Where Do Organisms Get Their Energy?

I've been thinking a lot about fundamental concepts in biochemistry. One of them has to be energy—where do cells and organisms get the energy to grow and divide?

Most of the metabolism section of biochemistry courses in North America are taught from an anthropomorphic, fuel metabolism perspective. That's understandable since the purpose of such courses is mostly to prepare students for the MCAT exam. (Medical school entrance exam.) I prefer an evolutionary approach to teaching biochemistry but that's not very popular these days.

By the time the course is over, students will have learned that humans get their energy from food, especially glucose. The next step is to ask where the glucose comes from. The simple answer is that food (i.e. glucose) comes from plants. The next question is where do plants get the energy to make glucose? The answer is, of course, sunlight. This should lead to an explanation of photosynthesis but that rarely happens in introductory biochemistry courses.

This description leads to the classic "food chain" as shown in the figure (above) from FT Exploring Science and Technology [The Flow of Energy Through Plants and Animals]. This is conceptually sound biochemistry as far as it goes. As long as students understand how sunlight can be used to make ATP and how ATP can be used to make macromolecules (including glucose), then they will understand that humans ultimately get their energy from sunlight. I would be happy if all biochemistry students could explain this food chain at the molecular level.

But in order to make sure that students really understand this process, I go one step further. I explain that there are many species of bacteria that are chemoautotrophs. Chemoautotrophs are incapable of photosynthesis yet they are able to grow and divide in the absence of any organic compounds. Their carbon source is CO₂, just like photosynthetic organisms. These bacteria have a basic metabolism that teaches us what primitive life forms must have been like. Knowing how they get their energy helps students understand evolution.

Where do chemoautotrophs get their energy? I'm interested in knowing how many readers have taken biochemistry and are able to answer that question. Please let me know in the comments before you read the answer in these posts [Carbon Dioxide Fixation in the Dark Ocean] [Core Concepts: Pathways and Transformations of Energy and Matter] [Ubiquinone and the Proton Pump].

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