Tukuk-OSOne line carries most life on Earth: carbon dioxide plus water, powered by light, becomes sugar plus oxygen. Inside chloroplasts, plants and algae capture sunlight with the green pigment chlorophyll and spend its energy tearing water apart — releasing the oxygen you are breathing as the exhaust of that step — then using the freed electrons to assemble carbon from CO₂ into sugars. Every calorie of food, and nearly every breath of oxygen, passed through this reaction at some point in its chain.
The efficiency is humbling: crops convert only a few percent of the sunlight that hits them into stored chemical energy, and even that small share is why a field feeds a family while a bare rock feeds nothing. The slow-motion version of the same equation is also why every fossil fuel is ancient stored sunlight: coal, oil and gas are photosynthetic products from past ecosystems, buried before decay could spend them back.
Who does the work matters for the air: ocean phytoplankton — microscopic drifting algae — contribute roughly half of global photosynthetic oxygen production, which is why the often-repeated claim that “the Amazon produces 20% of the world’s oxygen” misses the accounting: mature forests respire (burn back) nearly all the oxygen they make. The net gift to the atmosphere comes mostly from the sea and from young, growing wood.