your teacher help you). Make a wet mount of the red
onion epidermis.
2. Examine under low power. When you have a clear view of several cells,
switch to medium power. Make a drawing of ONE CELL in the circle to the
right. Use colored pencils; this will give you a record of the original
appearance of the onion cells. Label the cell wall, cell membrane, and
cytoplasm.
3. Take a dropper and add several drops of salt solution to one side of your
cover slip (not on top of the coverslip), then take a small piece of paper towel
and place it along the opposite edge of the cover slip. The paper should draw
out the water and draw in the salt solution.
4. Make sure you are looking at the cells nearest to where you put the salt
solution. Observe the effects of the saline (salt) solution on the onion cells.
Make a drawing of ONE CELL in the circle to the right. Label the cell wall, cell
membrane, and cytoplasm.
5. Using a pipette, flood the onion with distilled water in the same way that
the salt solution was added. Make a drawing of ONE CELL in the circle to the
right. Label the cell wall, cell membrane, and cytoplasm.
CONCLUSION QUESTIONS (answer in complete sentences!):
1. The picture below is of a cell in a solution. If osmosis is going to occur draw arrows telling me whether
water is going to move in or out of the cell. Then draw another picture to the right… of what that cell will
look like after osmosis occurs.
H2O
H2O
H2O
H22O
O
H
H2O
NaCl
NaCl
H2O
H2O
H2O
H2O
H2O
H2O
2. The picture below is of a cell in a solution. If osmosis is going to occur draw arrows telling me whether
water is going to move in or out of the cell. Then draw another picture to the right… of what that cell will
look like after osmosis occurs.
NaCl
NaCl
H2O
NaCl
H2O
H2O NaCl
H2O H2O
H2O H2O
H2O
NaCl
NaCl
3. Red blood cells (and other animal cells) placed in a distilled water solution usually swell up and burst.
What prevented the red onion cells from swelling up and bursting when they were placed in the distilled
water?
4.
What does this movement of water have to do with homeostasis in a cell?
5. What would happen to a plant cell when placed in a solution that matched
it’s internal salt content? Why?
6. How does the cell membrane help to maintain the homeostasis of water
content?
7. If a bowl of fresh strawberries is sprinkled with sugar, a few minutes later the berries will be
covered with juice. Why? Use the terms: osmosis, concentration gradient, high concentration, low
concentration and cell membrane to explain your answer.
8. Roads are sometimes salted to melt ice. What does this do to plants around the roadside and why? You
must be specific in terms of osmosis.
10. If a person drinks a very large amount of water in a short time without consuming any salt, this can result in
confusion, seizures, coma, or even death, due to abnormal functioning of nerve cells in the brain. Explain how
these problems could result from drinking too much water too rapidly.
11. What do you think is the reason that a person who is stranded at sea should not drink ocean water? How could
drinking salty water harm a person's cells? Be thorough in your explanation.