Exergonic and Endergonic Reactions

Summary


1. Exergonic reactions are those in which the free energy of the final state is less than the free energy of the initial state.

This represents energy that can be used to do biological work

2. Endergonic reactions are those in which the free energy of the initial state is less than the free energy of the final state.


Reactions

Reactions can be described as a change in state

Each state has an associated Entropy (S), Enthalpy (H) and Free Energy (G)


S1 = entrophy of state 1
S2 = entrophy of state 2
H1 = enthalpy of state 1
H2 = enthalpy of state 2
G1 = free energy of state 1
G2 = free energy of state 2

The relationship between these three parameters is given by the equation

(G2-G1) = (H2-H1) - T (S2-S1)

 

Exergonic reactions

If the free energy of state1 is greater that the free energy of state 2 then the value of the free energy change for the reaction is a negative number. (-G)

In an exergonic reaction the change is free energy is represented by a negative number (-G), indicating free energy is released during the reaction

This statement has the following implications

free energy is liberated during the reaction.

This represents energy that can be used to do biological work

the reaction will occur spontaneously. However, the reaction does not necessarily occur at a great rate. This is where enzyme come in.

 


Endergonic reactions

In an endergonic reaction the free energy of the initial state is lower than the free energy of the final state.

This kind of reaction is not termed a spontaneous reaction. In order to go from the initial state to the final state a considerable amount of energy must be imparted to the system. These kinds of reactions are associated with a positive number (+G).


In an exergonic reaction the change is free energy is represented by a negative number (-G), indicating free energy is released during the reaction

 

This statement has the following implications

free energy is liberated during the reaction.

This represents energy that can be used to do biological work

the reaction will occur spontaneously. However, the reaction does not necessarily occur at a great rate. This is where enzyme come in.


Enzymes increase the rates of exergonic reactions


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