Experiment showing a Hydrilla verticillata plant photosynthesising underwater. At the start the plant is in dim light, and there is a slow stream of bubbles coming from the stem. When the light is turned on, the stream of oxygen bubbles becomes much faster. This indicates that more photosynthesis is occurring. Photosynthesis is the biochemical process that plants use to produce sugars from atmospheric carbon dioxide, producing oxygen as a side_product. The process is driven as they harness energy from sunlight using the green pigment chlorophyll. All the oxygen in the atmosphere has been produced by photosynthesising plants.