Chemistry and The Salt Water Impact on Onion Cells

All organisms are composed of cells (prokaryotic or eukaryotic). Plant cells and animal cells are two different types of eukaryotic cells. Both similarly consist of cell membranes and vacuoles, but only plant cells involve a cell wall for supporting structure. Vacuoles are larger in plant cells, and result in turgor pressure when water is moved from the outside of the cell [low solute concentration] into a vacuole [high solute concentration]. Cell membranes are composed of phospholipid bilayers. Aquaporins are found in the cell membrane; these passive transporters, or proteins, conduct the movement of water across the membrane. Osmosis, the diffusion of water and a type of passive transport, allows water to move in and out of a cell. Cells control turgor pressure through osmosis.

The term “hypertonic” is used to describe a solution that consists of a bigger amount of solutes than the amount of solutes inside the cell. This results in plasymolysis of the cell; plasymolysis is when the cell shrinks due to loss of water through the process of osmosis. The term “isotonic” is used to describe a solution that consists of the equivalent amount of solutes as inside the cell. Onion cells, a type of plant cell, were used to conduct this experiment. When the cells were magnified and looked at carefully, hypertonic and isotonic solutions made a very big change on the cells.

The hypertonic solution [salt water] was used as the independent variable, whereas the isotonic solution [fresh water] was used as a control variable. The area of the cells was used as a dependent variable to compare the effects of the hypertonic and isotonic solution on the cell’s size.

I hypothesized that if salt water is applied to the onion cell, then the onion cell will shrink because the solution will be more concentrated than the cell. The salt water and the cell are not concentrated equivalently. Water will move out of the cell to balance the amount of solutes in the cell and solution.

A slide and slide cover was used to securely place the onion skin and solutions under the microscope. The microscope was used to view the onion skin under 400x magnification. The onion skin was used to compare the areas of the cells after the solutions were applied. The control variable, fresh water, and the independent variable, salt water, were used to apply over the onion cell. The constant variables of the lab were the microscope, magnification, field diameter of cell, and onion skin.

First, the onion skin was placed on the slide. Then, one drop of either fresh or salt water was applied on top of the onion skin. The slide cover was placed over the onion skin, and helped secure it in place.

Observation: It was harder to focus and view the onion cell with isotonic solution under the microscope. The amount of light from the microscope affected the detail of the onion skin when it was viewed. When the cells were magnified 400x, the microscope was blurry and took time to adjust.

The hypothesis was supported by the data as seen in figure 2. After conducting the experiment, the data was compared. The data showed that the salt water and fresh water had a major effect on the area of the onion cell. Like I has predicted, the salt water caused the cell to shrivel, whereas the cell in the fresh water seemed to be larger.

After comparing the data, the data in figure 2 revealed that area of the cell was impacted by both solutions. The salt water and fresh water had a difference of 30,000 µm. The area of the onion cell after the salt water was applied was 20,000 µm2. The area after the fresh water was applied was 50,000 µm2.

The area was minor with the hypertonic solution. This happened because the salt water was much more concentrated than the cell. So, the cell balanced the concentration and allowed water from the cell to exit. The salt water had less water, and more salt. The cell had more water, and less salt. The turgor pressure increased as the percentage of water inside the cell was higher than the percentage of water in the solution. The water from the cell went to the outside of the cell to maintain a balance between the cell’s concentration and the solution’s concentration.

On the other hand, the fresh water had as much water as the cell did. The same amount of water that left the cell entered the cell. This occurred because concentration of the isotonic water and the cell was already balanced. There was no need for the cell to lose, or gain water. Therefore, water moved in and out of the cell at the same rate, not impacting the cell, and causing no turgor pressure.

The lab could have been improved in various ways. First, accurate data would’ve improved the lab results. Comparing the volume of the cells after the solutions were applied would’ve helped in getting better data, rather than using the area. Second, a hypotonic solution should have been tested as well. Lastly, the lab should have been repeated at least 3 times using the same procedure to guarantee better data. The average of 3 trials would’ve ensured better results.