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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.
Energy is obtained more efficiently in the presence of oxygen via the process called aerobic respiration. When there is no oxygen available to break the sugar molecules, some cells are still able to produce energy by the process called fermentation or anaerobic cellular respiration or anaerobic glycolysis. Certain organisms do not need molecular oxygen to produce the energy molecules called ATP.