Plant and Animal Cell Tonicity

Purpose

To investigate the effects of osmosis and tonicity in plant and animal cells.

Field

Biology

Difficulty

Procedure: Medium

Concept: Medium

Background

You’ll need to read up on osmosis and tonicity.

In the readings focus on what happens to cells when put in isotonic solutions (solute concentrations in solution is equal to that of the cell), hypotonic (solute concentrations in solution is less than that of the cell), hypertonic (solute concentrations in solutions is greater than that of the cell). Don’t worry too much about all of the details.

http://en.wikipedia.org/wiki/Plant_cell
Basic cellular components of plant cells and animal cells, especially the plasma membrane
http://www.biologie.uni-hamburg.de/b-online/e22/22c.htm
Diffusion, Osmosis and hydrostatic homeostasis mechanism of the cell
http://www.college-cram.com/study/biology/presentations/422
Follow the tutorial for “Osmosis in an Animal Cell” to solidify your concepts

This figure sums up the basic concept of the effect of osmosis (water exchange) on cells due to their tonicity. Corresponding images of red blood cells are shown to illustrate what happens in real life. Make sure you understand what is going on in the images.

The three possible tonicities of an animal cell
Source: http://biology.unm.edu/ccouncil/Biology_124/Summaries/Membranes.html

Since you will be using a microscope, it is best if you get used to using it. You will just need to do basic magnification and fine tuning. A simple light microscope will do for this project.

Hypothesis

What do you expect will happen to the plant and animal cells in a hypotonic, isotonic and hypertonic solution?

Materials

Procedure

  1. Add 5ml of the distilled water to each of the graduated cylinders. Mix enough NaCl to each cylinder to make the solution 0%, 0.5%, 2% saline solution. Suggestion: To determine the tonicity of onion, it is suggested that more saline concentrations be investigated with smaller increments from 0% to 1%.
  2. Peel the skin, and cut three thin, small, and equally sized slices of the flesh of the onion, one for each cylinder. Feel the texture, rigidity and other characteristics of the onion slices. Immerse the slice and make sure it stays immersed for a few hours.
  3. Feel the slices again and make a note of how they feel compared to before they were immersed in the solutions.
  4. Also, with the help of the science teacher or an adult, make a microscope slide with the onion slices from each of the solutions and a normal onion slice not placed in solution (control). If cellular features are not clear, ask your science teacher if he/she could stain the onion cells for a better view. Observe and make note of the differences in cells. Pay special attention to the vacuole sizes and the cell boundaries.
  5. For each of the slices, also make note of the number of intact cells and the non-intact cells. Compare to the control (normal onion cells) that were not subject to the solutions. The number can just be for that that view of the microscope. Moving the slide in the microscope changes the cells under view. Count again. Make as many repetitions of this as possible counting each as an attempt. It would be best if you could repeat this for different slices and call that as an attempt.

Results

Qualitative

Quantitative

Discussion

Explain all of the results, both qualitative and quantitative. Pay special attention to the cell membrane (what is the special characteristic of the cell boundary in plants?), central vacuole, and osmosis or water flow between the cells and their surroundings.

Are the differences in the percentage of intact cells statistically significant between the solutions? Explain why or why not. Also discuss the limitations of this method in quantifying the differences. Discuss the possible causes for any unexpected or unreasonable observations.

Conclusion

What can you conclude about the effects of varying solute concentrations around a plant cell?

Extension

If you’re interested, you can also determine how animal cells respond to differing solute concentrations.

First and foremost, read through this project and talk to your science teacher about it before beginning. Ask if your teacher they have the time and resources to help you with this project.

⚠WARNING

  • Always use gloves on your hands and goggles for your eyes when dealing with blood.
  • Do all of these experiments under adult supervision.
  1. Ask your teacher to get you 3ml of red blood cells (centrifuged blood).
  2. Make salt solutions with the concentrations similar to the plant cells.

    Note: Since you will be adding 1ml of blood to your total of 5ml solutions, you have to calculate how much of salt you need to dissolve in the 4ml solution that you will be adding to the blood to make the total concentration of salt equal to 0%, 0.9%, and 5%. Carefully mix the blood gently in the solution without spilling. Make note of any differences in the brightness of color in each of the solutions compared to the pure red blood solution (control).

  3. With your science teacher’s help, prepare animal blood cell smears on microscope slides. Use one drop of the solution to make a very thin smear.
  4. Prepare a smear with just the pure red blood cells solution to serve as control.
  5. Observe the differences between the red blood cells from each of the slides under a microscope. Also count the number of intact red blood cells in each of the slides. Use gloves and goggles even when using the microscope to study the slides.
  6. Repeat this at least three times using the rest of the solutions.

How does the percentage of intact red blood cells in each slide vary? Is this difference statistically significant? Explain. What are the some of the limitations of this method?

Using these results, explain the difference between the effects of tonicity in animals cells in general compared to plant cells.