This energy can be used at many places in the cell, to synthesize a protein, to build a membrane, etc. Frying an egg is an example of an irreversible , endergonic reaction. Reversible reactions move toward an equilibrium , a state in which the reaction occurs at about the same rate in both directions. If all 686 kcal of energy were released at one time, it would be lethal. The energy released at any one step is much smaller than the total amount for the overall reaction, and the cell can deal with these smaller amounts without being killed.
Enzymes reduce the energy needed by the reactants to be chemically altered.
Enzymes usually will not function at incorrect pH or temperature. When product levels fall, enzyme can refold back to its active shape, and begin to function again.