Why is the word glucose problematic?
Cellular respiration, which is the oxidation of organic molecules for the production of ATP, is often defined via the summary: Glucose + oxygen --> water + carbon dioxide. But is this the best definition we can give to our secondary students?
Glucose is not the only molecule used in cellular respiration; indeed, many organic molecules are oxidised including fatty acids, amino acids, and ethanol. As such, we can create a discrepancy between common knowledge of diet and health and the 'fact' that respiration as the oxidation of glucose.
Let’s consider a handful of possible questions that may arise: Is sugar therefore completely essential in our diet? Why do we store copious amounts of energy as fats in adipose tissue? When we teach the macro-nutrients, why do we say that fat is used for energy if we teach them that it is glucose that undergoes oxidation? As the keto diet rises in popularity, how can it be that we can survive on oxidation of circulating ketone bodies primarily? Indeed, human babies are the ketogenic kings and often revert to ketosis.
How about the breakdown of muscle for energy production? Athletes and gym goers often understand such problems and ensure they include appropriate amounts of protein in their diet post-work out. Why do we lose muscle mass when fasting, or diet regimes? And how is it that ethanol in our alcoholic beverages can contain so much accessible energy, if it is only glucose that we oxidise?
We can’t allow exceptions to form the rule, but nor can we allow a single example to form the rule. Metabolic flexibility is not an exception, it is the norm. Humans certainly oxidise fatty acids all day long. Extending beyond human biology, cellular respiration can become much more flexible. If it’s through constructing connections that comprehension is obtained, then the sole use of the word glucose limits this process. It should be clear to students that glucose is an example of many.
Of course, one of the major benefits of using glucose in the definition is that it can be aligned with a summary of photosynthesis: Water + carbon dioxide --> glucose + oxygen, thus highlighting the important relationship between these reactions. However, if we substitute glucose for organic molecules, we gain in accuracy without losing our ability for comparison.
The catch, of course, is that we cannot commit to an (easily-learnt) balanced symbol equation, but how useful is this for true biological understanding? (Is it just to appease the chemists?). In post-16 education when biochemistry is studied in detail it may be beneficial to study a balanced equation, but what about before this level, at what stage in biology education is a balanced equation of photosynthesis and cellular respiration of conceptual usefulness?
Another example of its benefits is the relationship between the oxidation of glucose and how we teach the homeostasis of blood glucose. In this case it is highly useful, and I am not advocating the avoidance of the word glucose, just that it needs to be known that it is an example of molecules used in cellular respiration, and that we should consider when in biological education it is appropriate to introduce the term.
I often find that one of the misconceptions students have is that we eat food to obtain energy. This is partly true, but they often fail to understand the anabolic necessity of obtaining organic molecules for the production and maintenance of cells. This is equally true of misconceptions of photosynthesis as students fail to recognise that the products of the reaction are also used to build the cells of the organism. Photosynthesis and respiration are not just a tale of energy, but also of matter.
For my classes I teach the definition using the term organic molecules, and when we study cellular respiration, I point out that we are studying the oxidation of glucose in cell respiration as an (important) example, and that other molecules can be incorporated into the process.
In photosynthesis the ultimate product is not glucose, but triose phosphate, a precursor that photosynthetic organisms utilise to produce an array of other organic molecules. Again, glucose is only used in the equation to allow for a simpler connection with cell respiration (and that pesky balanced symbol equation).
I know that teachers will want to start students early in the definitions that they will need later in upper secondary, but is it useful to biological understanding? I think as students progress through their education in biology we can refine our terms to give extra scientific detail, but surely the central aim is to aid the comprehension of the biological relationship with energy and matter via photosynthesis and respiration.
What I have been doing with Year 7 (11-year-olds)
For the last few years in my year 7 classes I have simply substituted the word glucose for the much more concrete ‘food’. Qualitatively I would like to say that I have noticed an increase in understanding. I like the word food in this context (first year of secondary biology education), because it easily relates to their prior knowledge.
Humans breathe to obtain oxygen, humans eat to obtain food (organic molecules), but at this stage students often reply teleologically, such as ‘We breathe for our lungs’. It is during this first year that we need to connect these behaviours to the production of energy and the building of cells without limiting their understanding to just glucose. Using this image I can help students build a lot of conceptual understanding:
Image by Christian Moore Anderson
Students need to understand that they themselves are built from organic molecules (food), they are built from what they eat. If we can say that beef is food (mainly organic molecules and water) then logically so are we. When students are fairly secure with this concept (I mean over time, not in a single lesson), photosynthesis comes more easily, just a simple diagram of the cycling of the matter and input of energy is enough for intuitive understanding: Photosynthesis re-builds the food using energy from light, and we are part of this big cycle:
Image by Christian Moore Anderson
As their education progresses, other biochemical details can be added when the time is right. We can talk more of the macro and micronutrients and their roles and link them to cellular respiration. We can discuss organic molecules as being carbs, lipids, proteins, and nucleic acids.
Personally, I feel I have enabled much more conceptual understanding in my Year 7 students by avoiding using the word glucose, something that is easily added into their schemas later in their secondary education, as an example of cellular respiration.
As we know, respiration and photosynthesis are central concepts to an education in biology and so they should reappear regularly, which gives ample opportunities to refine our terms. Year 7 is not the time to define all the technical vocabulary we ultimately want students to obtain at the end of the secondary education, but the time to give a firm foundation of the big ideas in biology.
Transforming our students' view of nature is more important, and more difficult, than the accruing of technical vocabulary so early on. If you'd like to read more around this idea, check out this post: How excessive use of the respiration equation can cause problems. If you've enjoyed this—check out my book. Download chapter 1 here—English edition—edición española—or check out my other posts.
Christian Moore-Anderson
@CMooreAnderson (twitter)