How do plants make oxygen?

Plants make oxygen through photosynthesis, the process by which they convert sunlight, water and carbon dioxide into sugars for energy. Inside their leaf cells, structures called chloroplasts capture light energy and use it to split water molecules into hydrogen and oxygen. The hydrogen is used to help build sugars, while the oxygen is released into the air as a by-product — the very oxygen that most life on Earth depends on to breathe. Over billions of years, photosynthesis by plants, algae and cyanobacteria transformed Earths atmosphere, filling it with oxygen. Ocean phytoplankton, not forests, are thought to produce roughly half of the worlds oxygen.