The central theme of this lesson is to have students recognize and appreciate that cell membranes, which they see surrounding cells, are far more than a simple envelopes holding the cell’s contents together. Have students reflect on and list materials that must enter or leave cells, hence pass through the cell membrane. (It will be helpful to have students consider cells of particular organs in turn. For example: cells of inner parts of lungs, red blood cells, muscle cells, cells lining the gut, cells of salivary glands that secrete saliva, cells of kidneys, etc.) Don’t neglect the importance water itself, which perpetually both enters and leaves cells.
Then, turn students attention to the question: What is the nature of cell membranes that allows, facilitates, and/or promotes such transfers?
Recall simple diffusion. If necessary review:
We have seen that the movement of oxygen and carbon dioxide into or out of cells in the course of respiration and photosynthesis is always toward lower concentrations, i.e., down concentration gradients. Therefore, movement can be explained by simple diffusion, which requires no energy or special mechanism. The membrane only needs to be such that it allows these molecules to pass through.