Lab: Osmosis in Red Onion Cells
TOTTEN
SCIENCE
Name _______________________________ Date __________ Class/ HR ___________ Lab Minutes _________
INTRODUCTION
The passage of water molecules through a selectively permeable membrane is a special case of diffusion known as osmosis. Osmosis results in the
movement of water from an area of lower concentration of dissolved material to an area of higher concentration of dissolved material. For a
demonstration of this principle, a purple onion cell is going to be bathed in different concentrations of salt (NaCl) water.
BACKGROUND
In comparing two solutions of unequal solute concentration, the solution with a greater concentration of solutes is said to be hypertonic. The solution
with the lesser solute concentration is hypotonic. These are relative terms that are only meaningful in a comparative sense. For example, tap water is
hypertonic to distilled water, but hypotonic to seawater. In other words, tap water has a higher concentration of solutes than distilled water, but a lower
concentration than seawater. Solutions of equal solute concentration are said to be isotonic.
The diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis. In a sense, the water is
diffusing down its concentration gradient. The solution with the greater concentration of solute has the lesser concentration of solvent. The direction of
osmosis is determined only by a difference in total solute concentration, not by the nature of the solutes. Water will move from a hypotonic to a
hypertonic solution even if the hypotonic solution has more kinds of solutes. Seawater, which has a great variety of solutes, will lose water to a solution
of sugar that is very concentrated, because the total solute concentration of the seawater is less. It should be noted that there is no net movement of
water between isotonic solution separated by a membrane.
In this demonstration, the purple pigment visibly responds to the changes in the salt concentration. Sodium molecules are too large to pass through the
cell membrane so the water moves in and out of the cell to maintain the proper cell pressure.
Problem: How do solutions of various salt concentrations influence osmosis
in relation to an onion cell?
Hypothesis:
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Materials: (per student student group): red onion epidermis, forceps, dropper,
distilled water, 5% or high percentage sodium chloride (table salt) solution,
paper towels, microscope, slide, and cover slip
PROCEDURE:
1. 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 high power.
Make a labeled drawing, properly labeled in your data. This will give you a record of the original
appearance of the onion cells.
3. Take a dropper and add several drops of salt solution to one side of your cover slip while placing
a small piece of paper towel along the opposite edge of the cover slip. The paper should draw out
the water and draw in the salt solution.
4. Observe the effects of the saline (salt) solution on the onion cells. Make a properly labeled, careful drawing
of the cells' appearance in a second properly labeled carefully constructed drawing in your data.
5. Replace the sodium chloride solution with distilled water in the same way that the salt solution was added.
Make a properly labeled drawing of the cells' appearance in your data.
6. Answer the questions which follow your data.
DON'T FORGET TO INCLUDE YOUR DATA IN YOUR FORMAL REPORT
7. If you have time, prepare anew wet mount onion slide using the salt solution. Record your observations under high and low power.
8. Take a dropper of distilled water and add it to this slide, using the same procedure in # 3 above.. Record any changes you observe.
Observations:
Solution/ Object __________________
Solution _______________________
Magnification ______________
Magnification ____________________
Description ________________________________
Description _______________________
CONCLUSION QUESTIONS:
1. Draw a diagram to indicate the relative proportions of salt and water within the onion cells and outside
the onion cells when they were placed in the saline (salt) solution. Also use an arrow to properly indicate
the direction of osmosis.
2. Draw a diagram to indicate the relative proportions of salt and water within the onion cells and outside
the onion cells when they were placed in the distilled water solution. Also use an arrow to properly indicate
the direction of osmosis.
3. Red blood cells (and other animal cells) placed in a distilled water solution usually swell up and burst.
Why prevented the red onion cells from swelling up and bursting when they were placed in the distilled
water?
APPLICATION QUESTIONS (Place these answers in your conclusion please.).
4. Why do grocery store owners spray fresh fruits and vegetables with water?
5. Roads are sometimes salted to melt ice. What does this do to plants around the roadside and why?
6. If a shipwrecked crew drinks sea water, they will probably die. Why?
7. If a bowl of fresh strawberries is sprinkled with sugar, a few minutes later the berries will be
covered with juice. Why?