The process of photosynthesis converts CO₂ (carbon dioxide) to organic material by reducing this gas to carbohydrates in a complex set of reactions. Electrons for this reduction reaction ultimately come from water, which is then converted to oxygen and protons.

All the oxygen in Earth's atmosphere is believed to have come from photosynthesis. All respiring plants and animals use the oxygen and produce CO₂. Photosynthesis and respiration are similar in that both processes are dependant on the others product. On a global scale, about a trillion kg of dry organic matter is produced by photosynthesis per day, and likewise, the respiratory processes of the Earth's inhabitants convert about the same amount of organic matter to CO₂. The vast majority of this process happens in our oceans mostly by oxygenic procaryotes (bacterium or a blue-green alga).

Without plants, from alga to the trees in the rainforests, there is no photosynthesis. Without photosynthesis, the oxygen in the atmosphere would be gone in a few thousand years. Without oxygen . . .