An Introduction to Metabolism pt. 2 – Free Energy Change

Estimated Reading Time: 5 Minutes


Hey All!

This blog post is a part of STEMTalksNC’s ever-expanding General Biology Series.  In the next series of posts, we will discuss metabolism, which will help you understand how matter and energy flow during life’s processes and how that flow is regulated. This is the next post of this series on metabolism, discussing free energy!

The Free-Energy Change of a Reaction Tells Us Whether or Not the Reaction Occurs Spontaneously

It’s important to understand chemical reactions (whether they occur spontaneously or non-spontaneous) without assessing the energy and entropy changes in the entire universe for each reaction.

Free Energy change is a function that can determine whether the system is spontaneous without considering its surroundings. Free energy is the part of a system’s energy that can do work when pressure and temperature are uniform, as in a living cell. Free-energy change is calculated when a system changes- for example, during a chemical reaction. The change in free energy can be calculated for a chemical reaction using the formula:

Image result for free energy change formula

Once we know the free energy change, we can use it to predict the spontaneity of a reaction. Over a century of experiments have indicated that only processes with negative free energy change are spontaneous. This means that every spontaneous process decreases the system’s free energy. Another way to think of free energy change is the difference between the free energy of the final state and the free energy of the initial state.

Thus, free energy change can be negative only when the process involves loss of free energy (the free energy of the initial state is greater than the free energy of the final state). We can relate free energy to stability. Because a system has less free energy, the system in its final state is less likely to change and is, therefore, more stable than it was previously. In other words, unstable systems (higher free energy) move towards equilibrium to a more stable system (lower G). For a system to be at equilibrium (maximum stability), it has to be at the lowest free energy value.

Movements away from equilibrium are non-spontaneous and will have a positive free energy change (meaning it will be more unstable and have more free energy). A process is spontaneous and can perform work only when it is moving toward equilibrium (losing free energy).

Relating Free Energy and Metabolism

Based on their free-energy changes, chemical reactions are either Exergonic or Endergonic. Exergonic reactions proceed with a loss of free energy (free energy change is negative). The magnitude of free energy change for an Exergonic reaction represents the maximum amount of work the reaction can perform. The greater the decrease in free energy, the greater the amount of work can be done.

An Endergonic reaction absorbs free energy from its surroundings. This means that the reaction stores free energy in the molecules (free energy change is positive), and it is considered non-spontaneous because the magnitude of free energy change indicates how much energy is needed to “drive” the reaction. The greater the increase in free energy, the greater the energy is required to drive the reaction.

If a chemical process is Exergonic (downhill), releasing energy in one direction, then the reverse process must be Endergonic (uphill), consuming energy.

Image result for exergonic reaction

That’s all for this post. In the next post of this series on metabolism, we will discuss ATP’s role in Exergonic and Endergonic reactions! Feel free to share this post to people that might benefit!

-Tahmid Islam

Co-Founder and Editor of StemTalksNC



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