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 web 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.
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. An isolated understanding may be formed with some links to other polymers, but without links to larger scales of organisation.
Biology is much more meaningful at the organismal scale. Components can be easily contextualised by rearranging the sequence of biology curricula, so that they form part of explanatory questions: How? and Why?
Image by Christian Moore Anderson
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 could 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.
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.
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.
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Christian Moore-Anderson
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