How Do Plants Make Oxygen?

By Robert Korpella
People depend on plant life for the oxygen they breathe.

Oxygen is a byproduct released when plants engage in photosynthesis, the process they use to produce their own food. The chemical events that occur during photosynthesis are complex. The result is that six carbon dioxide molecules and six water molecules become six glucose molecules and six oxygen molecules. The word "photosynthesis" means “making things with light.”

Water and Nutrients

Plant roots absorb water and some nutrients from the ground. Water moves up the plant through xylem, which is a specialized tissue inside the plant. Water is needed as a means of transporting vital nutrients throughout the plant, and it becomes a reducing agent for the chemistry that occurs during the photosynthesis process. The plant uses water to break down carbon dioxide molecules before they can be reassembled into the sugar substance the plant uses as stored energy.

The Sun’s Energy

Photons in sunlight provide the energy required for photosynthesis to occur. The plant captures these photons with light-absorbing pigments, such as chlorophyll and carotenoids. These pigments are also responsible for the green leaves on plants. Chlorophyll and carotenoids do not effectively absorb green or yellow light from the color spectrum. As a result, these colors reflect away from the leaves, making them appear green to our eyes.

Carbon Dioxide

Plants absorb carbon dioxide from the air through tiny openings in the plant’s epidermis, or outer tissue layer. Called stomata, these microscopic pores open and close as the plant’s needs change from absorbing carbon dioxide to expelling oxygen and water. Carbon dioxide is present in the atmosphere from animal exhalation during respiration, as well as from decaying organic matter.

Making Glucose

Once all the necessary ingredients are in place, the plant tears apart some of the water it has absorbed through the roots and the carbon dioxide it has taken in from the atmosphere. A series of chemical reactions and the sun’s energy disassembles the molecules. Another series of chemical reactions, again using the sun’s energy, reassembles the resulting atoms into glucose molecules. The plant stores most of this simple sugar for growth and consumes a little during photosynthesis. The oxygen molecules are not needed by the plant, so they are expelled through the stomata. As a result of photosynthesis, animals live in a symbiotic relationship with all plant life on Earth. Plants require the carbon dioxide animals expel, while animals require the oxygen plants produce.

About the Author

Robert Korpella has been writing professionally since 2000. He is a certified Master Naturalist, regularly monitors stream water quality and is the editor of, a site exploring the Ozarks outdoors. Korpella's work has appeared in a variety of publications. He holds a bachelor's degree from the University of Arkansas.