All living cells have an outer layer known as the cell membrane. The cell membrane is an organelle that strictly controls what molecules can enter and leave the cell. Some molecules like sugar are too big to cross on their own, and some molecules like oxygen are tiny enough to squeeze through. This characteristic of the cell membrane is known as selective permeability.

When the cell membrane is placed in uneven concentrations of molecules (or solute), it attempts to balance those molecules out to reach an equilibrium. This automatic evening out of molecules from high to low concentrations is known as diffusion. For example, if the cell membrane has too many water molecules outside of its cell, then water will move inside the cell in hopes of reaching an equilibrium. This specific type of diffusion is known as osmosis, or the movement of water.

In this 4-day lab, an egg will serve as a model of a cell and its cell membrane. The egg will be placed in various liquid solutions, forcing molecules to diffuse inside and outside of the egg, depending on the type of solution.