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Investigating Cell Sizes
by Alton Biggs

Introduction:

How big is a cell?
From your reading about cells, you know that cell organelles have many different functions. Cells are specialized for particular functions that are carried on by their organelles. For instance, some nerve cells contain many long processes with a thickened plasma membrane covering. This covering maintains the internal biochemical potential that allows electrical impulses to travel from one place to another. Other cells have different structures that have evolved for a particular function.

Do all cells have the same size?
Is there an "average" cell size, and if so, what is it? Does the size of a cell depend on the kind of organism it is found in? You could use some common 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 such questions.

Your instructor has prepared several samples of cells that you can use. Work in groups of three or four classmates to carry out your procedure.

Problem:
In this laboratory, you will be a scientist who answers some question about cells. What are the average sizes of some cell types? Or, what is the average size of several cell types?

Safety Precautions:
Be sure to wash your hands after working with biological solutions and chemicals. Be careful when pipetting stains. When scraping the inside of your cheek, be careful not to gouge into your tissues.

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

Experimental Plan

  1. As a group, make a list of possible ways you might test your hypothesis using the materials your teacher has made available.

  2. Agree on one idea from your groupÍs 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.

  3. Following the style of a recipe, write a numbered list of directions that anyone could follow.

  4. 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 your 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. If your hypothesis was not supported, what new experiment might you design?
  4. Use your group's results and consult with other groups to answer the following questions: Of the cells you observed, do animal or plant cells appear to have a larger average size? What was the average size of all the cells?
  5. Do plants and animals appear to share similar cell organelles?
  6. Describe the organelles found in some of your cells, but not in others.

Higher Level Thinking

  1. Why are cells important?
  2. Why might living things have different average cell sizes?
  3. Do you think there are any cells the size of a large watermelon? Why?
  4. Write a brief conclusion to your experiment.
  5. 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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