Search
  • Christian Moore Anderson

Contextualising biological components: Cellulose, an example

Updated: Dec 3, 2020

Cellulose is an incredibly important component of cell walls, plant cells, plants, and whole ecosystems. However, cellulose can only be a working component of a system that is organised in such a manner that it can interact with it. The plant cells need to organised so that cellulose forms a cell wall in the correct place and thickness. Plants need to be organised so that enough cells (and their walls) are correctly placed to produced the desired structure, and in the correct orientation. A passing bovid can digest cellulose, but only with a system of correctly organised molecules, organs, systems, symbionts, et cetera. A component in the wrong place is pretty useless.


A schema of knowledge is similar; understanding emerges from its organisation and its correct connections. Isolated islands of knowledge can not contribute to a larger understanding until those connections are made. As such, biological components, and their mechanisms, only hold meaning when contextualised within their larger biological system. I have written in more depth about this here.


Let me give an example of how an abstract component, cellulose, can be contextualised by zooming in and out of the scales of biological organisation, in the same manner as a semantic wave, oscillating between the more concrete and the more abstract (Knippels 2002; Maton, 2013). In this example I have my IB biology curriculum in mind (A-level equivalent).

Cellulose is quite an abstract molecule for the beginning of a biology course. Mainly because it is a molecule with functions at the molecular scale, but also because it is typically taught without context.


Some teaching points for cellulose may be:

  • It is the principle component of plant cell walls & is not present in animal cells

  • It is formed by a polymer of beta-glucose monomers via 1-4 bonds

  • It is macro-structure differs from that of starches and glycogen due to the point above

  • It is considered fibre in human nutrition as we (and most organisms) lack cellulases


Most biology curricula begin their course with cell biology and molecular scale with little oscillation between different scales of organisation. The focus is placed solely at these scales of organisation, so much so that the topics become descriptive in nature, describing the what, and the what for.


If taught early in the course, typically alongside the teaching of amylose, amylopectin, and glycogen, it is likely that cellulose will remain isolated from meaning for the student. The learning of plant cells, and their unique components so early in the course gives them little opportunity to build on previously formed schema with concrete knowledge. A small but isolated schema may be formed with some links to other polymers, but without links to larger scales of organisation.


Biology is much more concrete at the organismal scale, much more accessible and meaningful to students. Components can be easily contextualised by rearranging the sequence of biology curricula, so that they form part of an explanatory question: how?


Image by Christian Moore Anderson


To give cellulose more meaning (to students), the learning should be adding to larger schema, one that connects the different scales of living systems to the more concrete scales.


But not only this, a biologist is someone who can zoom in and out of the scales of organisation to see how mechanisms at one scale have affects at other scales. I want to avoid a reductionist philosophy, suggesting that everything in biology can be explained by studying the smallest scales. I want to instil a philosophy in my students in which they see natural phenomena and seek explanations by zooming in and out of the scales. I want them to see it like a biologist.


Therefore, I recommend a late introduction in the curriculum for cellulose, when its structure and function can be appreciated within a narrative that includes its evolutionary and physiological significance on the larger scales of plants.



An example narrative for contextualising cellulose


An easily appreciated narrative is turgidity in plant cells, and how the collective turgor pressure of each cell enables non-woody plant components to grow against the pull of gravity. Wilted house-plants provide excellent concrete examples.


The narrative can begin with the evolutionary context to appreciate the problems faced by plants. I would begin with lessons of the evolutionary story of plants, focusing on key innovations and how they are classified. Aside from vascular tissue and seeds, some discussion could be held on the innovations required for green algae to grow on shorelines without the structural support of water.


The narrative has been set, a physical problem faced by plants, that was overcome, but what was the solution? We have covered the why, but now must descend from the macro scale to the smaller scales to appreciate the how.

Image by Christian Moore Anderson & Blanca Martínez Valiente


The introduction of cellulose as a key component of a mechanism that provides turgor pressure in conjunction with the large vacuole provides real contextual information for why the position of hydrogen bonds in cellulose are so well position for their function.


To zoom out again from the molecular scale, students must appreciate the result of the collective effect of each cell's turgor pressure, and the potential effects on a plant of dehydration (wilting).


Once fully contextualised, cellulose can be compared to its polysaccharide counterparts to consider how form affects function at the molecular, cellular and macro scales.


As the narrative ascends back up to the environmental scale, the course could begin entering a learning sequence on photosynthesis and energy & matter (with the importance of plant biomass) in ecosystems.


Image by Christian Moore Anderson


In this manner cellulose is no longer an abstract molecule devoid of meaning, but a well placed snippet of knowledge within a larger network of understanding.



Christian Moore Anderson

@CMooreAnderson (follow me on twitter)



More posts that you may enjoy:

Developing a biologist’s gaze: the organism in its environment

Classroom questioning for explicit networked-thinking in biology

Making the philosophy of the biology curriculum as explicit as the content: with examples of IB bio

References

Knippels, M.C., 2002. Coping with the Abstract and Complex Nature of Genetics in Biology Education: The Yo-Yo Learning and Teaching Strategy. Ph.D. Thesis, Utrecht University, Utrecht.


Maton, K., 2013. Making semantic waves: A key to cumulative knowledge-building. Linguistics & Education, 24(1), pp.8–23.



160 views0 comments