# Why are catabolic reactions exergonic and anabolic reactions endergonic?

Catabolism breaks apart large molecules and anabolism creates large molecules. Catabolism is supposed to be exergonic, while anabolism is supposed to be energonic. Id on't really understand this.

By definition a chemical bond is at an energy minimum. So if catabolism is taking apart large molecules with many bonds, which are at an energy minimum, how is this supposed to release energy (be exergonic) if the bonds are already at an energy minimum?

By contrast why is it that anabolism, which is putting creating a large molecule (which is a minimum energy state) from many atoms is enderognic and thus requires energy? I mean you're creating a minimum energy state, why would this require energy?

• Besides the biological aspects consider that a stable molecule is at an energy minimum when isolated. If you put it an environment than you must consider the global situation and how the energy in that system can get redistributed to attain an even lower energy state. " So if catabolism is taking apart large molecules with many bonds, which are at an energy minimum, how is this supposed to release energy (be exergonic) if the bonds are already at an energy minimum?" because the fragments are at an even lower minimum. – Alchimista Jan 8 at 9:17

You could say there are three reasons why we eat food: To directly use the substances we ingest (e.g. vitamins), to use the substances we ingest to make new substances we need (e.g. amino acids in proteins), and to oxidize substances (in humans with $$\ce{O_2}$$ as the terminal electron acceptor) and harvest the Gibbs free energy. We choose food accordingly (e.g. we don't eat sand). Also, we don't eat Grignard reagents or other highly reactive substances because they are usually not available and we are not equipped to handle very exothermic reactions in our bodies. So the food we eat is fairly stable, and often it requires Gibbs free energy to drive biosynthetic reactions (explaining why many anabolic reactions are endergonic)