What is the major atmospheric by-product of photosynthesis?

Photosynthesis is the process in which plants use light to convert carbon dioxide (CO2) and water (H2O) into simple sugars (C6H12O6) and oxygen (O2). Six molecules each of CO2 and H2O turn into one molecule of C6H12O6 for plant use and six molecules of O2 exhausted into the atmosphere.

Photosynthesis is one of the most important processes for the propagation of carbon-based life. It is responsible for a sizable percentage of the oxygen in Earth's atmosphere, it is a process by which inorganic carbon-based compounds (such as CO2) can be converted into organic compounds (such as sugars), and is the source of much of the energy necessary for life on Earth.