The answer is Carbon is a source of energy.
Plants absorb carbon dioxide and make carbohydrates in a process of photosynthesis. Carbohydrates have many C-H bonds which breaking down releases such amount of energy. Also, lipids are the macromolecules with the highest number of carbon-hydrogen (C-H) bonds, so they contain more C-H bonds than carbohydrates. So, carbon is the part of macromolecules containing potential energy in the form of C-H bonds. At the end, carbon is the source of energy.