Energy transformations are an important part of the functioning of ecosystems, and a key part of those energy transformations is photosynthesis. Photosynthesis is the process by which plants, bacteria, and other organisms use the energy of sunlight to manufacture food in the form of sugar. If it weren’t for photosynthesis, the energy source (sunlight) for all ecosystems would be useless. In other words, we’d all die, even though we might have really nice tans.