Cell respiration and its relationship to photosynthesis is one of the fundamental concepts of biology. It sits up high in the hierarchy of concepts, subsumed by the more encompassing concept of the Flow of Energy and Matter. Thus, its ubiquitous presence in the secondary curriculum is well deserved, but do these equations cause additional problems for a more holistic understanding of life? I think so.
The greatest simplification I use with my students is that organic molecules play two equally important roles in living organisms:
they provide the material for building structure
they provide a store of energy
My experience tells me that the latter is most easily understood, but the former is more tricky. Of course, the verbatim rote learning of this concept is easy, but connecting this knowledge, and for it to make meaning, is tricky.
This is where the equations of cell respiration and photosynthesis provide a problem. Simply put, they are an unwieldy simplification, especially when they are connected so that the products of photosynthesis are directly fed into cell respiration.
The simplification is useful when zooming so far out that we are concerned with the eventual fate of energy in living systems. The connection is key to understanding the flow of energy through ecological systems, but standing alone, and rote learnt for recall purposes, it hides a deeper truth.
Organic molecules form the basis of organismal structure. Organisms are organised matter of organic molecules, but also derive their energy from those same molecules, just like a car built from petrol.
An organic molecule produced by a long-lived organism may persist for centuries, even millennia as part of its structure. On the other hand an organic molecule may persist passing through many organisms, through a producer food chain, and then into a decomposer food chain. There is much to know about organic molecules in between their production and final destination.
But what's the problem with having students remember the cell respiration and photosynthesis equations? The simplifications may give way to partial understandings. Let me explore two that I find compelling:
Organic molecules function only to provide a store of energy and that organisms are made from some other substance that is not directly linked to the intake of molecules. This may take on an essentialist view point, that there is something fundamentally different about the essence of say a plant and an animal, rather than just a different organisation of the same organic molecules.
The function of photosynthesis is to provide food for animals (tied with the misconception that animals have mitochondria and plants have chloroplasts), rather than to provide the building material for the producer itself, and a store of energy.
If we go to its core, I find that students with these alternative conceptions have decoupled matter and energy. They can brandish around the term of energy loosely, they may score points on assessments that require verbatim recall, but make little meaning of the substance of their own bodies or those of others.
Ask a student how an adult can maintain their body mass for 20 years and you'll probably get the answer of 'exercise'.
Why exercise? They are resorting to folk knowledge, rather than their own mental models of the flow of matter through organisms. I find this answer in students of all ages. Probe them further and the folk knowledge will soon emerge, instead of robust biological models. Despite years of biology education and knowledge of photosynthesis and respiration, students may still make little meaning how matter and life are connected beyond just providing energy.
Ask a student what happens to the body mass that is lost during a diet regime of negative energy balance and you'll probably not get a coherent answer.
Rather than having students memorise and recall of chemical equations, students will require opportunities for cognitive conflict, the challenging of their mental models, and the need to construct new ones.
Some ideas
More complex diagrams & explanations:
I would recommend always offering and asking for both functions of organic molecules: structure and energy. Obviously, when the focus is on energy, stick to the typical, but don't let it be the only thing they learn. This may require new diagrams such as these possible ideas (see this post for my best diagram):
In my opinion, it is also worth deploying a stock and flow diagram to encourage students to consider both inflows and outflows of organic molecules from organisms, and to consider the factors that affect the rates of these flows. For example, metabolic rate, age, total biomass, exercise.
This helps students to visualise the flow of matter and how it accumulate in the body - showing a direct relationship between the inflow of molecules and structure. But also, to give more attention to outflows, which I find students often ignore. This shall help students develop an idea of how excretion and assimilation can be balanced and weight can be maintained, or out of balance so that total growth, or biomass loss can occur.
Reconsider the organisation of curriculum design and way of seeing:
Primarily, I would focus more on the deep similarities of life, such as the cellular nature of all life, and the need for organisms/cells to obtain organic molecules to build/maintain their structure, and as an energy source.
This is a pattern seen across life on Earth, we are all built from the same fundamental organic molecules and use them in similar ways. Only later would I focus on the ecological differences, which principally come down to how organisms obtain their organic molecules, contrasting autotrophs with heterotrophs.
Rather than focusing on the flow of energy (only) in food webs, emphasis should also be given to the flow of matter. Students take some time to come to terms with the fact that their structure is built from the food they consume. That to be an animal implies the procurement of organic molecules from other organisms; you are built from your food.
Finally, to really discover how your students see, give them a free writing task rather than a quiz asking for recall. See what mental models they think with.
Retrieval alone can help students remember details, but not necessarily help students make sense, make meaning, or construct their own understandings. 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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