Photosynthesis is important, in fact essential, to all life for a number of reasons. First, it makes it possible for plants to survive. Since almost the entire food chain depends on plant matter either directly or indirectly, life as we know it would come to an end without photosynthesis--with very few exceptions, both herbivores (animals that consume plants) and carnivores (animals that consume other animals) would starve. Only very simple life forms could live. Photosynthesis is important for another reason. It is a chemical reaction that uses carbon dioxide from the atmosphere, water, and light to create sugars that nourish plants. But it also produces oxygen, which is essential to almost all life. Because of photosynthesis, plants essentially function as filters that take carbon dioxide, which is poisonous to humans and many other life forms, from the atmosphere and replace it with oxygen, which makes life possible.