After they've completed the earlier experiments, show them this set up. Ask them why the water outside the bag did not turn blue also. This can lead to a discussion of membranes and how our cells, just like this bag, let some molecules pass through. It is the smaller molecules, not the polymers, that typically pass from the outside of a cell into the inside. Therefore, if we didn't break down our food into smaller molecules, we couldn't easily get the food inside our bodies.
Digestion is essential in helping nutrients be absorbed by our intestinal cells.
Hold a discussion about the lessons learned from this series of experiments to make sure that each student understands the concepts. This would further illustrate what is happening in Step 4. Having a concrete example to look at is often very helpful in learning a new concept. Therefore, build a life size model of the digestive system to illustrate the relationships of the various organs. Another great way to visualize the digestive system is to take advantage of software packages about the human body.