<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (September 2014) (Learn how and when to remove this template message) </Td> </Tr> <P> Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products . The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process, as weak so - called "high - energy" bonds are replaced by stronger bonds in the products . Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity . Cellular respiration is considered an exothermic redox reaction which releases heat . The overall reaction occurs in a series of biochemical steps, most of which are redox reactions themselves . Although technically, cellular respiration is a combustion reaction, it clearly does not resemble one when it occurs in a living cell because of the slow release of energy from the series of reactions . </P> <P> Nutrients that are commonly used by animal and plant cells in respiration include sugar, amino acids and fatty acids, and the most common oxidizing agent (electron acceptor) is molecular oxygen (O). The chemical energy stored in ATP (its third phosphate group is weakly bonded to the rest of the molecule and is cheaply broken allowing stronger bonds to form, thereby transferring energy for use by the cell) can then be used to drive processes requiring energy, including biosynthesis, locomotion or transportation of molecules across cell membranes . </P> <P> Aerobic respiration requires oxygen (O) in order to create ATP . Although carbohydrates, fats, and proteins are consumed as reactants, it is the preferred method of pyruvate breakdown in glycolysis and requires that pyruvate enter the mitochondria in order to be fully oxidized by the Krebs cycle . The products of this process are carbon dioxide and water, but the energy transferred is used to break bonds in ADP as the third phosphate group is added to form ATP (adenosine triphosphate), by substrate - level phosphorylation, NADH and FADH </P>

Why must combustion in cells take place in multiple steps