Guided Inquiry Skills Lab

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Guided Inquiry • Skills Lab

Chapter 7 Lab Detecting Diffusion

Problem How can you determine whether solutes are diffusing across a membrane?

Introduction A cell membrane is a selectively permeable barrier. In other words, some particles can pass through the cell membrane while other particles are held back. Solutes that can move across the membrane generally do so by diffusion. When solutes diffuse, they move from an area of high concentration to an area of lower concentration. In this lab, you will use a small plastic bag to represent the cell membrane. The bag has small openings, or pores, that allow the passage of relatively small molecules. You will use an indicator to determine which molecules diffuse through the pores.

Skills Focus Use Models, Analyze Data, Infer

Materials • 400-mL beaker • twist tie • 25-mL graduated cylinder • iodine solution • 1% starch solution • forceps • plastic sandwich bag

Build Vocabulary Term Definition permeable Letting liquids or gases pass through, or permeate, a barrier such as a membrane selective Able to choose, or select, some options and reject others solute A substance that dissolves in another substance, such as water, to form a solution indicator Something that is used to show, or indicate, the presence of a solute in a solution 45 Name Class Date

Safety Iodine solution can irritate the eyes and skin and can stain clothing. Wear safety goggles, gloves, and a laboratory apron while handling any solution that contains iodine. Rinse off any solution that spills on your skin or clothing. Wash your hands thoroughly with soap and warm water before leaving the lab. Pre-Lab Questions 1. Draw Conclusions How will you know whether starch has diffused across the membrane?

2. Draw Conclusions How will you know whether iodine has diffused across the membrane?

3. Use Analogies How is a window screen similar to a cell membrane?

Procedure You will use iodine as an indicator to determine whether the plastic bag is permeable to starch. Iodine will turn blue-black when it comes into contact with starch. 1. Put on your goggles, apron, and gloves. 2. Add about 200 mL of water to a 400-mL beaker. 3. Pour 25 mL of starch solution into the plastic sandwich bag. 4. Use a twist tie to tightly seal the bag. 5. Use tap water to thoroughly rinse the outside of the bag in case any starch solution spilled onto the outside of the bag. Be sure to rinse the twist tie as well.

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6. Place the plastic bag in the beaker so that the bag is completely covered with water. 7. Add 8 drops of iodine solution to the water in the beaker. Record your initial observations in the data table. Wait 15-20 minutes, and then record your final observations. 8. Use a forceps to remove and dispose of the plastic bag as instructed by your teacher.

Data Table Inside Bag Outside Bag

Color Is starch Is iodine Color Is starch Is iodine present? present? present? present?

Initial Final

Analyze and Conclude 1. Infer After you placed the plastic bag in the beaker, what happened to the iodine? What happened to the starch?

2. Apply Concepts Use what you know about the structure of starch molecules to explain your results.

3. Infer Did water move into the bag or out of the bag? Why?

4. Perform Error Analysis When Alyssa did the experiment, she observed the solution outside the plastic bag turn black. What might have happened?

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5. Use Models Describe two functions of a cell membrane that cannot be modeled with a plastic bag.

6. Predict Red blood cells are placed in water that has been distilled so that there are no dissolved substances in the water. Are the red blood cells likely to swell up or shrink? Why?

Build Science Skills Is a sandwich bag permeable to glucose? Describe how you could modify the procedure in this experiment to answer this question. What could you use to test for the presence of glucose?