Bond Energy vs. Metabolism

Two key concepts that are taught in chemistry and biology are described below:

How can we consolidate these two seemingly contradictory ideas? The key is the scale at which we view these two concepts.

The second law of thermodynamics asserts that the entropy of the universe is always increasing. In other words, the universe is constantly approaching disorder. So, does bond formation violate the second law of thermodynamics?

There are two components to the universe, the system and the surroundings. Here, the system includes the chemical transformation and the surroundings includes everything else. During bond formation, two atoms combine into a larger molecule, increasing the system's order (i.e. decreasing entropy). This transformation releases energy, which may be used to break bonds in other molecules, thereby decreasing the surrounding's order (i.e. increasing entropy). As long as the entropy of the surroundings increases more than the entropy of the system decreases, bond formation can occur.

The energy released from bond formation can be used to power other chemical transformations. We call this energy the "free energy," or the energy to do work. Free energy is the net energy transferred over the course of an entire chemical reaction, including all the bonds broken and formed.

During anabolism, bond formation still produces energy and bond breaking still requires energy. However, anabolic reactions require energy input (+ΔG), and catabolic reactions release energy (-ΔG).

Some catabolic reactions are highly thermodynamically favorable. In other words, the formation of very stable products releases a large amount of energy, greatly exceeding the energy required to break the reactant's bonds. Using enzymes, we can harness this energy release to power the formation of high energy molecules in a process called reaction coupling. For example, an enzyme may couple a catabolic reaction to ATP synthesis. The energy released by the catabolic reaction is sufficient to form ATP.

Molecules are considered to be high in energy due to the presence of unstable bonds. These unstable bonds are high in energy and little energy is needed to break them. For example, the three negatively-charged phosphates on ATP repel one another, making the terminal bond unstable. When phosphate is released, the resulting products are much more stable. Energy released from the formation of stable bonds in the products will exceed the energy input needed to break the unstable bond in the reactant. Ultimately, potential energy that was stored in the unstable bond is used to power anabolic reactions.

In summary, bond energy describes the energy transfer of a single bond, free energy describes the energy transfer over a chemical reaction, and metabolism describes the total energy transfer over many chemical reactions in a biological system.