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... organism to do work endergonic reactions must be linked with exergonic reactions. The energy that the body requires is provided by respiration where the aerobic oxidation of a glucose molecule provides 2870 kJ of energy. Respiration is not just one big reaction but is in-fact a series of small steps with each reactions being regulated by enzymes. The enzymes help to lower the activation energy of glucose so that aerobic respiration can take place relatively easily. Theoretically, the energy released from respiration could be harnessed in the cell so that it can be used to do work directly. However, a much more flexible system actually occurs where exergonic reactions in all organisms are linked to the production of an intermediary molecule, ATP (adenosine triphosphate). The structure of ATP consists of the purine base, adenine, a 5C ribose and 3 phosphate groups. When ATP is hydrolysed, a phosphate Pi is lost ...