Energy for Life
What Is the Difference between Endergonic and Exergonic Reactions?
Endergonic and Exergonic Reactions
The laws of thermodynamicsglossary term (opens in a new window) can be used to classify chemical reactions into two categories: reactions that take in energyglossary term (opens in a new window) from their surroundings and reactions that release energy into their surroundings.
Endergonic reactions require a net input of energy from the surroundings. In a biological system, free energy (G) is energy that can do work if temperature and pressure are held constant. The change in free energy (ΔG) of these reactions is positive because the products of the reaction contain more energy than the reactants. Because these reactions have a positive ΔG, they are not spontaneous; that is, they are not energetically favorable and require the input of energy to proceed.
Exergonic reactions release energy. The ΔG of these reactions is negative because the products of the reaction contain less energy than the reactants. This energy is released into the surroundings and can be used to power endergonic reactions. Because these reactions have a negative ΔG, they are spontaneous; that is, they are energetically favorable and occur without the input of energy.
In cellular processes, endergonic reactions are often linked to exergonic reactions. A common exergonic reactionglossary term (opens in a new window) that is used to power a variety of cellular processes is the breakdown of ATP into ADP (adenosine diphosphate). This breaking down of larger molecules into smaller molecules is exergonic and can be used to power endergonic reactions.
Cells regenerate their ATP store by adding a third phosphate to ADP; synthesizing larger molecules from smaller subunits is endergonic. Cells can break down food molecules (an exergonic process) to release the needed energy to build ATP (an endergonic process). ATP is then stored until its chemical energyglossary term (opens in a new window) is needed to do work.
Teacher Note
Emphasize to students that endergonic and exergonic reactions are not the same as endothermic and exothermic reactions. Although many exothermic reactions are also exergonic, such as burning wood, many are not, such as dissolving table salt in water. Although dissolving salt in water is spontaneous, it is endothermic. Have students demonstrate their understanding of these reactions using the Snowball Fight strategy. Students should write an example of a reaction on a scrap of paper, then crumple that paper and toss it in the air. Students then grab a new scrap of paper, uncrumple it, and try to correctly name the type of reaction described in the example. Access this strategy by clicking on the Professional Learning tab on Science Techbook. Click on Strategies & Resources, then Spotlight on Strategies (SOS). Snowball Fight is found underneath “Key Ideas and Details.”
