Photosynthesis is the process that drives nearly all of the energy economies on Earth. It is the mechanism by which plants and green algae do the work of converting sunlight into an effective fuel for driving protein and carbohydrate synthesis. These biomolecules are then used by the plant for its own growth and reproduction.
The useful organic compounds synthesized by plants eventually find their way into the food web. Herbivores, also called primary consumers, acquire the nutrients produced by photosynthesis via ingestion. The herbivores are then often eaten by carnivores, and the carnivores' bodies are eventually broken down by decomposers. New energy almost never enters the web at any stage, making the photosynthetic ability of the plants native to the ecosystem the essential first step in supporting life at every level.
Photosynthesis also aids animal life by producing molecular oxygen as a waste product. As this gas is secreted through pores on the plant's leaves, it becomes available in the atmosphere to be taken up by animals. Without the oxygen produced in this way, animal metabolism would be unlikely to function at the level it does, and it is possible that the continued existence of large, multicellular life forms would be impossible.