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.
Carbon gives the best stored energy to weight ratio of any element.
Because oxidized carbon is not readily found in the environment, it puts a limit on how much energy plants can produce. The problem isn't in bad in water, where carbon dioxide an disassociate into carbonic acid. Although water can hold about the same amount of carbon dioxide as is present in the air, the formation of carbonic acid creates a sink for inorganic carbon. As aquatic plants use of the carbon dioxide in water, some of it is replaced by the carbonic acid, making the effective concentration about 100 to 200 times that in the air. Thus, aquatic plants tend to be more efficient at converting solar energy into reduced carbon than terrestrial plants.