What is the free energy released upon combustion of sugar?

Like humans, bacteria have preferences about what they eat.  In a series of beautiful and insightful experiments, Jacques Monod showed that when bacteria are offered different carbon sources, they would first use their preferred carbon source before even turning on the genes to use others. The substrate of choice for bacteria such as E. coli is glucose, a molecule known to every biochemistry student as the starting point for the famed reactions of glycolysis.

The free energy released in oxidizing glucose by oxygen is ≈ -3000 kJ/mol (BNID 103388 and http://equilibrator.weizmann.ac.il/classic_reactions). Expressed in other units this is ≈ -700 kcal/mol, or ≈ -1200 k_BT, where a kcal is what people often count Calories (capitalized). As is clear from the schematic showing the range of biological energy scales at the beginning of this chapter, this energy is at the high end of the scale of molecular energies. To get a better idea for how much energy this is, let’s think about the delivery of useful work from such reactions.  One of the ways of reckoning the potential for useful work embodied in this energy release is by examining the number of ATP molecules that are produced (from ADP and Pi) in the series of reactions tied to combustion of sugar, also known in biochemical lingo as cellular aerobic respiration. The cell’s metabolic pathways of glycolysis, the TCA cycle and the electron transfer chain couple the energy release from combustion of a single molecule of glucose to the production of roughly 30 ATP molecules (BNID 101778), sufficient energy to permit several steps of the molecular motors that drive our muscles or to polymerize a few more amino acids into a nascent polypeptide.

We learn from the labels on our cereal boxes that a human daily caloric intake is recommended to consist of 2000 kcal. If supplied only through glucose that would require about 3 mol of glucose. From the chemical formula of glucose, namely, C₆H₁₂O₆ the molecular weight of this sugar is 180 Da and thus 3 mol corresponds to ≈500 g. So half a kg of pure sugar (whether it is glucose, sucrose or as is often common today, high fructose corn sugar, so called HFCS) would supply the required energy of combustion to fuel all the processes undertaken by an “average” person in a single day, though not in a nutritionally recommended fashion.

To get a better sense of the energetic value of all of this glucose, we now consider what would happen if the body did not conduct the heat of combustion of these recommended 2000 kcal into the environment, but rather used that energy to heat the water in our bodies. A calorie is defined as the energy required to increase the temperature of 1 g of water by 1^OC (denoted by c below). For a human with a mass (m) of 70 kg, the potential increase in temperature resulting from the energy released in combustion (∆Q) over a day can be estimated by the relation

∆T = ∆Q/(cxm) = 2×10⁶ cal / (1 cal/^OC x gram) (70*10³ gram) ≈ 30^OC,

illustrating that the energy associated with our daily diet is has a lot of heating capacity.