Overall, plant photosynthesis converts carbon dioxide from the air into simple organic molecules such as sugar. These are subsequently burnt by the plant in its mitochondria to produce more ATP , and converted into the wealth of carbohydrates, lipids, proteins and nucleic acids that make up life. We met the enzyme that cobbles hydrogen onto carbon dioxide rubisco , the most abundant enzyme on the planet. But rubisco needs to be spoon-fed with its raw material . Carbon comes from the air, or is dissolved in the oceans so that is easy. Hydrogen on the other hand, is not readily available - it reacts quickly (especially with oxygen to form water) and is so light that it can evaporate away into outer space. Hydrogen therefore needs a dedicated supply system of its own. This is in fact the key to photosynthesis, but for many years the look resisted picking. Ironically, the mechanism only become clear when researchers finally understood where the oxygen waste came from.