The very first time you learned about carbon dioxide was probably in grade school: We breathe in oxygen and breathe out carbon dioxide. Any eight-year-old can rattle off this fact.

More specifically, the mitochondria within our cells perform cellular respiration: they burn carbohydrates (in the example shown below, glucose) in the oxygen that we breathe in to yield carbon dioxide and water, which we exhale as waste products, as well as energy, which is required to maintain our bodily processes and keep us alive.

C₆H₁₂O₆ 6O₂ ? 6CO₂ 6H₂O energy

carbohydrates oxygen ? carbon dixoide water energy

It should come as no surprise that, when confronted with the challenge of reducing our carbon emissions from the burning of fossil fuels, some people angrily proclaim, "Why should we bother? Even breathing out creates carbon emissions!"

This statement fails to take into account the other half of the carbon cycle. As you also learned in grade school, plants are the opposite to animals in this respect: Through photosynthesis, they take in carbon dioxide and release oxygen, in a chemical equation opposite to the one above. (They also perform some respiration, because they need to eat as well, but it is outweighed by the photosynthesis.) The carbon they collect from the CO₂ in the air forms their tissues - roots, stems, leaves, and fruit.

These tissues form the base of the food chain, as they are eaten by animals, which are eaten by other animals, and so on. As humans, we are part of this food chain. All the carbon in our body comes either directly or indirectly from plants, which took it out of the air only recently.

Therefore, when we breathe out, all the carbon dioxide we exhale has already been accounted for. By performing cellular respiration, we are simply returning to the air the same carbon that was there to begin with. Remember, it's a carbon cycle, not a straight line - and a good thing, too!

We exhale carbon and that carbon is sequestered in the next plant we eat.

Here's a simple question: Why do you weigh more when you go to sleep than when you wake up? Because you do... You can check this yourself. Somehow, while doing absolutely nothing all night but sleep, you will wake up lighter.

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All night long, every time you breathe out, a bunch of carbon atoms, formerly inside your body, leave your insides and take off into the night air. You breathe in oxygen, O2. You breathe out carbon dioxide, (two oxygen atoms with a carbon atom attached), so there's an extra carbon atom leaving in every round trip.

Each of those carbon atoms weighs almost nothing, a fraction of a fraction of a gram. But every breath expels roughly 10,000,000,000,000,000,000,000,000,000,000, or ten billion trillion atoms, so add up all the atoms coming from all the breaths you take all night long ... and — could it be this simple? — you wake up carbon-depleted, more than a pound lighter.

Over the course of the night, through respiration, you lose a pound of weight to the carbon atoms in Carbon Dioxide.