Monday January 26, 2015

LT: I can explain cell diffusion.

Name: ________________________ Date:__________________ Class Period:__________________

Semi-Permeable Membrane Lab

WARNING: Iodine will stain clothes and skin. Make sure you follow all lab procedures and do not get any on you! Safety goggles must be worn for the whole lab*****

Diffusion is the movement of molecules from a concentrated (crowded or more densely spaced) area to a less concentrated (less densely spaced) area.

Draw a picture of to represent diffusion:

High Concentration                                              Low Concentration

Osmosis is a special type of diffusion. Osmosis is the movement of water molecules through a membrane (from a crowded area to a less crowded area).

         Draw a picture to represent osmosis:

Background Information:

A plastic bag is called semi-permeable which means it has many microscopic openings that allow small amounts of air or water to enter and leave. Larger things cannot pass through these openings.

Corn starch when it comes in contact with iodine turns purple.

Problem: How does the thickness of a plastic bag affect the movement or diffusion of molecules?

What is your independent variable?

What is your dependent variable?

What is your hypothesis? (If, then, because…)

Materials:

• (1) plastic bag                      • corn starch                           • (1) 250mL beaker

• iodine                                   • spoon/stir rod              • (1) safety goggles                  • eye dropper/pippet                                                    

Procedure:

1. Fill the 250 mL beaker with 100 mL of water.

2. Put 4 drops of iodine into the water.

3. Put one spoonful of corn starch into the plastic bag.

4. Pour 100 mL of clean warm water into the plastic bag.

         *Take most of the air out of the bag and close it so it is sealed well.

5. Rinse off the outside of the bag to make sure there is no cornstarch on it.

6. Place the bag into the beaker with the iodine and let it sit.

Sketch Your Lab Set Up (use color)

What are your constants? (Things that stay the same?)           

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Make Some Predictions

1. If the baggie was permeable to starch, which way would the starch move, into the bag or out of the bag?

2. If the baggie was permeable to iodine, which way would the iodine move, into or out of the bag?

3. If the baggie was permeable to iodine, what color would you expect the solution in the baggie to turn?    

         What about the solution in the beaker?

4. If the baggie was permeable to starch, what color would you expect the solution in the baggie to turn?   

         What about the solution in the beaker?

5. Make a prediction about what you think will happen with regard to molecules moving and color change

Questions:

1.     Molecules tend to move from areas of _______ concentration to areas of ______ concentration.

2.     What cell structure does the bag represent?

3.     What cell structure does the starch solution represent?

What's in the Bag?

1. Is the baggie or beaker more concentrated in starch?

2.     Is the baggie or beaker more concentrated in iodine?

Post Lab Analysis

1.     Based on your observations, which substance moved, the iodine or the starch?

2.     Draw a small diagram showing which substance diffused through the plastic baggie.

3. How did you determine this? _____________________________________________________________________________ ______________________________________________________________________________

4. The plastic baggie was permeable to which substance (which substance went through it)? ________________________________________________________________________________

5. Does the plastic bag only let certain substances through? (Selectively Permeable) How do you know? _________________________________________________________________________________

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6.What would happen if you did an experiment in which:

the iodine solution was placed in the baggie, and the starch solution was in the beaker? Be detailed in your description.

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7. Based on this lab, why is it not a good idea to store iodine in a plastic bag?

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8. How does this lab represent cell diffusion?

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