Molecules of chlorophyll capture the energy of sunlight (mostly wavelengths of blue, violet and red; green is mostly reflected). The chlorophylls use this energy to bust up molecules of water, and to power reactions that break up carbon dioxide. From the atoms in water and carbon dioxide, a plant builds molecules of sugar. And from sugar, it makes virtually everything else it needs (and we need) Ð fats, proteins, carbohydrates.

From water, the plant needs only the hydrogen atoms. The rest Ð atoms of oxygen Ð it releases as waste. The products of photosynthesis Ð sugars, fats, proteins, complex carbs - thus contain less oxygen than the molecules for which they came Ð water and carbon dioxide.

It takes energy to bust up carbon dioxide and water. Pulling a similar trick backward, when carbs or fats re-combine with oxygen, we get energy back out (often as heat and/or light). Molecules of carbohydrate and fat "want" to re-combine with oxygen, to re-form molecules of carbon dioxide and water.

The oxygen we breathe was once bound up in water, released when the water was broken up by chlorophyll. Given half a chance, sugars caramelize (turn brown, like an old newspaper) ... fats become rancid ... wood burns ... all are combining with oxygen, making their way back to carbon dioxide and water.

Why is there any oxygen left in the air to breathe?

The only reason there is any oxygen left in the air is that, before they have a chance to recombine, some of life's molecules are sequestered, hidden away from the air. Algae die and fall to the low-oxygen seafloor. Land-plants, growing in wetlands, die and fall into the water; they begin to rot (to be consumed by microbes), but rotting soon uses up all the oxygen dissolved in the water.

Streams flowing into low-lying wetlands carry sediment; a "snow" of sediment similarly blankets the seafloor. In the ocean or on the land, the remains of dead plants are soon buried, partitioned off from the air.

As dead plants on the floors of the seas and the swamps are buried by ever-more sediment, they're pushed downward by its weight.

The sediment slowly turns to rock.

Squeezed by the rock above, cooked by heat from below, the atoms in the buried molecules of life re-combine. Hydrogen and oxygen , which "want" to bond, do so, forming water.

Plants in the ocean and on land build themselves out of pretty much the same molecules, but in different proportions. Suspended in water Ð and, for the most part, composed of just single cells Ð ocean plants don't need as much of the cellulose fiber that land plants use to hold themselves upright.

A count of its atoms reveals that the cellulose molecule is hydrogen-poor, relative to the molecules in sea-plants. Cooked and squeezed below the surface, the hydrogen of cellulose-rich land plants is used up; what remains is largely carbon: coal.

With less cellulose, and more hydrogen atoms to start with, sea plants, cooked and squeezed, instead run out of oxygens. The remaining hydrogen and carbon atoms rearrange to form hydrocarbons: oil.

Cooked and squeezed still more, oil molecules break down to form natural gas.

Fats and carbs are energy-rich because they have fewer oxygen atoms than water and carbon dioxide (to which they would "like" to revert). With less oxygen yet, coal and oil and gas are phenomenally energy-dense ... and valuable - so valuable that, every day, we haul tens of millions of tons of these molecules to the surface, to allow them to re-combine with oxygen, to re-form water and carbon dioxide.

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Alan Stahler trained as a biologist and is an amateur astronomer. He teaches private enrichment classes for students of all ages. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).