Experiment showing an eelgrass plant Vallisneria sp. photosynthesising underwater. At the start the plant is in dim light, and no bubbling is seen. When the light is turned on, bubbles of oxygen are seen coming from the leaves. This indicates that 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.