Onion Cells

Problem

What will happen to an onion cell in a high salt environment compared to an onion cell in regular tap water?

Hypothesis

I think the onion cell will shrink when exposed to a higher salt environment compared to a controlled environment. This is because cells are 95% water and a little bit salt, while salt water is only a little bit water and mostly salt. Therefore, I think water will move out of the cell to gain a higher concentration of water on the outside in a process called osmosis.

Experimental Design

Materials

• Onion
• Microscope
• Razor
• Slide and cover slip
• Dye
• Eye dropper
• Tweezers
• Salt

Variables
Constant Variables

• Same onion and the same layer
• Amount of dye
• The room temperature
• The microscope

Independent Variable

• The amount of salt in the environment

Dependent Variable

• What happens to the onion cells

Procedure
To get to the center of the onion where the moist onion cells were, the first few layers of the onion was pealed. A razor and tweezers were used to peal a thin layer of skin from the onion. After that, the onion skin was placed on a slide and the eye dropper was used to apply one drop of water on the slide followed by a cover slip. The slide was examined to see the onion cells. Dye was then added to the slide. By looking through the microscope at the second lowest intensity, the nucleus was found and the cell bounders were found. Then the onion cell in a high salt environment slide was prepared by first adding one tablespoon of salt to a glass water to make a salt water mixture. A new piece of skin was pealed from the onion and mounted on a slide with the salt water mixture and dye. It was examine with the microscope at all the intensities. To record the image, a picture was drawn to match the what was seen through the microscope for both the cells.

Observation

The experiment went as smoothly as planed. I successfully created two slides -- one of the onion skin in tap water and one of the onion skin in salt water. I examined both of them through the microscope. I was new to using a microscope, and I had only used it a few times prior to this project. Before actually examining the slides I had learn how to use the microscope properly. It was pretty simple once I learned how to do it. In order to look at the slides properly, I had to place one slide on the stage and clip it in. I started off with the onion skin in tap water, and examined it through the first intensity. Once it was focused with the fine focus, I could only see a faint outline of the onion cells, so I moved onto the next intensity. Through that lens I could see the nucleolus and each individual cell. To demonstrate what I saw, I drew the diagram above. Following that, I looked at the onion skin in salt water through the second intensity as I did with the other one. Just by adding salt water the onion skin had changed very much. I could see all the little organelles clumping around each nucleolus and all the cell's vacuoles were now visible. To match what I saw, I drew another picture and labeled the differences between the two pictures. Just so there was a clear picture of what I actually saw through the microscope when looking at the onion cell in tap water, I include a photographed image of it.

Though I didn't have any trouble creating the slides and examining them, the precise method of creating a slide could be affected by so many external factors it was very difficult to get them just perfect. First, I had to make sure that slide and cover sheet I was using had no dust of other residue on them because that could potentially blur what I saw through the microscope. Next, I had to be very precise when pealing the onion skin and adding the same amount of dye to each slide. At the same time, I was making sure I didn't get any finger prints on the slide. I pealed the onion skin from the same layer and used tweezers to do it each time so my fingers never came in contact with the slide or the onion. Overall, I eliminated all the external variables that I could. Though when looking through the microscope, I saw some air bubbles that blocked me from seeing some of the onion cells, it didn't affect the rest of the cells.

Conclusion

Though my hypothesis wasn't quite right, in theory I had the right concepts. You see, it really wasn't the onion cell that shrunk, but the vacuole in the onion cell. When I looked through the microscope at the original onion skin, before I added the salt water, I couldn't see the vacuoles at all because they were lined up with the cell walls. Once the salt water was added, I could actually see the vacuoles pulling all the organelles to the center as they were shrinking in. This process is called osmosis and it happens because the concentration of water in salt water is lower than the concentration of water in the onion cell. Therefore to make up for the low amount of concentration on the outside of the onion cell, the water molecules in the vacuole moved through the membrane to the outside of the onion. This causes the vacuole to shrink. Therefore the organelles were all grouped in the center because they got pulled in as the vacuole got closer to the center. For most polar molecules, such as sugar and protein, this process doesn't work. Even though water molecules are polar, they are tiny enough to slip through the membrane. Therefore, the onion cells didn't shrink, but the vacuoles in the onion cells did.

Bibliography

"Animation: How Osmosis Works." Your Page Title. N.p., n.d. Web. 16 Dec. 2011. <http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

"Osmosis." arbl.cvmbs.colostate.edu. N.p., n.d. Web. 15 Dec. 2011. <http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html>.

"Osmosis." Molecular Physiology & Biophysics / University of Vermont. N.p., n.d. Web. 18 Dec. 2011. <http://physioweb.uvm.edu/bodyfluids/osmosis.htm>.