Students are conducting an investigation. They place a white starch solution into dialysis tubing and tie knots in each end of the tubing. Students place the dialysis tubing containing starch solution into a beaker of water. They use a dropper to place several drops of Lugol’s iodine solution in the beaker. As the iodine solution dissolves in the water, the red-orange color of the iodine is observed throughout water in the beaker. After 10 minutes, the white starch solution turns black, and the iodine solution remains red-orange.

Question

How do the results of the investigation relate to the transport of molecules through cell membranes?

dialysis exp.PNG

Students are conducting an investigation. They place a white starch solution into dialysis tubing and tie knots in each end of the tubing. Students place the dialysis tubing containing starch solution into a beaker of water. They use a dropper to place several drops of Lugol’s iodine solution in the beaker. As the iodine solution dissolves in the water, the red-orange color of the iodine is observed throughout water in the beaker. After 10 minutes, the white starch solution turns black, and the iodine solution remains red-orange.

How do the results of the investigation relate to the transport of molecules through cell membranes?

A cell membrane will actively transport needed molecules into the cell. The dialysis tubing actively transported the iodine inside the tubing to react with the starch.

Diffusion of small molecules occurs through semipermeable cell membranes. The iodine solution passed through the dialysis tubing, but the starch solution remained inside the tubing.

Membrane proteins allow large molecules to pass through the cell membranes. The holes in the dialysis tubing allowed the starch solution to move outside of the tube to react with iodine.

Protein pumps within the cell membrane actively increase the concentration of chemicals in the cell. The dialysis tubing increased the concentration of iodine inside of the tube.

Answer 1

The scenario described in the experiment illustrates important concepts related to the transport of molecules across cell membranes, particularly focusing on diffusion and selective permeability.

1. Diffusion of Small Molecules: The key takeaway from the experiment is that the iodine solution (Lugol's iodine) is able to diffuse through the dialysis tubing, which simulates the behavior of a cell membrane. The dialysis tubing acts as a semipermeable membrane, allowing smaller molecules like iodine to pass through while retaining larger molecules like starch. This is analogous to how small solutes can diffuse across biological membranes, while larger molecules often cannot.

2. Selectivity of Membranes: The experiment highlights the selective permeability characteristic of cell membranes. The iodine solution, which is small enough to pass through the pores of the dialysis tubing, diffused into the starch solution, reacting with it and turning the solution black. Conversely, the starch molecules, being larger, could not pass through the membrane, remaining inside the tubing. This illustrates how cells selectively allow certain substances to enter or exit based on size and characteristics.

3. No Active Transport in This Case: It’s important to note that the experiment does not support the notion of active transport since both iodine and starch were involved in passive diffusion processes rather than any form of active transport mechanisms (such as protein pumps). Active transport requires energy to move molecules against their concentration gradient, which is not demonstrated in this experiment.

In summary, the most accurate interpretation of the results of this investigation is that diffusion of small molecules occurs through semipermeable cell membranes. The iodine solution passed through the dialysis tubing, but the starch solution remained inside the tubing. This reflects the fundamental principles of how substances move across cell membranes—larger molecules are restricted while smaller ones can diffuse freely, demonstrating the selective nature of cellular membranes.