At Georgia Tech, we are revising our teaching of molecules, cells and metabolism (our Module 3) to follow an evolutionary perspective. It strikes me that although we talk about the importance of evolution as an organizing principle of life, and may quote Dobzhansky that “nothing in biology makes sense except in the light of evolution,” we too often ignore evolution in teaching much of introductory biology. A prime example has been our teaching of this module, which pretty much follows the treatment in all the standard textbooks. We present animal and plant cells with all of their organelles and membrane systems, focusing on their structure and function, without addressing their evolutionary origins or connections. We present respiration and photosynthesis, starting with the most complex systems in mammalian cells and flowering plants, with no hint of how such complexity evolved, other than the endosymbiotic theory for mitochondria and plastids.
I hypothesize that such an approach underserves our students in at least three ways. First, we overwhelm students with the most complex systems. This cannot but help reinforce the perception that biology is a bunch of facts to memorize. Students struggle to absorb the material without an underlying framework. Second, we may be inadvertently creating misconceptions, such as that only animals respire, that all living cells require oxygen for respiration, and that only plants photosynthesize, or that all types of photosynthesis generate oxygen. Third, in ignoring evolution in teaching these concepts we may be sending our students a message that evolution doesn’t matter, in terms of understanding these structures and their functions. This creates difficulties in understanding the molecular and metabolic diversity of life, and the evolutionary history of life.
In refashioning our approach, we rethought our student learning goals. Overall, we de-emphasize specialized parts and pathways, and emphasize what is common to all life, and how later specializations may have evolved through innovation and adaptation. The following is a list of the broader, overarching goals for this module (some to be reinforced and extended in the next module on genetics and DNA):
1) Students should be able to identify what types of macromolecules, structures and metabolic pathways were present in the last universal common ancestor, and thus articulate some of the major evidence that all life on Earth has a common ancestry.
2) Students should be able to identify key differences between Bacteria and Archaea – this is definitely an underserved topic in all major textbooks.
3) Students should identify which organelles in eukaryotes originated as bacterial endosymbionts, and cite the evidence for the endosymbiotic theory.
4) Students should identify which aspects of eukaryotic cell biology have antecedents in prokaryotes.
5) Students should identify eukaryotic adaptations and innovations forced by cell size (3 orders of magnitude increase in cell volume).
6) Students should identify which metabolic pathways operated prior to the oxygenation of the Earth atmosphere.
7) Students should be able to compare and contrast anaerobic and aerobic metabolism.
Each lecture in this module has its own learning goals/outcomes, that serve these broader goals. I have not listed thermodynamic principles and macromolecular structure/function relationships and other important learning goals, which are included in the individual lectures.
A specific example of what we hope to achieve, is that students understand that the electron transport transport chain already existed in the LUCA, on the plasma membrane, with alternative electron acceptors, and it powered chemisosmotic ATP synthesis and active transport. When a respiratory bacterium became an endosymbiont, and evolved into mitochondria, then its plasma membrane became the mitochondrial inner membrane, explaining the location of the ETC in mitochondria. Eukayotes did not have to invent chemisomotic ATP synthesis, or the citric acid cycle. In fact, eukaryotes lost the ability to use alternative electron acceptors other than oxygen.
This approach should not increase the amount of material presented to the students – it reorders and reorganizes approximately the same amount of material. I hope that students will find this approach somewhat more digestible, by starting with a simpler, primitive prokaryotic system. In particular, starting photosynthesis with green-sulfur bacteria and single photosystem may reduce the apparent complexity of the light reactions.
Although we have implemented some of this piecemeal this academic year, we will be launching the fully recast version this fall, with a pre- and post-test. I expect that we won’t get this working to our satisfaction until our 3rd or 4th iteration. I welcome feedback, comments and suggestions. I was fascinated by what I learned in researching the evolution of metabolism and question of eukaryotic origins. Please inform me if you know of helpful resource materials, or know of others who have tried this.
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