Labs & Activities › Membrane permeability lab

Membrane Lab: What Passes Through?

Use dialysis tubing as a model cell membrane. Test which substances pass through it, then measure how the concentration inside changes the movement of water.

• 45–60 minutes (Part B needs 30 min soak or overnight)
• Groups of 3–4
• Low cost
• Biology BIO.1D.1, BIO.1D.2
• Foundations FB.3.3

Question

Is the membrane selectively permeable, and how does solute concentration affect the movement of water across it?

Background

A cell membrane lets some substances pass and blocks others. This is called selective permeability. Small molecules such as water can cross easily. Large molecules such as starch cannot. Diffusion moves a substance from where it is concentrated to where it is less concentrated. Osmosis is the diffusion of water across a membrane. Water moves toward the side with more dissolved solute (a hypertonic solution) until the sides balance. Dialysis tubing is a manufactured membrane with tiny pores, so it is a useful model.

• selectively permeable
• diffusion
• osmosis
• concentration gradient
• hypertonic
• hypotonic
• isotonic

Safety

Materials

• Dialysis tubing, 10–12 cm pieces, soaked in water
• Starch solution (1%), iodine solution (Lugol's), distilled water
• Sucrose solutions: 0%, 10%, 20%, 40%
• Beakers or clear cups, string or clips, balance, paper towels, graduated cylinder

Part A: Which substances pass through?

Hypothesis: Write an If … then … because … statement predicting what happens to the color inside the bag and in the beaker.

1. Tie off one end of a piece of tubing. Open the other end by rubbing it between your fingers.
2. Pour about 10 mL of starch solution into the bag. Tie off the top and rinse the outside under water.
3. Fill a beaker with water and add enough iodine to make it a pale amber color.
4. Put the bag in the beaker. Record the colors of the bag contents and the beaker liquid right now.
5. After 15–20 minutes, record the colors again.

Iodine turns blue-black when it meets starch.

Part B: Does concentration change the movement of water?

1. Prepare four bags. Fill each with about 10 mL of a different sucrose solution (0%, 10%, 20%, 40%). Squeeze out the extra air and tie them off.
2. Rinse and blot each bag gently. Measure the mass of each and record it as the starting mass.
3. Place each bag in its own beaker of distilled water for 30 minutes (or overnight in a refrigerator).
4. Remove each bag, blot it, and measure the mass again.
5. Subtract to find the mass gained. If a bag lost mass, record 0. Enter your group’s results below. If your class has several groups, each group is one trial.

Analysis

1. Part A: What happened to the color inside the bag? What does that show about which molecules (iodine or starch) can cross the membrane?
2. Why does this show that the membrane is selectively permeable?
3. Part B: Describe the trend in mass gained as the sucrose concentration increased. Use your data as evidence.
4. Use the words osmosis and hypertonic to explain which direction the water moved in the 40% bag.
5. What was the independent variable in Part B? What was the dependent variable? Name two things you kept constant.
6. A red blood cell is placed in pure water. Predict what happens and explain, using what you learned about the bags.