Free energy
The energy in a system that is available to do work, written as G. A reaction is spontaneous if the change in free energy (delta G) is negative.
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Analogy
A ball on a hill: it rolls down by itself, but needs a push to go up.
Example
Delta G below 0: exergonic, releases energy.Delta G above 0: endergonic, needs energy.Spontaneous does not mean fast.
In practice
Burning glucose releases energy, so delta G is negative. Making glucose from carbon dioxide needs energy input, so delta G is positive.
Practice
Flashcards
- What does a negative delta G tell you?The reaction is spontaneous (exergonic) and releases free energy. It says nothing about how fast it goes.
Interview questions
- Explain how an enzyme speeds up a reaction without changing the free energy change of that reaction.
- Lowers activation energy
- Stabilises the transition state
- Does not change start or end energy levels
- Is not consumed
- Describe how ATP lets a cell carry out reactions that would not happen by themselves.
- Hydrolysis has large negative delta G
- Coupling makes overall delta G negative
- ATP is regenerated from ADP
Questions people ask
What is the difference between exergonic and endergonic?
An exergonic reaction releases free energy and has a negative delta G, such as burning sugar. An endergonic reaction absorbs free energy and has a positive delta G, such as building a protein from amino acids. Cells run endergonic reactions by pairing them with exergonic ones.
If a reaction is spontaneous, why might it not happen quickly?
Spontaneous only says the reaction is energetically downhill. It says nothing about speed. A hurdle called activation energy may be in the way. Sugar and oxygen react spontaneously, yet a bag of sugar sits safely in a cupboard until a spark or an enzyme gets it going.