The Oxidation of Glucose

With glucose being one of the most common sugars in the biosphere (and certainly the most common in labs), the oxidation of glucose is a major source of chemical energy for many organisms. This process - the conservation of energy through complete oxidation of glucose using oxygen - is called “respiration.” Much of the energy produced during glucose oxidation is stored as ATP. If we take a look at the full oxidation of glucose to carbon dioxide and water

Glucose + 6 O2 ⇌ 6 CO2 + 6 H2O

we find that ΔrG’° is about -2930 kJ / mol. In this case the effect of correcting for a more typical biological concentration of 1 mM is almost negligible with ΔrG’m ≈ -2910 kJ / mol. According to our calculation from above, that’s enough energy to make ~65 ATP (in theory). But most organisms generate 30 ATP or less through the oxidization of glucose (Nelson et al., 2008; Flamholz et al., 2014). [1]

Biochemistry textbooks often write this pathway as generating 36 ATP

Glucose + 6 O2 + 36 ADP + 36 Pi ⇌ 6 CO2 + 42 H2O + 36 ATP

giving a ΔrG’m of around -1350 kJ / mol. [2] In other words, there’s more free energy! Cells could, in principle, make a lot more ATP than they do in practice. Which begs the question: why not make more ATP? If you’re interested in learning more, consider looking up “chemical motive force” for reversible reactions.

[1]See BioNumbers ID 111475, which discusses ATP yield from glucose in yeast, for example.
[2]Remember ΔrG’m assumes all concentrations are 1 mM.