The primary source of energy for photosynthetic organisms is sunlight. Through the process of photosynthesis, these organisms, such as plants and algae, convert light energy into chemical energy stored in glucose. This process occurs in chloroplasts, where chlorophyll absorbs sunlight to drive the conversion of carbon dioxide and water into glucose and oxygen. This energy conversion is crucial for the entire food web, as it supports not only the primary producers but also the consumers and decomposers in the ecosystem.