<Table> <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> </Table> <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 cellular respiration is technically 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>

Where does o2 come from in cellular respiration