The Egg and I – A Lesson on Diffusion
Concept or conceptual understanding: Using eggs and Eloedea as models, students will explore how molecules diffuse across a membrane. They will predict which direction the fluids will diffuse and later explain what happened at the molecular level.
Real world connections (Relevancy):
Using an egg as a model which is a familiar object, students will be able to visualize how the process of diffusion works. Since diffusion is critical to cell health, they will begin to understand how their own body works. As they do the microscope portion of the activity which uses Eloedea, they will see that plants use the same process. In the lab writeup they are given several real life scenarios in which they are asked to apply the knowledge they have gained in the lab.
AZ standards: (Science Standards Articulated by Grade Level – High School)
S1C1PO 4. Predict the outcome of an investigation based on prior evidence, probability, and/or modeling (not guessing or inferring).
S1C4PO 4. Support conclusions with logical scientific arguments.
S4C1PO 3. Explain the importance of water to cells.
S4C1PO 4. Analyze mechanisms of transport of materials (e.g., water, ions, macromolecules) into and out of cells:
· passive transport
· active transport
Incorporation of inquiry:
Students will need to predict the flow of water in three different scenarios – distilled water, Karo syrup and saline solution. They will also predict what will happen to their Eloedea sample when they add salt water or distilled water to the cell.
Behavior objectives:
Set up models of diffusion and predict the direction of the flow of water in those models
Use the microscope to observe diffusion in plant cells
Infer how diffusion could lead to dehydration
Prior knowledge necessary:
Eggs are models of (and examples of) cells and as such can be used to observe the process of diffusion. Plant and animal cells both use diffusion as a method to transfer substances.
Identify possible student preconceptions:
Eggs are baby chickens. Eggs are impermeable. The egg in Karo syrup will increase in size because the sugar will “suck up” all the water from the egg. People dehydrate because the salt sucks up water.
Materials:
Important note! You must dissolve the outer shell of the egg at least 48 hours prior to the lab. This involves covering the eggs with vinegar overnight, rinsing and covering again, and then rinsing and covering with saline solution overnight so that the eggs equilibrate in the saline solution. Don’t procrastinate about this – it takes some time to prepare! If you don’t let the eggs equilibrate, that particular part of the lab doesn’t work well and students draw incorrect conclusion.
Each group will need 3 eggs from which the shells have been removed.
Small cups or beakers to hold the eggs
Distilled water – enough to completely cover one of each group’s eggs
7% saline solution - enough to completely cover one of each group’s eggs
Karo syrup- enough to completely cover one of each group’s eggs
Triple beam balances
Eloedea (purchased from a pet store)
Microscopes
Slides and cover slips
Safety:
Students should observe the normal precautions necessary for using a microscope and slides. Since the eggs are fragile, students should be advised to take care in handling them and the eggs should not be poured down the sink when the experiment is complete.
Lesson Description:
Students place eggs whose shells have been dissolved into distilled water, Karo syrup and saline solution overnight to observe weight gain. They weight the systems before and after and make predictions about which way the water will travel. While the eggs are sitting, they use microscopes to observe the process of osmosis in Eloedea. See attached lab for a complete description.
Assessment:
The lab handout has the student predict, record data, draw pictures and analyze the results of the experiment as well as taking it to the next level in having the student apply the information learned in several real life scenarios.