The Adventure of a Carbon Molecule: Where Does Our Energy Come From?

When I was younger, I thought I’d become a doctor. I’ve always been fascinated by science, especially the way food becomes fuel, and how every system in the body links together. That curiosity never left me, not during my years of professional triathlon, and certainly not now.

This post is about one small but mighty traveller: a single carbon atom. Let’s follow it on its journey, from the air to your plate, through your bloodstream, and finally into the energy that powers your workouts.

Because understanding where your energy comes from isn’t just for scientists. It’s for athletes who want to fuel smarter and go further.

From Atmosphere to Plant: Capturing Sunlight on a Plate

Our carbon atom begins its journey as part of a CO₂ molecule floating in the atmosphere. It’s just hanging out—until a plant grabs it.

Thanks to photosynthesis, plants absorb CO₂ and water, then use sunlight to turn it into glucose, a simple sugar:

6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂

That glucose becomes plant fuel. And when you eat plants—or animals that ate plants—you’re basically capturing a piece of sunlight. Think of your pre-race porridge as edible sunshine.

Digestion: Breaking Down the Fuel

Now let’s imagine you eat a banana 30 minutes before a ride. Inside your digestive system, enzymes get to work, breaking down starches into glucose molecules.

That glucose gets absorbed in your small intestine and released into your bloodstream, ready to be delivered to your cells. Your body is fuelling up.

Cellular Respiration: Where the Magic (and ATP) Happens

Once glucose enters your cells, the real energy production begins. Picture it like a three-stage triathlon:

Swim (Glycolysis – in the cytoplasm): Glucose is split into two pyruvate molecules, producing a small splash of ATP (2 net ATP) and NADH (2 NADH).
Bike (Pyruvate Oxidation & Krebs Cycle – in the mitochondria): Pyruvate is converted to acetyl-CoA, releasing CO₂, and enters the Krebs Cycle. This cycle further breaks it down, releasing more CO₂ and generating NADH, FADH₂, and a bit of ATP (2 ATP).
Run (Electron Transport Chain): The final sprint. NADH and FADH₂ donate electrons to the ETC, driving a proton gradient that powers ATP synthase to produce a surge of ATP, around 30–32 ATP per glucose.

The energy powering your bike ride or hill sprint? It’s this beautifully efficient chemical triathlon happening in your cells!

Energy Storage: The Athlete’s Battery Pack

Not every gram of glucose is used right away. Some is tucked away for later, like extra tools in your race belt.

Glycogen is stored in your liver and muscles, ready to be tapped during long sessions.
Fat: If your glycogen stores are full, glucose is converted into fatty acids and stored as triglycerides.

This is your body’s equivalent of having aero bottles and a backup gel taped to your top tube.

Fun fact: When you “burn” fat, most of the mass leaves your body as CO₂ and water—you breathe it out.

Back to the Atmosphere: The Carbon Atom Exhales

Once energy is extracted, our carbon atom hops back onto a CO₂ molecule and gets exhaled. Every breath during a long run is your body releasing the carbon it once stored from food.

The cycle continues—plants inhale what we exhale. The same carbon might fuel a blade of grass next week, or become the oats in your breakfast next month.

Big Picture: Performance is Just Solar Power in Motion

Understanding the journey of this carbon atom makes nutrition feel a lot more powerful—and personal.

“There is something nice about thinking about energy starting with sunlight and ending with effort. And thinking about how every bite relates to every breath, and every training session.”

— Alistair Brownlee

So, next time you’re fuelling before a long ride or wondering if that gel is doing anything, remember: you’re not just eating. You’re unlocking a chain reaction that turns light into motion.

If you're curious about how to use this energy wisely in training, check out our blog on how to fuel smarter with modular carb strategies.

Key References

Taiz, L. & Zeiger, E. (2010). Plant Physiology, 5th ed. Sinauer Associates.
Guyton, A.C. & Hall, J.E. (2020). Textbook of Medical Physiology, 14th ed. Elsevier.
Nelson, D.L., Cox, M.M. & Hoskins, A.A. (2021). Lehninger Principles of Biochemistry, 8th ed.
Berg, J.M., Tymoczko, J.L. & Stryer, L. (2019). Biochemistry, 9th ed.
Campbell, N.A. & Reece, J.B. (2017). Biology, 10th ed. Pearson Education.

About the Author

Alistair Brownlee is a two-time Olympic gold medallist, Ironman Champion, and co-founder of Truefuels. He is driven by a belief in science-backed training, clear structure, and removing friction from performance.

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