Name: ________________________________ Class: ________ Date: ______________
Osmosis Lab – Red Onion Cells
INTRODUCTION:
Individual cells exist in a LIQUID environment of about 75% water. This environment
makes it easier for materials like food, water, and oxygen to move into and out of a cell.
The movement of molecules from an area of HIGH concentration to an area of LOW
concentration is known as
. The movement of water molecules
through a selectively permeable membrane is known as
.
1. What two factors determine whether diffusion will occur?
a.
b.
_
_
2. What is a selectively permeable membrane?
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CONCEPT: Cell membranes are selectively (or semi) permeable to water.
OBJECTIVES: The student will be able to conclude from their own data that cell
membranes are selectively permeable and that water moves from areas of high
concentration to areas of low concentration within cells.
MATERIALS:
Microscope
Glass slide
Cover slip
Paper towel
Eyedropper
Red onion
20% NaCl Solution
Distilled (pure) water
Part 1 – Red Onion Cell
1. Obtain a piece of red onion epidermal tissue and wet mount it on a glass slide. Cover
it carefully with a cover slip. AVOID MAKING AIR BUBBLES!!!
2. Place the slide on the stage. Center the specimen and then locate the red onion cells
using the scanning objective. Focus on the onion, and then switch to low and then
finally high power. Be sure NOT to use the coarse adjustment knob while working at
high power.
3. Draw a picture of five (5) of the onion cells in high power. Pay close attention to the
cell walls and the vacuole (red portion) of each cell. Label the cell wall, cytoplasm
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Name: ________________________________ Class: ________ Date: ______________
and vacuole of one of the cells you have drawn. Make the labels neat and easy to
read. Use a ruler to connect the label to the cell parts.
4. Remove the slide from the stage, and carefully add 3 or 4 drops of 20% NaCl solution
to one side of the cover slip. Use a small piece of paper towel to soak up the liquid at
the other end of the cover slip.
5. Place the slide back on the stage. Locate the cells using the scanning objective lens
and refocus. Then switch back up to low and then high power. Watch the cells for
several minutes to see what happens to the red portion. CAUTION: Keep the
objective lens, slide and the stage dry!
6. Draw a picture of five (5) of the onion cells and what you see. Label the cell wall,
cytoplasm and vacuole of one of the cells in each drawing. Make the labels neat and
easy to read. Use a ruler to connect the label to the cell part.
7. Fill the eyedropper with pure H2O and carefully rinse the NaCl solution away from
the slide by adding 3 to 4 drops of pure water. Refocus on the onion cells. Watch the
red onion cells for several minutes to see what happens to the red portion of the cells.
8. Draw a picture of five (5) of the onion cells and what you see. Label the cell wall,
cytoplasm and vacuole of one of the cells in each drawing. Make the labels neat and
easy to read. Use a ruler to connect the label to the cell part.
Lab Observations & Data
Red Onion Cell ( _____ x)
Red Onion Cells
(initially)
Red Onion
(in 20% NaCl soln)
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Red Onion
(in pure H2O)
Name: ________________________________ Class: ________ Date: ______________
1. What is an independent variable?
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2. What is an dependent variable?
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3. What is the independent variable in this experiment? __________________________
4. What is the dependent variable in this experiment? ___________________________
5. Define the following terms:
a. Hypotonic solution: ______________________________________________
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b. Hypertonic solution: _____________________________________________
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c. Isotonic solution: ________________________________________________
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d. Plasmolysis: ____________________________________________________
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6. Which solution added to the red onion cells would correctly be described as the
hypertonic solution? ________________________________ The hypotonic solution?
_________________________________
7. Using the concept of osmosis and diffusion:
a. Explain what caused the vacuole (the red part of the onion) to change shape
when the 20% salt solution was added.
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b. Use a picture to illustrate what happened to the red onion cell in the 20% salt
solution.
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Name: ________________________________ Class: ________ Date: ______________
c. Explain what caused the vacuole (the red part of the onion) to change shape
when the The pure H2O was added.
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d. Use a picture to illustrate what happened to the red onion cell in the distilled
water.
8. With this knowledge of the effect that salt has on the cells of plants, explain how
plant growth on the sides of the highway system be effected by the salting of the roads
during snowy, winter months.
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9. What is likely to happen to forested, coastal areas if sea levels rise due to the effects
of global warming?
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