Two laws of thermodynamics govern energy transformations in biological systems. A biochemical reaction can release or consume energy, and it may not run to completion, but instead end up at a point of equilibrium.
learning outcomes
You should be able to:
Apply the second law of thermodynamics to biological systems.
Differentiate between exergonic and endergonic reactions.
What is the difference between endergonic and exergonic reactions, and what is the importance of positive and negative ΔG?
Exergonic reactions release free energy because the energy of the reactants is greater than that of the products. The reverse is true for endergonic reactions, which require an input of energy. ΔG is the free energy change of a reaction—
What makes it possible for endergonic reactions to proceed in organisms?
Endergonic reactions require the input of energy to create more ordered molecules. The second law of thermodynamics states that order tends to increase in the universe. Endergonic reactions are coupled in time and space with exergonic reactions, which increase the disorder and release the energy needed for the endergonic reactions to proceed. Overall, organisms need to take in energy from their environment continually to maintain these reactions.
The principles of thermodynamics that we have been discussing apply to all energy transformations in the universe, so they are very powerful and useful. Now let’s apply them to reactions in cells that involve the currency of biological energy, ATP.