Mini 3:1
Activity Overview
Key concepts:
Water is essential to life. Our cells are largely water, being surrounded in water and using water to transport essential dissolved materials. Every cell can be modeled as an aquatic solution of inorganic and organic compounds separated from the outside world by a thin membrane. This membrane regulates the flow of water and dissolved materials in and out of the cell.
In this activity, students explore living cell-related phenomena that they will model and study over the week. Working in a simulated Web-based lab, students add salt or distilled water to an aquatic solution surrounding an erythrocyte, and observe the response by the erythrocyte to changes in its environment. (The erythrocyte shrinks or swells, or even bursts.) Then students begin to reconstruct and model the sequence of events from the time salt or distilled water was added somewhere into the water surrounding the erythrocyte, to the moment cell shrinks, swells or collapses.Learning Objectives
Students will:
1. experience the phenomena of an erythrocyte responding to solutions of various salinities and will begin to consider what microscopic phenomena account for these changes;2. become acquainted with the key pieces of a model;
3. understand that some diseases, such as cholera , result from interference with normal functioning of cell membranes, with the result that cellular solution is lost.
Macro to Micro Connection
In the activity students are introduced to the macroscopic world of membrane-altering disease, and cellular reactions to the level of salinity in surrounding liquids. Throughout the unit students will return to these macroscopic phenomena while studying the microscopic world of polar water molecules, hydrogen bonds, dissolving, diffusion, and osmosis -- all underlying the macroscopic phenomena.
Conceptual Prologue
The focus of this "kick off" activity is to prepare students to embark on the exploration of one of the most essential properties of water its ability to dissolve ionic substances, such as salts, and polar substances, such as sugars, and move them about solutions and across semipermeable barriers, such as cell membrane. Students observe the dramatic effect of changing salinity on erythrocyte. They are challenged to figure out the origin of the forces that results in the shrinkage, swelling or collapse of the cell.
A larger context for the activity is water's ability to dissolve and move various substances, and the role this played in the formation of life. Our cells, which have evolved in water, are largely composed of water, and depend upon nutrients that are dissolved in the water. Cells carefully regulate the fluids and materials in solution both inside and outside their membranes. In addition, the structures of the cells are shaped by the pressure developed by water.
Students explore images and a model of a plasma membrane, the thin layer of lipids and proteins, which works as a barrier to the mixing of inner and outer solutions, at the same time allowing transport into the cell of necessary nutrients and out of the cell waste products. To provide motivation and the sense of mystery for the unit's work, students discuss "What if the membrane is damaged?" and learn about a membrane-related illness cholera. In cholera, a great amount of liquid is lost, and health workers have labored to find out the right salt-sugar balance to rehydrate patients.
Activity Design and Execution
Major Science Concepts Membrane, solution, cystic fibrosis Assumed Previous Knowledge: None Time: 50 minute class Materials: Model Worksheet (Student)
Model Workheet (Teacher)
Cholera story (if printed version necessary)Advanced preparation: Obtain and open "Zoom In";
ftp://ftp.concord.org/pub/workbench/zoomit-v19.zipYou also need a VRML browser plugin available at: http://www.parallelgraphics.com/products/cortona/ Practice going to Solutions Room.
Print the pretests and Worksheets, if necessary.Steps:
1. Hand out a printed pretest for the unit, or have students at their computers fill-in.
2. Distribute the printed Model Worksheet, or open and have available to students on their computers.
3.
Students should open the Zoom-It virtual laboratory and look at an erythrocyte to explore how it responds to changing salinity. Students will observe what happens to the red blood cells when one adds more salt or more water? [As cells absorb water, they swell and collapse. As they lose water, they become puckered.] Students will be guided in their work and helped to build a model by their Model Worksheets.
Pose the question to your students, while acknowledging that they will be better able to answer the question towards the end of the week: Why does adding salt to liquid in a container with a cell make that cell shrink? Why adding more water into the container make the cell swell? How does a cell "know" to respond to the changing concentrations in the surrounding liquid?
4. (Could be homework) Students read a short introduction to membranes, either on their computers or in print, Encourage them to read it, even if it is not all clear at first.
5. (Could be homework) Students read, possibly aloud to one another, either on their computers or in print, about Cholera, an example of how illnesses occur when a membrane's functions are altered (e.g. membrane-attacking toxins). Cholera can only be understood as the bacteria-caused loss of cellular solution.
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