Endergonic Reactions in Overall Exergonic Environments

Background

When talking about endergonic and exergonic reactions you learn that endergonic reactions consume energy and store it; as in making bonds. On the other hand, exergonic reactions will release energy by breaking bonds and usually release heat at the same time.

Endergonic Reactions in Exergonic Environments

When you first learned about endergonic and exergonic reactions they were presented without any energy requirements. Now ask yourself where do these reactions get their energy?

• Exergonic  reaction: They break a bond that has stored energy. So the energy is in the bonds already.
• Endergonic reaction: They create bonds that store energy from…(where does the energy come from!)

Endergonic reactions must be coupled or attached to exergonic reactions that have an excess of energy. Otherwise where would the energy come from?

Endergonic Reactions Attached To Exergonic Reactions

Endergonic reactions DO NOT happen spontaneously so they must be coupled or attached to exergonic reactions that will happen at will. Remember endergonic reactions need an energy source to occur.

For example linking amino acids together to form proteins (translation) is a common endergonic reaction. From the point of view of the “reactants” ,the actual amino acids pieced together, there is an endergonic reaction. From the point of view of the surrounding, the “cell” , there will be an overall exergonic reaction because the endergonic reaction was attached to another more powerful exergonic reaction. The exergonic reaction has enough energy to fuel itself AND the endergonic reaction, otherwise the endergonic reaction just wont happen.

Real example

An example of endergonic reactions attached to exergonic reactions is the production of glucose-6P. Below is the amount of energy released(negative number) or consumed (positive number). The overall negative means the reaction will happen spontaneously or it has enough energy to happen at that very moment.

Bioluminescence:

Bioluminescence is another example of an endergonic reaction, but it is attached to ATP release and many other steps that are are exergonic reactions. Sample principle as above.

In fireflies the molecule involved in the endothermic portion of bioluminescence is called Luciferin. Luciferin is attached to ATP release and there is enough energy left over to produce light :). Why use Luciferin? It can be easily bound to ATP, then regenerated back to “plain old luciferin” without ATP. So light can be produced over and over again.

Summary

1. Endergonic reactions need an external energy source to occur
2. More powerful exergonic reactions must be coupled to endergonic reactions
3. Bioluminescence involves an endergonic reaction that is attached to a more powerful exergonic reaction