• Christian Moore Anderson

Unifying & discerning organisms through a diagram

Updated: May 8

Whilst reading through a rather old book called Life Cycles by Peter Calow (1978), I came across a diagram that instantly struck me as having great pedagogical value. Here it is in Figure 1, below:

Figure 1. Calow's (1978) flow diagram of energy efficiency in heterotrophs.

If you've read my posts or my paper, you'll know that I think understanding the organism is central to biology education, and that entails comprehending the flow of energy and matter that sustains the organism.

It's about seeing how this activity is shared across all life; the procurement of organic molecules (and others) to provide for both the structure of the organism, and as an energy source.

I love the orientation of Calow's diagram above, representing a light-to-right flow. In this case it is focused on energy flow, but with some matter also. The organism sets up this flow to resist entropy and persist through time.

However, as a model for my IB biology (KS5) the image needed some work. For one, I wanted more emphasis on the matter flows, linked with the energy flows. Among these principal processes of organisms I also incorporated a Venn diagram to show the similarities and differences between autotrophs and heterotrophs.

Another decision was to omit metabolism but keep work. This is because the whole diagram, and therefore organisms, can be subsumed under metabolism (see Nicholson, 2018). Despite the term work not being completely satisfactory, I think the trade off in this model is acceptable.

I designed it to be memorable and drawable, and it worked beautifully with my IB students as I constructed the diagram section by section with continual questioning and interaction.

Here is my highly modified version (Figure 2, below).

Figure 2: A pedagogical graphic serving as a model of the organism and the flow of energy and matter through it, by Christian Moore Anderson.

This is an idealised model, of course. It focuses mainly of photoautotrophs, ignoring chemoautotrophs, mixotrophs, and diazotrophs (nitrogen-fixing bacteria). It also omits the additional step of amino acid synthesis.

It's a simplification that omits certain details to ensure that it is generalisable and is a good starting point for understanding the exceptions. It is difficult to put such a non-linear processes into a simple linear diagram.

The graphic clearly shows to students that life on Earth is essentially carrying out the same processes, with the major difference being how organic molecules, and thus energy and matter, are procured. A central concept to understanding biology and the organism.

Read a summary of my paper on designing biology education around the organism here.

Christian Moore Anderson

@CMooreAnderson (follow me on twitter)

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Calow, P. 1978. Life Cycles: An evolutionary approach to the physiology of reproduction, development and ageing. London: Chapman & Hall.

Nicholson, D. 2018. “Reconceptualizing the Organism From Complex Machine to Flowing Stream.” in Everything Flows, edited by D. Nicholson and J. Dupré, 3-48. UK: Oxford University Press.

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