One of the plant kingdom's most important roles---at least as far as human and other animal life is concerned---involves the production of oxygen through the process of photosynthesis. Light drives photosynthesis, making life on Earth dependent on energy from the sun. With plants, and without light to power photosynthesis, the continued renewal of oxygen supplies would not occur.
Photosynthesis begins when plants take in two compounds: water absorbed through their roots and carbon dioxide taken in from the air. The atoms in these molecules could be recombined to form sugar and oxygen, but breaking and reassembling chemical bonds requires energy. Here, light comes in. Light provides the energy needed to power the transformation of water and carbon dioxide into sugar and oxygen. The plant uses the sugar for energy and releases the oxygen back into the air.
To use chemical energy produced during photosynthesis by plants or consumed by animals, all organisms engage in cellular respiration, a process by which energy-rich molecules are broken down and the energy released. Respiration requires oxygen, as explained by biology professor John W. Kimball, so plants play an essential role in supporting not only their ability to live, but the lives of other organisms as well.
Photosynthesis increases with light intensity, to a point, meaning that as light brightens, oxygen production rises. For example, a plant grown in bright sun produces more oxygen than a plant kept in a shadowy corner in your house. You can only observe this effect to a point, however. Once light gets too bright, photosynthesis---and, therefore, oxygen production---levels off, as shown in an online lecture from Texas A&M University.
Light also varies in terms of color, or wavelength, with longer wavelengths appearing red and shorter wavelengths appearing blue or violet. Plants use all wavelengths of light except for green light when undertaking photosynthesis. Therefore, exposure to all wavelengths of light except green causes plants to produce oxygen.
The role of plants in not only producing oxygen but also eliminating carbon dioxide from the atmosphere makes them vital to solving environmental problems. A conservative estimate of plants' oxygen-producing abilities, presented by researchers Nowak, Hoehn and Crane in the journal "Aboriculture and Urban Forestry," finds that an acre of trees provides enough oxygen for eight people. Other estimates almost triple that amount.