Egg Osmometers: Teacher Notes
Nancy Iversen, Cooperstown, New York
Abstract: Students construct osmometers using raw eggs and other readily obtainable materials. Osmometers are used to examine the phenomena of concentration gradients, diffusion, osmosis, osmotic pressure, passive transport, and active transport.
Type of Entry: lab activity.
Type of Activity: designed as a hands-on, open-ended inquiry lab, but can be easily modified to for use as any of the following:
- authentic assessment
- take home assignment/lab
- group/cooperative activity
Target Audience: high school biology at all levels, but can be easily modified for others including:
- middle school life science
- A.P. biology
- integrated sciences
Teacher Preparation Time: Raw eggs must be purchased in advance. 10 minutes to assemble the necessary materials. 10 minutes to clean at the end of the day.
Class Time Required: 40-60 minutes to set up the experiment, and 10-15 minutes the next day to record results, to tear down, and wash beakers.
Background Information for teachers:
Aside from water, most of the other molecules in a fresh hen's egg are macromolecules that do not diffuse through most semi-permeable membranes. The egg yolk consists mostly of lipids and polysaccharides, whereas the albumin is protein. The shell membrane is not permeable to the macromolecules of the yolk or egg albumin.
In this activity, the contents of the egg will be maintained on one side of the shell membrane, and water on the other side. By doing so, a steep concentration gradient will be set up across the shell membrane. The macromolecules will be in higher concentration inside the egg and the water will be in higher concentration outside the egg. Since the macromolecules cannot diffuse across the membrane to reach equilibrium, the water will diffuse in. Because the shell and shell membrane at the top of the egg will be opened, displacement will occur through this opening, forcing the contents of the egg up through a straw which will be sealed in place at the opening.
The egg osmometer can be used to set up inquiry discussions of diffusion and concentration gradients at the very beginning of a discussion on transport or membrane function, or it can be used as a verification activity after the concepts of diffusion and membrane function have been taught in class. It can also be used as an assessment activity.
Although the activity is written for high school biology classes, it can be modified for physiology, physics or A.P. biology classes by introducing osmotic pressure, and by having the class come up with a good estimate of the actual pressure on the membrane. To do this the students will have to estimate the surface area of the exposed membrane and the volume (and therefore the mass) of the water that diffused into the egg across the exposed membrane.
Student Preparation: If the activity is used to set up an inquiry into diffusion and membrane function, students should have some basic knowledge of macromolecules. If the activity is to be used for verification or for assessment, the students should have an understanding of concentration gradients, diffusion, and osmotic pressure in addition to basic knowledge about macromolecules.