# 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.

Biology (university) · Part 02: Enzymes and energy · Card 3 of 9 · Last updated 2026-10-19

## Analogy

A ball on a hill: it rolls down by itself, but needs a push to go up.

## Example

**Reading delta G**

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.

## Related concepts

- [ATP](https://studyvisually.com/learn/science/biology-university/biology-university-part-2/biology-university-part-2-concept-2)
- [Energy coupling](https://studyvisually.com/learn/science/biology-university/biology-university-part-2/biology-university-part-2-concept-4)
- [Enzyme](https://studyvisually.com/learn/science/biology-university/biology-university-part-2/biology-university-part-2-concept-5)
- [Metabolism](https://studyvisually.com/learn/science/biology-university/biology-university-part-2/biology-university-part-2-concept-1)

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