In a series of choreographed chemical reactions, the free energy in each ATP molecule is released and used to split the carbon and oxygen in atmospheric carbon dioxide and repackage them into molecules known as carbohydrates. This is known as “fixing” carbon and it has two main purposes. First, carbohydrates provide building block materials such as cellulose, which give the plant structure. Second, making carbohydrates doesn’t use up all the energy stored in the ATP. The unused energy is, in effect, transferred into the carbohydrate molecules.
