Nearly all of the fixed carbon on earth is due to photosynthesis, and the atmosphere of the earth has been transformed from one of reducing power to one of oxidizing power.
Carbon dioxide is the product of combustion of hydrocarbons and carbohydrates.
Our sun supplies the energy to reverse the process of combustion and drive the synthesis of complex organic molecules. A summary is then made of the electron transfer that shortly follows the excitation, and that supplies the energy to fix atmospheric carbon dioxide, a very stable molecule, into sugars and starches. There are approximately 250 antenna chlorophyll molecules for every reaction center.