How does a leaf turn light and air into sugar?

A leaf makes sugar in two stages. In the first, light is caught and turned into chemical energy. In the second, that energy is spent to turn carbon dioxide from the air into sugar.

The first stage happens inside chloroplasts, tiny compartments in leaf cells; a typical plant cell holds about 10 to 100 of them. Inside, stacks of flattened disks called thylakoids carry the green pigment chlorophyll. When chlorophyll absorbs a photon, it loses an electron. That electron races down a chain of molecules and ends up on a carrier called NADPH, while the same flow builds up a gradient that an enzyme uses to make ATP, the cell's energy-storage molecule. Chlorophyll then gets its electron back by splitting a water molecule, and that split is where the oxygen we breathe comes from. For the plant, oxygen is just a waste product.

The second stage happens in the watery fluid around the thylakoids, called the stroma. There an enzyme named RuBisCO grabs CO2 from the air, and a loop of reactions called the Calvin cycle uses the ATP and NADPH from stage one to turn it into three-carbon sugars. Those are later combined into sucrose and starch.

This second stage is often called the "dark reaction", which sounds like it runs at night. It does not: it needs NADPH, which is short-lived and comes straight from the light reactions. So a leaf is not two separate machines taking turns on a day-night schedule, but one production line where the first stage feeds the second.