Diffusion Model

Introduction

Use this model to graphically simulate the net movement of diffusing molecules through a semipermeable membrane.

Concepts

• Diffusion

• Semipermeable membrane
• Concentration gradient

Background

Molecules are in constant, spontaneous and random motion. This spontaneous and random motion in a closed system results in the eventual even distribution of the molecules throughout the system. Often this results in a net movement of some molecules from an initial area of high concentration to an area of lower concentration. If there is a difference in concentration across a distance, the measure of this difference is called a concentration gradient. Because the net movement of the molecules is from a region of higher concentration to a region of lower concentration, they are described as moving down their concentration gradient.

How does this occur? The molecules in a gas or a liquid are in constant motion. Moving molecules continually collide, and the higher the concentration of molecules, the greater the number of collisions. These collisions cause the molecules to change direction and to spread out until they eventually become uniformly distributed. When the molecules are “evenly” distributed it is important to remember that they continue to move, collide and redistribute themselves. Once equilibrium is reached, however, there is no net movement of molecules down a concentration gradient. Molecular movement does not occur only when a concentration gradient exists nor does it cease when equilibrium is reached.

Materials

Safety Precautions

Handle the diffusion demonstration model with care. This model may break if dropped. When shaking the model, be careful to avoid striking any hard objects.

Disposal

No disposal necessary. Store for future use.

Procedure

1. Hold the model on one side so that all of the small beads can pass through the “membrane” to the side opposite the large beads. Shake the model in a slow, sifting-type motion until the two bead types are completely separated on opposite sides of the membrane.
2. Stand the model on its base and compare the “volume” of the two simulated molecules (beads) on each side of the membrane. The volumes should be nearly equal on each side of the membrane. (Note: Since the volumes are equal it means that there actually are more small beads than large beads and thus the concentration per unit volume of small beads is actually greater than that of large beads. However, this need not be considered as part of the demonstration.)
3. After observing the equal volumes, shake the model in a rigorous and random fashion for several minutes. Be sure to include some sideways shaking of the model. Stand the model on its base again and compare the volumes of beads on both sides of the membrane. The total volume on the side with the large beads should increase and the volume on the side with the small beads should decrease. The random distribution of the small beads has allowed some small beads to leave one side of the model and move to the other side. The membrane has blocked the passage of the large beads in the other direction. Thus, there is a total increase in volume of molecules on one side of the membrane and a decrease in the volume on the other.
4. Discuss the random motion of molecules and the redistribution due to molecular collisions as illustrated by the small beads. The net effect of a semipermeable membrane preventing the random distribution of some molecules (large beads) across the membrane should be discussed. What happens with continued shaking? Is equilibrium reached?

Teacher Tips

• The model can be used to illustrate diffusion principles prior to diffusion lab experiments or it can be used to summarize and visualize experimental results gathered in the laboratory. The model can also be used as a quiz stimulus. Demonstrate the model “before” and “after” and ask students to write an essay explaining the model using the following terms in their essay—molecules, random motion, diffusion, equilibrium, semipermeable membrane and diffusion gradient.

• The model should be kept closed to avoid losing any beads. The model is set up so that the total volume of the large and small beads is approximately equal.
• The volume difference observed in step 3 of the procedure can be used to describe and illustrate osmotic pressure.