Reactions that require energy to initiate the reaction are known as endergonic reactions. Image from W.H. Freeman and Sinauer Associates, used by permission.
Reduction is the gain of an electron.
Such coupled reactions are referred to as redox reactions. Image from W.H. Freeman and Sinauer Associates, used by permission. Image from W.H. Freeman and Sinauer Associates, used by permission. For example humans can synthesize 12 of the 20 amino acids, we must obtain the other 8 in our diet. Catabolism is the series of chemical reactions that breakdown larger molecules.