Plants make oxygen by using light energy to convert water and carbon dioxide into sugar and oxygen molecules. This occurs during the process of photosynthesis.
Plant cells contain organelles called chloroplasts. These organelles contain chlorophyll, which helps absorb energy from the sun. During photosynthesis, plants use this energy to convert carbon dioxide and water into sugar. The end result of this reaction is one molecule of sugar and six molecules of oxygen, according to Janet L. Stein Carter of the University of Cincinnati-Clermont College.
When photosynthesis is complete, plants release oxygen into the air. Animals breathe in this oxygen when they inhale and release carbon dioxide when they exhale. Plants use the carbon dioxide exhaled by animals to continue the oxygen cycle.