To help students criticize and revise their models, you are going to use the model competition strategy. If you need further help with this strategy, review the video.

1. As students are talking together in their small groups to come up with a model for the placement of capillaries in the small intestines for glucose absorption, have students think about how close the capillaries need to be, how materials move in and out of capillaries, and other factors in nutrient exchange.
2. As students suggest ideas in small group and draw these, take special note of various ideas although not challenge any at this time.
3. You may either post several competing models you have noted during group work or have individuals come up and draw competing models on a transparency, whiteboard, or large paper for all to see. (If students are having trouble drawing their model or seem to only come up with one model, you may use the drawings provided below and tell students that these are models other students came up with).
4. See drawing below. This is an actual example of three different models students constructed to explain the placement of capillaries in the small intestines.

1. Point out features of each model then ask students to compare the three ideas by examining their efficiency in absorbing nutrients within the small intestine" Even though you think you know which model would be the best, try to think of pros and cons for each model. Which one would be most efficient? "
2. " Let's look at Element A. Tell me how this placement would work. Do you see any drawbacks or pluses to this model? When you think back to the capillaries out in the body, how close did they lie to the cells? Why was this important? "
   • Answer = least efficient because of its placement on the diagram; the capillaries hypothesized to be too far away from the small intestine. If capillaries are located in" A " , the nutrients have to travel the longest distance to be absorbed.
3. " Which model would put the nutrients in the closest contact with the capillaries? " (Answer = B)
4. " Even though B would be closest for diffusion of nutrients between the intestines and the capillaries, what problems would be cause by this placement? " (Answer = hostile environment for capillaries). You may have to step students through a discussion to help them remember that the interior of the intestines contains enzymes and gastric juices that help in the digestion process. What effect would this have on fragile capillary walls? In addition, if the capillaries lined the inside of the intestines, what is the necessity for the folded configuration of the villi?
5. " Now let's look at Element C. What can you say about this model? Describe how this model might work. I see the capillary come into the villi and then branching off. What happens to the blood that comes in through this capillary? Remember back to the model we had of the nutrient exchange in the capillaries out in the body. How does this model compare to that model? "
6. " Now let's vote on the model you now think might be the most efficient and effective. How many think A; B; C? "
7. Based on vote you might say," Most people now have chosen C but this is still not completely compatible with the target model. " It is now important to encourage students to review and repair this model." Let's look closer at this model and see if there is anything you would like to improve in the model. Think back on the model of nutrient exchange at the cellular level. What happens to the blood that passes through the capillary? Where does it go? Is there a difference in the blood as it moves through the capillary? "
8. " In your group, draw your final model of the circulation in the villi. Explain to your teammates why the model you have chosen is most efficient and how it works. "

What is your model now? Look back to your original model.