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Investigating Isotonic, Hypotonic, and Hypertonic Solutions

by Alton Biggs



How can a cell keep from bursting or shriveling?

From your studies about cells, you know that plasma membranes have many different functions. Since animal cells do not have cell walls to prevent the cell from enlarging too far, the plasma membrane must regulate the flow of ions into and out of the cell. When you look at human red blood cells in isotonic solutions on a microscope slide, the cells appear to be biconcave discs. These cells swell and burst when placed in distilled water; they shrivel in salty water. Obviously hypotonic and hypertonic solutions have an effect on animal cells.

Just as animal cells have plasma membranes that regulate the flow of ions, plant cells also have plasma membranes. In addition, plant cells have cell walls that keep the cell sizes constant. How do plant cells look in hypotonic, hypertonic, and isotonic solutions. You might use Elodea or onion cells to help you find out. In this laboratory exercise you will be a scientist who will design and conduct an experiment to determine the answer to this question.

In addition to Elodea and onions, you will be provided with distilled water, salt, methylene blue, graduated cylinders, microscopes, and balances. Work in groups of three or four to carry out your procedure.


Problem:

Do plant cells appear the same in hypotonic, isotonic, and hypertonic solutions?


Safety Precautions:

Be sure to wash your hands after working with biological materials, solutions and chemicals.


Hypothesis:

What is your group's hypothesis? Explain your reasons for forming this hypothesis.


Experimental Plan:

  • As a group, make a list of possible ways you might test your hypothesis using the materials your teacher has made available.
  • Agree on one idea from your group's list that can be investigated in one lab period.
  • Design an experiment that will test one variable at a time. Plan to collect quantitative data.
  • Following the style of a recipe, write a numbered list of directions that anyone could follow.
  • Make a list of materials and the quantities you will need.

Checking the Plan: Discuss the following points with other group members to decide the final procedures for your experiment.
  • What variables will need to be controlled?
  • What is your control?
  • What will you measure? What size samples will you use?
  • How many trials will you carry out?
  • Are you certain you are testing only one variable?
  • Is the environment constant for each trial and for the control?
Make sure your teacher has approved your experimental plan before you proceed further.


Data and Observations: Carry out your experiment, make yourm measurements, and complete your data table. Make a graph of your results.


Analysis and Conclusions

  1. What conditions did you keep constant between experimental groups?
  2. What types of differences between experimental groups did you look for?
  3. Was your hypothesis supported by your data? Explain.
  4. Explain your graph in a brief paragraph.
  5. If your hypothesis was not supported, what new experiment might you design?
  6. Use your group's results and consult with other groups to answer the following questions: Of the reactions you observed, do plant cells appear change when ion concentrations change? What happens when you reduce the ion concentration by adding distilled water? What happens when you increase the ion concentration in the plant cells by adding salt?
  7. Do plant cells appear to be able to maintain their shape in a variety of salt solutions?
  8. Describe the shape of the cells you observed as conditions changed from isotonic, to hypotonic or hypertonic, and back again.

Higher Level Thinking
  1. Why are ion concentrations important to cells?
  2. Why might living things need to have a plasma membrane surrounding their cells?
  3. Write a brief conclusion to your experiment.
  4. Draw and label the cells as you observed them in isotonic, hypotonic, and hypertonic solutions. Each drawing should be large, in color, and properly drawn to scale. Include a scale that shows the size of the cells in each drawing. Be sure to include the magnification at which you observed the cells.
  5. Why might a salmon have homeosmotic problems when returning to a fresh water stream?
  6. In the poem, "The Rhyme of the Ancient Mariner," is a line "Water, water, everywhere and not a drop to drink." In terms of this investigation, what does this line mean?
  7. Based on this lab experience, design another experiment that would help you to answer another question that arose from your work. Assume that you will have unlimited resources and will not be conducting this lab in class.



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