Photosynthesis is the quiet chemistry in every green leaf that turns sunlight, water, and carbon dioxide into sugar and releases oxygen. It is the engine on which nearly all life runs: almost every food chain begins with an organism doing it, and the breathable air is its by-product. It also caused Earth's first great pollution crisis, the Great Oxidation, when the oxygen early microbes poured out was poisonous to most life of the time. Fossil fuels are ancient stored photosynthesis, so the modern carbon and climate problem is, at its root, downstream of this single reaction.
Almost everything you have ever eaten began as sunlight. A blade of grass, a stalk of wheat, the plankton a fish once grazed: each was built by a reaction inside green cells that takes sunlight, water, and carbon dioxide and turns them into sugar, breathing out oxygen as it goes. That oxygen is the gas you are using to read this sentence. Yet when the first microbes learned the trick, billions of years ago, the oxygen they released was a poison that killed most of the life around them. Photosynthesis is the slow engine that has run the living world ever since, and it built the air before anything could breathe it.