Photosynthesis is the process in which plants use carbon dioxide and water, in the presence of sun light, to make food. If we mix carbon dioxide and water in a test tube and kept it in the sunlight, we will not see any photosynthesis. This process requires plants, more specifically plants containing a special pigment called chlorophyll. Without this chemical, we cannot replicate photosynthesis process. This process can be chemically summarized as:
`6CO_2 + 12 H_2O + light -> C_6H_(12)O_6 + 6O_2 + 6H_2O`
Photosynthesis is the process that generates food for all the higher life forms, thus plants are the main producers in an ecosystem. That and oxygen production are the chief reasons we need green plants. If replication of photosynthesis was so easy as mixing carbon dioxide and water in a test tube, we could have done away with plants and life would have very easy, esp. for astronauts (who have to carry food with them).
Hope this helps.