Respiration is one of the key ways a cell releases energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are redox reactions. Although respiration is technically a combustion reaction, it clearly doesn't resemble one when it occurs during a living cell due to the slow, controlled release of energy from the series of reactions. Cellular respiration may be a set of metabolic reactions and processes that happen within the cells of organisms to convert energy from oxygen molecules or nutrients into ATP (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy because weak high-energy bonds, in particular in molecular oxygen are replaced by stronger bonds in the products.