Some reactions take place in a single step. We can represent this using an energy profile . An activated complex (or transition state ) forms between reactant and product. This is not a 'real' substance in the sense that it can be isolated and put in a test tube. But based on various pieces of experimental evidence it is the chemist's model of how the reaction occurs. The energy 'hump' shows how much energy reacting molecules must have for a 'successful' collision, . one that leads to reaction. The formation of an activated complex requires energy to bring molecules together in the correct orientation. Therefore, it is always an endergonic reaction. The energy required is called the activation energy (E a ).
ATP is a 5-carbon sugar (ribose) attached to a nitrogenous base (adenine; recall our discussion of the nucleotides DNA and RNA) and a group of three phosphates. The three phosphates are the triphosphate component of adenosine triphosphate, and they are very unstable. This instability is due to the three negative charges that induce an intramolecular strain in one area of the molecule. Most reactions that involve ATP depend on the hydrolysis of the third phosphate to liberate the potential energy that can be used to do work. In addition to free energy that can do work, ADP (adenosine diphosphate) and inorganic phosphate (PO 4 ; abbreviated P i or a circled P) are released during hydrolysis. The overall reaction is summarized in the figure above. The amount of energy that is made available to do work during this process is variable (depending on a number of factors, including temperature).