Green plants do not eat in the way animals do. They build sugar from air and water using light. Inside leaves, chloroplasts hold chlorophyll, a pigment that absorbs red and blue wavelengths and reflects green, which is why foliage looks the color it does. Energy from those absorbed photons splits water, releasing oxygen as a waste gas. The hydrogen and the carbon from carbon dioxide are assembled into glucose. That reaction is photosynthesis, and it is the entry point of almost all food energy on land and in the sunlit sea.

The oxygen in every breath is a byproduct of this chemistry, accumulated over geological time. Early Earth had little free oxygen. When photosynthetic microbes became abundant, the atmosphere changed, rusted the iron in the oceans, and made complex animal life possible. Forests, grasslands, phytoplankton, and crops are all running versions of the same invention.

Photosynthesis is not evenly spread. It is strongest where light, water, and nutrients meet: rainforests, estuaries, spring blooms in cold seas. Deserts and the deep ocean produce little. Humans now harvest a large share of land photosynthesis as food, timber, and fuel. The carbon plants lock away is the same carbon that industry returns to the air when those stores are burned. The leaf is still the planet’s most important factory, quiet and everywhere at once.
