HOW COME? Plants' role in creating oxygen

Why and how do plants make oxygen? asks a reader.

Billions of human beings, and quintillions of other animals (including insects) continuously take in oxygen and release carbon dioxide. Meanwhile, fires burn day and night all over the planet, consuming oxygen as they do. So why doesn't carbon dioxide gas build up in the air? And why doesn't oxygen disappear over time?

It took scientists centuries to solve the puzzle. In 1771, British scientist Joseph Priestley noted that a candle flame would go out if covered by a glass jar. But when he put a green plant under the jar, the candle would stay lit. Green plants seemed to give off something that a flame takes away. That "something" turned out to be oxygen. But how do plants do it -- and why?

All over the world, oxygen is escaping into the air -- from trees in the forest, algae in the park pond, the oceans' tiny phytoplankton. (Underwater, we can see bubbles of oxygen seep out of waving plants.) The result? Earth's air is about one-fifth oxygen gas.

Here's how it works. Green plants use energy from the sun to make their own food-sugar. To make it, they also pull water from the soil and carbon dioxide from the air. Oxygen is released as plants make dinner, like the lovely scent wafting from a bakery making bread.

The process is called "photosynthesis," from Greek words meaning "putting together with light." And it's the pigment that makes plant leaves green that starts the process. Green chlorophyll uses energy from sunlight (mainly its red and blue wavelengths) to split H2O molecules apart, leaving oxygen, hydrogen ions, and electrons.

Plant cells use these atomic raw materials in a series of complex chemical steps. A compound called ATP is formed, and a molecule in the plant called NADP is altered. In the end, carbon dioxide is used to make a carbohydrate -- glucose. Presto: plant food.

Leftover oxygen escapes into the air. And we breathe easy.

Nearly 70 percent of the oxygen we breathe comes from ocean-dwelling plants, with 28 percent supplied by rain forests. The photosynthesis of plants and the respiration of animals tend to balance each other out. But since the 1800s, CO2 levels have been rising, mainly from the burning of fuels like coal, oil and gas.

Carbon dioxide traps heat from the sun, and too much CO2 creates a greenhouse effect, raising Earth's average temperatures. The result is melting ice sheets and rising seas, extended heat waves and extreme weather, with more flooding in some places and long droughts in others.