Units on the cell will also include lessons about how the cell transports materials across the membrane.  For beginning biology students who don’t have a strong foundation in physical science, the ideas of entropy, solutions, and concentrations can be overwhelming.   Diffusion is easy to explain with a perfume bottle or food coloring.    

How diffusion can occur across a membrane and why it moves from areas of high concentration to low concentration can be a difficult concept for many students.    One way to help students visualize these processes is to create a model of the cell using a cheap plastic sandwich bag filled with a solution of water and corn starch.    

The bag seems solid to students, but after this activity they will understand that the bag is also semi-permeable and will let some things across it.   The bag is placed in an iodine solution and within about 15 minutes the starch solution will turn purple, iodine is an indicator for starch.   This activity can be done as a demonstration, and I find it helpful to guide the students through each step.

Topics addressed:

Concentration gradient
Diffusion / Osmosis
Solutions
Indicators
Permeable

Scientific Modeling
Molecules and Matter (in explaining how iodine can cross the plastic)

HS-LS1-2 Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms