As shown above, the reactants (a and BC)are at a higher energy level than the products(AB and C). However, for the reaction to occur, a and bc need to go through a high energy"transition state" before forming the products. The activation energy, Ea, is the energy difference between the reactants and the transition state. Thus, for the reaction to proceed, energy is needed to push the reactants over the Ea hump. Due to random fluctuations in energy caused by molecular motion,some of the reactant molecules may posses just enough energy to reach the transition state and form the products however the higher the ea hump is the less likely the reactants can do so when a thermodynamically favorable reaction fails to occur spontaneously due to a high Ea, the reaction is kinetically trapped. what can we do to make reactions occur faster? If you are thinking about catalysts, you are absolutely right. Catalysts make reactions more reach the transition state and form products. See figure below where an enzyme lowers cules to favorable by lowering the activation energy. As Ea is lowered, it is easier for reactant mole Transition state cu Ea without enzym g A+BC Products Figure by MIT OCW Now consider the reverse reaction Ab+C)A+BC This reaction is endergonic. Also, the activation energy in this reverse reaction is greater than that in the forward reaction. the energy difference between(AB+c)and the transition state is greater than the energy difference between(A+ BC)and the transition state. Notice that the enzyme lowers Ea in both the forward and the reverse reactions by the same magnitude. Also, the enzyme does not change the energy levels of AB, C, A, and BC. Thus, catalysts hasten reaction rates in both the forward and reverse directions by lowering energy of activation. Catalysts do not change the thermodynamic properties of reactions, such as the equilibrium, which dictates the relative concentrations of reactants and products So why do we care? Most biological reactions(the ones needed to sustain life and to adapt to environmental changes)do not occur fast enough without catalysts. Furthermore, some very important reactions are endergonic. Even if the Ea is considerably lowered by an enzyme, additional energy is still required for reactants to form products efficiently. So how does a cell do so? This brings us to the example Glycol 7.012 Fall 2004 lecture 5 notes2 As shown above, the reactants (A and BC) are at a higher energy level than the products (AB and C). However, for the reaction to occur, A and BC need to go through a high energy “transition state” before forming the products. The activation energy, Ea, is the energy difference between the reactants and the transition state. Thus, for the reaction to proceed, energy is needed to push the reactants over the Ea hump. Due to random fluctuations in energy caused by molecular motion, some of the reactant molecules may posses just enough energy to reach the transition state and form the products. However, the higher the Ea hump is, the less likely the reactants can do so. When a thermodynamically favorable reaction fails to occur spontaneously due to a high Ea, the reaction is kinetically trapped. What can we do to make reactions occur faster? If you are thinking about catalysts, you are absolutely right. Catalysts make reactions more favorable by lowering the activation energy. As Ea is lowered, it is easier for reactant molecules to reach the transition state and form products. See figure below where an enzyme lowers Ea. Now consider the reverse reaction: AB + C  A + BC This reaction is endergonic. Also, the activation energy in this reverse reaction is greater than that in the forward reaction. The energy difference between (AB + C) and the transition state is greater than the energy difference between (A + BC) and the transition state. Notice that the enzyme lowers Ea in both the forward and the reverse reactions by the same magnitude. Also, the enzyme does not change the energy levels of AB, C, A, and BC. Thus, catalysts hasten reaction rates in both the forward and reverse directions by lowering energy of activation. Catalysts do not change the thermodynamic properties of reactions, such as the equilibrium, which dictates the relative concentrations of reactants and products. So why do we care? Most biological reactions (the ones needed to sustain life and to adapt to environmental changes) do not occur fast enough without catalysts. Furthermore, some very important reactions are endergonic. Even if the Ea is considerably lowered by an enzyme, additional energy is still required for reactants to form products efficiently. So how does a cell do so? This brings us to the example of II. Glycolysis 7.012 Fall 2004 lecture 5 notes A B C Transition state A + BC Reactants Free Energy AB + C Products Ea without enzyme Ea with enzyme Figure by MIT OCW