Where should a topic on the digestive system begin, let's say, for a GCSE class (ages 14-16)? Should it be with the general structure of the system, the anatomy? Should it be with the structure & function of enzymes? Or should it be with the system as a whole?
In my view, the teaching of the digestive system isn't just another topic of here's how the body works, but another opportunity to establish identity—thinking about what we and others are.
The fundamental insight to the digestive system is how it functions to meet the needs of the whole organism. Yet, students can often learn the digestive system and answer short-answer questions well without being able to think about the big picture and why organic molecules are digested and absorbed.
To address this, we must first begin with the whole, but we must contrast it with other systems—we have to see what we're not to see what we are.
With this in mind, let me show you a lesson I have taught:
I began with a contrast between autotrophs and heterotrophs, and
I then compared fungi, Hydra, and humans.
Contrast
I built this diagram step by step in dialogue with the class. I began with the autotroph, before passing to the heterotroph. The contrast here was made explicit—autotrophs readily take in small molecules then synthesise large organic molecules within their cells.
Heterotrophs must obtain their organic molecules from other organisms, but by doing so are confronted with the challenge of absorbing the large organic molecules. Heterotrophs must break the polymers down into monomers, absorb these smaller molecules, and then synthesise the large organic molecules once within the cell. Additionally, they also want to synthesise the organic molecules true to their species, rather than simply obtain those of others.
This discussion helps address another issue. It contrasts the two general functions of enzymes—catabolism and anabolism. This distinction is generally lost when enzymes are only taught within the context of the digestive system (i.e. where only catabolism is considered).
At this point some students asked me why we don't just obtain monomers in the first place, and this sets up a nice ecological discussion of how humans would have lived forty thousand years ago. And, how the cultural evolution of cooking & processing has enhanced this prospect.
Comparison
The contrast set the scene for the next stage in the lesson. I wanted to consolidate this abstraction by showing more examples of it—I wanted to generalise the concept across diverse examples. To do this, I taught it with fungi, Hydra, and animals with an anus. Here's what I drew:
First, I drew the fungus and establish the term of secretion. As this example is very similar to the previous, I moved on to include Hydra. I search for images of Hydra on google to show how these species—as with other cnidarians—have a primitive digestive system without an anus. I made explicit how the examples of fungi and Hydra, show the same concept.
But here, there are more questions.
Fungi and animals are both heterotrophs, but one has an digestive system, while the other doesn't? Why? This is where we can start pushing students to infer in terms of ecology and evolution. I did't expect students to come up with the correct answers, but I wanted them to think about the whole organism in its environment and how the digestion contributes to its life.
I had students suggest how it's beneficial to have a digestive system if you can move, and therefore take a digesting meal with you. Other answers I had were that animals can be less vulnerable while digesting food, as they can have specialised organs for digesting, and others for forming the external barrier of the animal. These discussions took maybe five minutes.
Finally, I added the diagram of the animal with an anus. Again I made explicit that it is the same concept being shown. I have found that it's really important to highlight here that what is in the digestive system is in a tube running through you, and is not considered 'you'.
The interesting question now pertained to the ecological evolution of the anus. Some students came to the answer—with more or less guidance depending on the class—of how an anus allows several meals to be processed at once, at different stages. Without an anus, it's one meal at a time. Related to this, some students saw how Hydra's simple system is a generalist. Having a mouth and anus has favoured the evolution of specialist organs that can process foods in different ways.
From here, I asked students to carry out self-explanation, one of Fiorella and Mayer's eight generative learning strategies (2015). This activity involves students explaining the diagrams to themselves as if they were explaining it to a classmate who had missed a lesson. I then took any questions when students realised they had missing information. If you've liked this then check out my book. Download chapter 1 here—English edition—edición española—or check out my other posts.
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References
Fiorella, L., and Mayer, R. 2015. Learning as a Generative Activity: Eight learning strategies that promote understanding. UK: Cambridge University Press.