You have been studying examples of interactions between internal and external environments and the boundaries that separate these environments. These boundaries exist in every organism, from the smallest cell to large plants and animals. In the last activity, cells served as good examples of compartments with membrane boundaries . Although you experimented with different external environments for the cells, you were unable to manipulate the contents of the internal environments directly.
In this activity, you will use a synthetic membrane to construct yet another cell model, but this time are able to manipulate the internal environment. The synthetic membrane is dialysis tubing (used for kidney dialysis) and you will create an internal environment using several different solutions to build your cell model. In addition, you will have indicators to help you detect what substances can and cannot pass through the membrane. Indicators are chemicals that show the presence of certain substances through a chemical reaction that takes place when mixed together. Using this model, you will continue your study of how cell boundaries affect the internal cellular environment. Your job is to develop an explanation for how boundaries and compartments help living systems maintain and regulate the conditions necessary for life. (from pg.161, BSCS Biology: A Human Approach )
1. Read the green box entitled,
“Background Information” on p.162 and fill in the following box. These, and cups, are your lab materials.
Description Diagram of what this might look like
(see p.161, bottom right)
2. Visit a lab station and test the indicators. a. Fill a glass tube 1/3 full with starch solution . Test for the presence of glucose by placing a glucose test strip into the solution for 10 seconds and then waiting for 1-2 minutes for a color change (will turn brownish if glucose is present). b. Test the starch solution again but this time for the presence of starch (what do you expect to see? ) by dropping
12 drops of Lugol’s Iodine solution into the container. Bluish black liquid indicates the presence of starch. c. Repeat steps a and b with glucose solution . Be sure to use a new glass container to avoid contamination. d. Rinse both containers thoroughly and dry them upside down over a paper towel.
3. Read about the size of a molecule (bottom of green box). Show the difference between a simple sugar and starch.
Refer to p.335 of your textbook for the structure of both molecules, and draw them below.
Starch Simple Sugar / Glucose
What is the relationship between simple sugars (glucose) and starch?
4. Consider the lab materials you can use (see box for #1 above).Generate two testable questions around the ability of various substances to pass through the synthetic membrane that will serve as a model for the cell membrane.
Plan to incorporate the use of every single material in that list in your experiment. You will eventually set your models up based on one of the questions and see what happens overnight.
Testable Question #1
Testable Question #2
5. With your group, share all of your ideas. Decide on which testable question you would like to use for your experiment.
The goal is to determine what molecules may/may not pass through the cell using all of the materials described above.
Question we will investigate:
6. Design an experiment to test the question you as a group chose. Your design must use all of the following:
- Dialysis Tubing to create at least two cells
- Starch solution (a mixture dissolved in water)
- Glucose solution (a mixture dissolved in water)
- Appropriate indicators that test for the presence of the above molecules
Form your prediction here:
Briefly describe (or diagram) below what you plan to do to conduct your experiment. Include as much detail as possible.
Predict what will happen before you conduct your experiment.
(Are you using indicators to know if substances passed?)
Explanation for Prediction
Data and Observations
Create a data table in the space below to record the information you will need to answer your question. Create a well organized diagram of your observations and results in addition to it.
Which substance(s) could pass through the membrane?
What is your evidence?
List all of the substances used in this lab in order from smallest to largest.
What is your evidence?
Why were indicators necessary in the design of your experiment?
Explain how a cell membrane behaves as a barrier and how the dialysis tubing served as a model for the cell.
How is the model or this lab connected to your cell diagram/drawing?