Why do catabolic reactions release heat despite that breaking bonds absorbs heat?

Question

According to Campbell, the definition of a catabolic reaction is:

Some metabolic pathways release energy by breaking down complex molecules to simpler compounds. these degradative processes are called catabolic pathways, or breakdown pathways. ... Energy released from downhill reactions of catabolic pathways can be stored and then used to drive uphill reactions of anabolic pathways.

The book also mentions that:

It is important to realize that the breaking of bonds does not release energy; on the contrary, as you will soon see, it requires energy. The phrase "energy stored in bonds" is short-hand for the potential energy that can be released when new bonds are formed after the original bonds breaking, as long as the products are of lower free energy than the reactants.

Can someone help me link these two ideas considering the apparent "contradiction"? How is it that catabolic pathways, which release energy, work on breaking bonds, which require energy? Are catabolic pathways energy-releasing because of the "net" energy difference because the heat required to break bonds exceeds the energy required to break them? And how are those bonds broken? I think the book was attempting to answer my question in that second excerpt, but I could not fully grasp it from their wording. Thank you!

Answer 1

The book is right, but you are missing quite a big chunk of information inbetween. In a sentence, catabolism is fairly complex and it cannot be seen reaction by reaction but as whole. Because, in order to produce ATP from glucose so that it can be used to drive reactions in anabolic processes, the metabolism of glucose undergoes multiple processes (or phases) with distinct goals: glycolysis, krebs cycle and oxidative phosphorylation (OP).

• Glycolysis, or the lysis (as in breaking down) of glucose is a preparation phase. In this phase, glucose is metabolized in a way of preparing it to eventually generate energy. This phase barely generates any energy (net +2 ATP per glucose molecule), but the initial molecule (glucose) undergoes quite a transformation before being converted into pyruvate.

• Krebs cycle is a continuation of the preparation phase where, essentially, carbon is converted into electrons. In other words, one full cycle in the Krebs cycle entails Acetyl-CoA (2 carbons) entering the cycle and the full oxidation of 2 carbons into CO2. With this cycle, several molecules of FAD and NAD+ are reduced into FADH2 and NADH, respectively, using the electrons of the oxidative reactions. Per glucose molecule, only 2 GTP molecules are produced as well.

• OP is where energy is effectively generated. O2 is reduced into H2O, NADH/FADH2 are oxidized into NAD+/FAD and ADP is converted into ATP by harnessing energy from a proton gradient inside the mitochondria. Per molecule of glucose, and depending on efficiency, 30-36 molecules of ATP are produced in this phase.

Of course, multiple reactions in these processes release energy (-ΔG'^o), while others actually take energy (+ΔG'^o). The only way the cells can make up for this is by adjusting the concentrations of products and reagents to drive the reactions in the direct reaction and not the other way around by resulting in a negative ΔG, according to the following equation:

ΔG = ΔG’^o + RT * ln([products]/[reagents])

By doing this, metabolic reactions flow in a way that the overall process of glucose metabolism will ultimately result in energy generation, despite some of the reactions barely generating any energy, while some others actually take up energy.