What gas is released as a byproduct of photosynthesis?

The gas released as a byproduct of photosynthesis is oxygen.

During the light-dependent reactions, chlorophyll absorbs light energy and drives the splitting of water molecules. This process supplies electrons and hydrogen ions for the photosynthetic reactions, while molecular oxygen is released from the water-splitting reaction.

A simplified overall equation is six molecules of carbon dioxide plus six molecules of water producing glucose and six molecules of oxygen, using light energy. Plants use the glucose and related carbohydrates as chemical energy and building material, while much of the oxygen diffuses out through microscopic leaf pores called stomata.

A frequent misconception is that the released oxygen comes directly from carbon dioxide. Isotope-tracing experiments show that the oxygen gas released during oxygenic photosynthesis comes from water. Carbon dioxide supplies the carbon incorporated into sugars. Oxygenic photosynthesis by plants, algae, and cyanobacteria is responsible for most of the oxygen accumulated in Earth's atmosphere.