The Reaction That Changed Everything
Photosynthesis captures light and stores it as chemical bonds:
It runs in two connected stages. The light reactions, in the thylakoid membranes, split water, release oxygen, and produce ATP and NADPH. The Calvin cycle, in the stroma, spends that ATP and NADPH to fix carbon dioxide into sugar. The second stage is often called light-independent, which is true only in the narrow sense that light is not directly required, it stops within minutes in the dark, once the ATP and NADPH run out.
The Calvin cycle's arithmetic is worth knowing because it explains the cost. Fixing 3 molecules of CO₂ yields one three-carbon sugar and consumes 9 ATP and 6 NADPH, so a full six-carbon glucose costs 18 ATP and 12 NADPH.
The oxygen released is a waste product, and it transformed the planet. The Great Oxidation Event, about 2.4 billion years ago, was arguably the largest pollution event in Earth's history: oxygen was toxic to most existing anaerobes and drove a mass extinction. But it also made aerobic respiration possible, which yields far more energy per glucose than fermentation, roughly 30-32 ATP against 2, and that energy budget is what large, complex, active organisms run on.
Oxygen also produced the ozone layer, which absorbs ultraviolet radiation. Until it existed, the land surface was effectively sterilised by UV, and life was confined to water. The colonisation of land had to wait for a by-product of photosynthesis to build a shield.
Common pitfall: treating the Calvin cycle as something that happens at night. It needs ATP and NADPH from the light reactions, so it stops shortly after darkness falls. "Light-independent" means it does not use photons directly, not that it runs without light.