How does photosynthesis work?

Chlorophyll and associated pigments in chloroplasts absorb light, principally in the blue and red parts of the spectrum, reflecting green, which is why leaves look green. That captured energy drives two linked stages. In the light-dependent reactions, water molecules are split, releasing oxygen as waste and generating energy carriers. In the Calvin cycle, those carriers power the assembly of carbon dioxide drawn from the air into sugar molecules. The overall summary is that carbon dioxide plus water plus light yields glucose plus oxygen, though the actual pathway involves dozens of enzymatic steps.

The scale of the consequence is difficult to overstate. Atmospheric oxygen is almost entirely biological in origin, produced over billions of years by photosynthetic organisms, beginning with cyanobacteria whose activity caused the Great Oxidation Event roughly 2.4 billion years ago and transformed the planet. Marine phytoplankton, not forests, are responsible for a very large share of current oxygen production. And essentially every food chain begins with photosynthesis, since animals ultimately eat plants or eat animals that ate plants, and fossil fuels are stored photosynthetic energy from ancient organisms.

Two refinements matter agriculturally. Most plants use the C3 pathway, which loses efficiency in hot dry conditions because the key enzyme sometimes binds oxygen instead of carbon dioxide in a wasteful process called photorespiration. Corn, sugarcane, and many tropical grasses evolved the C4 pathway, which concentrates carbon dioxide and largely avoids that loss, making them more productive in heat. Succulents use a third variant, CAM, opening their pores only at night to conserve water. Improving photosynthetic efficiency in staple crops is an active research area with substantial implications for food supply.