This post is an explainer to go along with this week’s It’s Okay To Be Smart video, an animated ode to the cycles that oxygen and carbon take through the biosphere. Click here to watch it.
I’ve always been fascinated with the elegant cycle that oxygen takes through our bodies and through the biosphere. While equally elegant, the biological cycle of carbon is a lot more straightforward, so I’m not going to talk about it today. My apologies to Team Carbon :)
If you ask me, more than any other, your life depends on the following two chemical equations:
What isn’t immediately obvious when you look just at the equations is why these connections exist. Where exactly do the oxygen atoms that a plant exhales come from? Out of CO2, or water? And does the oxygen we breathe end up in CO2, or H2O? There is little elegance in an equation, only simplicity.
A beautiful recycled chain of oxygen chemistry supports a vast majority of Earth’s living universe upon its back. While it is certainly poetic in its recursive harmony, we’re not here to view it only as art. It is because of decades of scientific research that we have unlocked the beautiful secrets of the living oxygen cycle.
Take a deep breath, and join me…
When we inhale oxygen gas, it diffuses into our blood via the alveoli of our lungs. Inside our red blood cells, that dissolved gas is caged by iron-containing hemoglobin proteins that shuttle it to hungry cells throughout your body. As oxygen-rich capillaries pass near oxygen-starved cells, the double-O’s diffuse across the cell membrane.
Inside your cells, that oxygen makes its way to the mitochondria. In the early 1960’s, it was discovered that those cellular powerhouses use diatomic oxygen, the stuff you breathe, as an “electron acceptor" during the electron transport chain, the reactions that drive ATP production in our cells. Thanks to biochemistry, we know without a doubt that the oxygen you breathe ends up as water, and not CO2. You’d never learn that from the equation.
What happens to that water? You’ll be reminded next time you go to the bathroom.
Eventually, the H2O you “release” joins with rivers and rainclouds, which deliver it back to thirsty plants. Within their veins, water molecules (some containing oxygen atoms that were once breathed in by a living creature) are delivered to chloroplasts, where they begin the next phase of their cyclical journey.
We know that photosynthesis eats up light, water, and carbon dioxide in order to produce oxygen gas and sugars. But what are the fates of those atoms? Biologists had figured out the basics of photosynthesis by the early 1800’s, but argued for decades about the detailed atomic journeys within a leaf.
In 1941, at the age of just 27, a biologist named Sam Ruben wanted to find out once and for all if the oxygen that plants exhaled came from CO2, or from H2O. Again, the equation fails to tell the story. Ruben fed plants both water and carbon dioxide that contained a heavy isotope of oxygen. Only when the heavy oxygen began as water did he find it in oxygen gas, meaning the O you breathe comes from entirely from water!
That oxygen eventually makes its way back to us, along a long and frantic journey through the atmosphere, where some of it is now entering your lungs, ready to fuel the same curious brain that now understands the cycle of the breath that feeds it. Seems like we’re finding cycles within cycles now, eh?
The living world, at least according to the oxygen cycle, seems to be a very elaborate means to trade electrons between photosynthetic and respiratory branches of the Tree of Life. Richard Feynman once said that “all life is fermentation” … perhaps he should have said all life is electricity.
Breathe that in, and stay curious :)