Concentrating on Equilibrium

Introduction

Molecules are in constant motion, and continue to move in and out of cells even when cellular equilibrium is reached. Do students really grasp this concept of dynamic equilibrium? In this class participation demonstration, students manipulate “molecules” in both directions across a model cell membrane to visualize how dynamic equilibrium is achieved through molecular motion and diffusion.

Concepts

• Concentration gradient
• Equilibrium
• Diffusion
• Molecular motion

Materials

Safety Precautions

Although the materials used in this demonstration are considered nonhazardous, please observe all normal classroom safety guidelines.

Prelab Preparation

1. Prepare model cells by counting and placing 30 bingo chips in each Petri dish. Prepare one model cell for each student group plus one for the instructor to demonstrate.
2. Make a copy of the Concentrating on Equilibrium Worksheet for each student.

Procedure

1. 1Distribute one Petri dish with 30 bingo chips to each student group. Keep one Petri dish with chips as an instructor model.
2. Distribute the Concentrating on Equilibrium Worksheet to each student.
3. Explain that each dish represents a cell with a semipermeable membrane and that the chips are molecules.
4. The chips will be moved into and out of the cell in a series of steps modeling the process of molecular motion and diffusion.
5. Place the demonstration Petri dish on the overhead projector. Scatter the 30 chips around the outside of the dish.
6. Guide the students through the transfers according to steps of the Procedure on the student PDF.
7. Fill in the data table transparency sheet for each transfer round.
8. Once an equal number of chips is on each side of the “membrane,” complete at least one more transfer to show that the concentration of molecules in the cell has now reached dynamic equilibrium.

Student Worksheet PDF

10858_Teacher1.pdf

10858_Student1.pdf

Teacher Tips

• This kit contains enough reusable materials for the instructor and 30 students working in pairs to perform the class activity: 500 bingo chips, 16 Petri dishes and a data table transparency sheet. All materials are reusable.
• This simulation 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.
• Once students have been guided through two transfers, they may be able to complete the rest of the procedure on their own. The most important point students have to remember is to remove ⅓ of the chips from each side of the “membrane” first, and then make the transfer to determine the new total number of molecules inside and outside the cell.
• After the fifth transfer, students should grasp the idea that even though the concentration of molecules remains the same, molecules continue to move back and forth across the membrane. Another transfer may be done to reinforce this concept.
• This activity will work with any number of initial chips inside or outside the Petri dish. For example, begin with 5 chips outside the dish and 25 chips inside the dish. Following the same general procedure, equilibrium will be reached in four transfers.
• The student laboratory kit, Introduction to Osmosis and Diffusion (Flinn Catalog No. FB1870), can be used to further explore diffusion and equilibrium.

Sample Data

Answers to Questions

1. Based on the results of this activity, describe the changes observed in the number of molecules inside and outside the cell over time.

   The number of molecules outside the cell decreased and the number of molecules inside the cell increased until the amounts remained equal.

2. Write a definition of cellular equilibrium based on the answer to Question 1.

   Equilibrium is achieved when the molecules both inside and outside the cell are evenly distributed.

3. Why do molecules move across the cell membrane in both directions?

   Molecules move in a spontaneous and random manner, and when collisions occur, some molecules will move across the membrane in either direction.

4. Since the chips representing molecules move in both directions across the Petri dish “membrane,” explain why a net movement of molecules occurs in one direction for the first four transfers.

   A net movement of molecules occurs because a higher concentration of molecules exists on one side of the membrane. More collisions occur than in the region of lower concentration. This results in a greater number of molecules moving from the region of higher concentration to the region of lower concentration than vise versa.

5. The state of cellular equilibrium is best described as a dynamic condition, not static. What does this mean?

   Once equilibrium is reached, molecules do not stop moving. Collisions from random molecular motion cause continuous movement of molecules across the cell membrane in both directions, with no overall change in the concentration.

6. Which figure most accurately represents the movement of molecules in the state of equilibrium?
   {10858_Answers_Figure_1}

Discussion

Students often have misconceptions relating to the concept of diffusion and equilibrium. One common misconception is the idea that molecules cease to move once cellular equilibrium is reached. Having students physically transfer “molecules” from an area of higher concentration to lower concentration as well as in the reverse direction reinforces the concept that individual molecules are constantly moving in a spontaneous and random manner. When a higher concentration of a particular molecule exists, the net movement of those molecules is directional. Thus, diffusion (the net movement of molecules) is said to occur down a concentration gradient—from high to low due to the greater number of collisions in the region of higher concentration. When the molecules become evenly distributed on either side of the cell membrane, they are at cellular equilibrium. The continued transfer of molecules in this activity supports the concept of dynamic equilibrium, where the molecules continue to move, collide and redistribute themselves, but there is no net movement of molecules in either direction and thus no change in the concentration.