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Photosynthesis

Photosynthesis is how plants turn sunlight into food. Inside the chloroplasts, chlorophyll captures light energy and uses it to combine carbon dioxide from the air with water drawn up from the roots. The result is glucose, a sugar the plant uses for energy, and oxygen, which is released back into the atmosphere for the rest of us to breathe.

The process happens in two connected stages. In the light-dependent reactions, energy from sunlight is used to split water molecules, releasing oxygen and storing energy in two carrier molecules, ATP and NADPH. These reactions take place in the thylakoid membranes stacked inside each chloroplast.

In the second stage, the Calvin cycle, the plant uses that stored energy to build sugar from carbon dioxide. This stage does not need light directly, so it is sometimes called the light-independent reaction. Enzymes fix the carbon into a stable molecule that is gradually rearranged into glucose.

Together these stages let a plant capture a tiny fraction of the sunlight that lands on its leaves and lock it away as chemical energy — the same energy that eventually powers nearly every living thing on Earth, from the insects that eat the plant to the animals that eat the insects.