Maintaining Homeostasis (Part 1/2)
Lesson 1 of 5
Objective: Students will explore several processes that help cells maintain homeostasis.
In today's lesson students will look at the role the cell membrane plays in maintaining balance in the cell. Here is an overview of what students will learn.
Show students two cell systems. In the first cell system, a dialysis tubing bag is filled with 10% starch water and placed in a beaker of iodine solution. In the second cell system, a dialysis tubing bag is filled with iodine solution and placed in a beaker of 10% starch solution. Inform students that these represent models of cells. Using the provided handout, ask students to make sketches of the current cell systems and make predictions of the appearance of those cell systems after five days. Students should explain their reasoning for their predictions.
(Note: Students come to biology with a variety of prior knowledge. Most students will know something about the cell. However, depending how rigorous their elementary and middle school science was, their depth of understanding may be limited. It is important to determine what they know before proceeding with the rest of the lesson. I use data from an experiment that students would have completed as middle school students. Students are to look at the data and then explain what it is telling us about the cell.)
Equipment Needed (per lab group)
- 9 50-mL beakers
- Distilled water
- Corn syrup
- Clear sports drink like Gatorade Glacial Cherry
- Graduated cylinder
- Tare Scale
- Weighing trays
- Gummy Worms
Using gummy worms as a model of a cell, students are to explore the effects of hypertonic, isotonic, and hypotonic solutions on cells. First, students should to weigh three gummy worms. They should record the masses in the table provided. Then they should measure 100 mL of distilled water, 100 mL of Gatorade, and 100 mL of corn syrup and placed each solution into its beaker. A gummy worm should be placed into each beaker. Beakers should be covered with parafilm and allowed to sit overnight. In their lab notebooks or using the provided handout, students should predict what they think will happen.
Next, students should calculate the concentration density of sugar solutions (aka Gatorade and corn syrup) using the labels from the corn syrup and Gatorade bottles. They should divide the amounts of sugars contained in a serving by the volume used. Students should record the concentration density in the table provided.
After finishing their laboratory set-up, students will complete the POGIL, Transport in Cells, in their lab groups. Using this POGILs, students will explore the several models to determine role of several methods of passive transport in maintaining cell homeostasis. Students will use this information in the analysis of the Gummy Worm Osmosis results tomorrow.
(Note: I use this POGIL as a method of allowing student to quickly extend or reinforce their knowledge about the type of passive transport in the cell. POGILs are designed for students to complete alone or in groups with little assistance from the teacher. This allows me to assist students that are still finishing the laboratory setup while keeping the entire class busy. Students that do not complete the POGIL in class will finish it as homework.)
While students are completing the POGIL, open the Membrane Channels simulation and project it for the entire class to view.
Bring students back together. Students should open a web browser and navigate to the Membrane Channels simulation found at pHet interactive simulations. Next students should select Download. Allow the students a minute or two to explore the tools in the simulation and see how they work. Move about the room and assist students with technical issues. After two minutes, ask students to select the Reset All icon.
Next, explain the parameters of the simulation to the students. For the purposes of this activity, students should
- consider the upper part of the simulation as the area of lower concentration and the lower part of the simulation as the area of higher concentration.
- assume that the blue diamonds represent water (the solvent).
- assume that the green circles represent potassium ions (the solute).
Students should add 1 green gated channel and 1 blue gated channel. On the upper part (area of lower concentration), they should add 20 blue diamonds. On the lower part (area of higher concentration), they should add 20 green diamonds. Students should select Show Concentrations. Students should then open both channels and allow the simulation to run for 30 seconds. They should observe the simulation and make five observations which they should record in their lab notebook or the provided handout. Students should also make a sketch of the graph before and after the simulation.
After a few minutes, ask the students to select Clear Particles. On the upper part (area of lower concentration), they should add 20 blue diamonds. On the lower part (area of higher concentration), add 20 green diamonds. Students should then open only the blue gated channel and allow the simulation to run for 30 seconds. Students should make three observations and consider if, based on their observations, the solution is hypertonic, hypotonic, or isotonic? Students should also make a sketch of the concentration graph after the simulation. Allow students time to record their observations and conclusions.
Ask the students to again select Clear Particles. Students should set up the simulation exactly as before. Students should then open only the green gated channel and allow the simulation to run for 30 seconds. Students should make three observations and consider if, based on their observations, the solution is hypertonic, hypotonic, or isotonic? Students should also make a sketch of the concentration graph after the simulation. Allow students time to record their observations and conclusions.
Finally, ask students to slow down the animation by selecting the Sim Speed and dragging it to slow. Students should describe the movement of the green circles and the blue diamonds.
Once students will complete the Membrane Channels simulation, they will individually work on another POGIL, Membrane Structure and Function. Using this POGIL, students will explore several models to that compare and contrast several methods of passive and active transport in maintaining cell homeostasis. Students will use this information in writing their final report explaining the results of their Gummy Osmosis lab.
(Note: I use this POGIL as a method of allowing student to quickly extend or reinforce their knowledge about the type of active transport in the cell. POGILs are designed for students to complete alone or in groups with little assistance from the teacher. This allows me to assist students that are confused about the Phet simulation or maybe having technical difficulties. Students that do not complete the POGIL in class will finish it as homework.)
Bring the students back together to play The Endocytosis/Exocytosis game as a class. To demonstrate how cell get large molecules in and out of the cell, have all students, but one, join hands. Have one student volunteer remain outside of the circle. Ask the students to try to get the student outside of the circle inside of the circle without breaking hands or lifting their hands in the air. This demonstrates endocytosis. (Note: To understand how the game works, please see the video in The Lysosome from The Cell Walk, Part 3/3).
Once the student volunteer is successfully inside the cell contained in a vacuole, then ask the students in a circle to expel the student volunteer. This demonstrates exocytosis.
Summarize the results of the game by having students draw pictures of the steps in the process using a comic book storyboard. Students should complete one for endocytosis and one for exocytosis.