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WHAT IS IT?
Photosynthesis is the process by which plants, algae, and some bacteria use light to convert carbon dioxide from the air and water from the soil into food. Chlorophyll, which gives plants their green color, absorbs light energy and uses it to rearrange CO2 and H2O molecules into the carbohydrate called glucose, a sugar. Photosynthesis also produces oxygen gas (O2), whichis returned to the atmosphere. This chemical process is extremely important to the biosphere.

WHY IS IT IMPORTANT?

• Oxygen in the atmosphere is due to photosynthesis. The Great Oxygenation Event, which occurred about 2.4 billion years ago, was caused by the first photosynthesizing organisms, cyanobacteria; this led to extinction of most forms of anaerobic (oxygen-intolerant) lifeand the evolution of most life forms we know today.

• Virtually all of life on Earth depends on the food created by photosynthesis; even animals (like tigers and wolves) that eat only meat require photosynthesis, because their prey (like antelope and rabbits) survive on plant material.

• Photosynthesis moderates global warming. Carbon dioxide in the atmosphere retains heat from the Sun that would otherwise bounce off the Earth and back into outer space. Plants remove that carbon dioxide. Deforestation (the clearing of large forest tracts, as in the Amazon, to create farmland) intensifies global warming by releasing the carbon stored within trees back into the atmosphere.

HOW DO WE KNOW ABOUT IT?

• It was originally believed that plants derived all of their nourishment from the soil. In the mid-17th century, Belgian researcher Jan van Helmont grew a willow tree in a pot ofsoil, adding only water. In five years, he found that the tree had gained 75 kilograms while the soil had lost only 60 grams. He correctly concluded that a plant does not get its substance from the soil, but wrongly believed it came entirely from the water.

• A century later, English researcher Joseph Priestly discovered that, while a mouse withina closed space quickly dies due to lack of oxygen, it continues to live if a plant is alsoplaced within the space. He rightly concluded that the plant restored the oxygen that the mouse consumed, but was unable to consistently repeat his experiments.

• Just a few years later, in 1778, the Dutch physician Jan Ingenhousz demonstrated thatall Priestly’s plant required to restore the oxygen that the mouse consumed was regular exposure to sufficient light. With light, both plant and mouse survive.

• Many researchers have since added to our understanding of photosynthesis