Initially, most of this carbon was in an inorganic state, but life has transformed much of that carbon into organic forms. However, plants still need inorganic carbon for photosynthesis. They usually obtain that carbon in the form of carbon dioxide, which is an oxidized form of the atom. The carbon dioxide comes from the atmosphere, where it is found in very low concentrations. This makes carbon dioxide a major limitation of photosynthesis, having all the sunlight in the world can't make a plant fix more carbon if there isn't enough carbon dioxide.
Carbon is fairly unique when it comes to elements, it can bind to four other atoms.
These bonds are fairly strong, meaning they can store lots of energy. That means there is the maximum amount of energy stored in the molecules. Other elements, like silicon, can also make four bonds and store lots of energy. However, each atom of silicon weight over twice as much as carbon. That is a lot of weight to carry around as an energy source.