Photosynthesis is the process by which plants effectively capture energy from the sun and store it for later use by organisms. It's the fundamental process that enables the entire food chain. Actual photosynthesis is a highly complex process involving many steps and many different chemicals. But the idea of what is happening to the overall energy balance can be seen even in simpler reactions. Let's consider a toy model that shows how an input of energy (for example from the sun) could be used to combine water and carbon dioxide to create an organic molecule (formaldehyde) and oxygen and effectively "store" that energy for use later. The figure below shows a the simplest possible such process. The potential energy (binding deficit) of each bond is shown on the bonds.