Suppose you are stranded on a small island in the middle of the ocean. As the sun beats down you would become very thirsty. There is no fresh water source on the island. Should you drink the salt water in order to survive? Believe it or not, if you do drink the salt water, you could die of thirst.

Sea water has a higher solute (salt) concentration than the water inside the cells in your body. To equalize the solute concentration on both sides of the cell membrane, water diffuses through the cell membrane, from the inside to the outside of the cell. This process is called osmosis. When too much water has diffused outside the cell membrane, the cells collapse.

## Solution Equilibrium

In this activity, you will explore how water particles move through the cell membrane to achieve equilibrium of solute concentration.

1. In the Gizmo™, observe the cell in a solution. (The cell is the square in the middle of the SIMULATION pane. The smaller, turquoise particles are water molecules. The larger, dark blue particles are the solute, dissolved in water.) Be sure that Solute outside is set to 5, and the Initial cell volume is set to 30%.
1. How many solute particles are inside the cell? How many solvent (water) particles are inside and outside the cell? (Look on the DESCRIPTION pane for these values.) What are the concentrations of solute inside and outside the cell? (In general, "concentration" means "concentration of a solute dissolved in a solvent.")
2. Click Play (). Run the simulation for 60 seconds. Then click Pause. How did the size of the cell change?
3. How did the solute, solvent, and concentration values change? What values stayed the same?
4. What value is equal in equilibrium that was NOT equal before you clicked Play?
5. According to what you have seen, which particles can move through the cell membrane? Which particles cannot? Explain.
2. Click Reset (). Be sure that Solute outside is still set to 5 and Initial cell volume is still set to 30%. Click the BAR CHART tab. Then click Play. Watch the bar chart as the experiment runs. After 60 seconds, click Pause ().
1. How did the solute concentration inside the cell compare to outside the cell as the experiment ran?
2. How did the cell volume change? What is the final cell volume?
3. Click Reset. Click on the GRAPH tab. Be sure that Concentration vs. Time is displayed below the graph. With the same Gizmo settings still, click Play. When it appears the two lines on the graph have merged to form one line, click Pause.
1. What does the graph tell you about the concentrations inside and outside the cell?
2. How long did it take for equilibrium to occur at these settings?
4. Click Reset. Click Play. While the simulation plays, switch back and forth between the Concentration vs. Time graph and the Volume vs. Time graph. As the graphs of the concentrations converge, what happens to the graph of the volume of the cell?

## The Effects of Initial Cell Volume

In this activity, you will explore how the cell's initial volume affects the solute concentrations inside and outside the cell, and whether the cell grows or shrinks to reach equilibrium.

1. Click Reset. Set the Solute outside to 5, set the Initial cell volume to 20%. Click on the BAR CHART tab. Check the Show numerical values box. Click Play.
1. How long did it take for the solution to reach equilibrium?
2. How do you know it has reached equilibrium by observing the bars of the bar chart?
3. What is the cell volume on the bar chart?
4. Did the cell get bigger or smaller?
2. Click Reset. Set the Initial cell volume to 60%. Be sure the Solute outside is still set to 5. With the BAR CHART pane visible, click Play.
1. Why does the cell get smaller? Describe the movement of particles that cause the cell to get smaller.
2. How does the final cell volume compare to the final cell volume in the experiment in the previous step? In step 2b in the first activity?
3. Write a conclusion: How does the initial cell volume affect the final cell volume in equilibrium?
4. With Solute outside set to 5, set Initial cell volume to 40%. Form a hypothesis for what the final volume of the cell, in equilibrium, will be. Then use the Gizmo to test your hypothesis. Describe your findings.

## The Effects of Solute Concentration

In this activity, you will explore how the initial solute concentration affects whether the cell will change size. You will then determine the final volume of the cell after reaching equilibrium.

1. Click Reset. Click the BAR CHART tab and select Show numerical values. Set Initial cell volume to 50% and set Solute outside to 1.
1. What are the initial solute concentrations inside and outside the cell?
2. Click Play and wait for the solution to reach equilibrium. Does the cell grow or shrink?
3. Once the solution is in equilibrium, what are the solute concentrations, inside and outside the cell? What is the cell volume?
2. Click Reset. Set Solute outside to 10. Be sure the Initial cell volume is still set to 50%.
1. What are the initial solute concentrations inside and outside the cell? What is the initial cell volume?
2. Click Play and wait for the solution to reach equilibrium. Does the cell grow or shrink?
3. Once the solution is in equilibrium, what are the solute concentrations inside and outside the cell? What is the cell volume?
3. Click Reset. With Initial cell volume still set to 50%, set Solute outside to 3.
1. Estimate what the final volume of the cell will be, in equilibrium.
2. Click Play to test your hypothesis. What did you find?
3. In general, how does the amount of solute outside the cell affect the size of the cell in equilibrium? Explain.
4. Click Reset. Adjust the Initial cell volume and the Solute outside to create a stable environment in which you think the cell will not change size. Click Play.
1. Did the cell stay the same size as you thought it would? If not, why not?
2. What settings resulted in stability? This situation is called a dynamic equilibrium because even though the water particles are free to cross the cell membrane, the total numbers of molecules that cross in each direction in a given time is expected to be equal, so the size of the cell does not change.
3. When Solute outside is set to 8, what Initial cell volume results in equilibrium?
4. When Solute outside is set to 4, what Initial cell volume results in equilibrium?
5. How can you tell before you click Play whether the cell and its surroundings are in equilibrium or not? (Hint: Think about what values become equal as equilibrium is reached.)