photosynthesis

Almost all the energy in the living world traces back to sunlight, but animals cannot eat light. Something has to catch sunlight and pack it into food. That something is photosynthesis, the process by which plants, algae, and some bacteria turn light, water, and carbon dioxide from the air into sugar. It is, in effect, how the world's food and most of its oxygen get made.

The overall reaction is the mirror image of respiration: carbon dioxide plus water, powered by light, yields sugar plus oxygen. Photosynthesis happens in two linked stages inside the chloroplast. First, the light reactions capture sunlight and use it to make energy carriers (ATP and a carrier called NADPH) while splitting water and releasing oxygen. Second, the Calvin cycle uses that ATP and NADPH to build sugar from carbon dioxide, with no light needed for those building steps themselves. So light reactions catch the energy; the Calvin cycle spends it to make food.

Photosynthesis is the foundation of nearly every food chain and the source of the oxygen we breathe; it also pulls carbon dioxide out of the air, which ties it directly to climate. A common misconception is that plants "breathe in" carbon dioxide instead of oxygen as their respiration — in fact plants do cellular respiration too, using oxygen like we do; photosynthesis is an additional, separate process that captures far more energy than the plant burns.