Which of the following describes the given graph correctly ?

NCERT CLASS-XI – Page No. 156.

 The ES complex formation is a transient phenomenon. After the ES complex formation,a new structure of the substrate called transition state structure is formed. There could be many more ‘altered structural states’ between the stable substrate and the product. All these are unstable in terms of energy. Stability is something related to the energy status of the molecule or its structure.

Enzymes eventually bring down this energy barrier, making the transition from ‘S’ to ‘P’ easier.

In the graph, the y-axis represents the potential energy content. 

The x-axis represents the progression of the structural transformation of states through the ‘transition state’. 

 Two things to be noticed in the graph. The energy level difference between S and P. If ‘P’ is at a lower level than ‘S’, the reaction is an exothermic reaction. One need not supply energy (by heating) in order to form the product. However, whether it is an exothermic or spontaneous reaction or an endothermic or energy requiring reaction, the ‘S’ has to go through a much higher energy state or transition state. The difference in average energy content of ‘S’ from that of this transition state is called ‘activation energy’.