Is the formation of starch from glucose in plants an endothermic or exothermic reaction?

Question

If I understand it correctly photosynthesis is an endothermic reaction. Energy from the sun is used to form glucose.

Next the glucose is turned into starch. Would this be a combination reaction? If so, is that then an exothermic reaction?

Answer 1

The formation of 1,4-alpha glycosidic bond between two glucose monomers in starch, and more generally of glycosidic bonds between monosaccharide monomers to form polysaccharides, involves a dehydration reaction, a type of condensation reaction, but is not a combination reaction since two products form.

A condensation step in the polymerization may be written schematically as

$$ \ce{-(C5H10O4)C(H)-\color{red}{OH} + \color{red}{H}-OC(H)(C5H10O5) -> -(C5H10O4)C(H)-O-C(H)(C5H10O5) + \color{red}{H2O}}$$

Note what happens:

• one C-O bond is cleaved (dehydration)
• one OH bond is cleaved (dehydration)
• one C-O bond forms (the bridging glycosidic ether bond)
• one OH bond forms (forming water)

So no net bonds are formed.

You can estimate the enthalpy from the difference in bond dissociation energies $^1$ for OH in water and in methanol, 463.5 kJ/mol versus 437.6 kJ/mol, respectively, and assuming the CO bonds have similar energies. The difference is -25.9 kJ/mol, so slightly exothermic.

You can also estimate the enthalpy change from heats of combustion$^2$ for glucose and maltose (or starch of course), ie the following reactions:

$ \ce{2 C6H12O6(s) + 12 O2(g) -> 12 CO2(g) + 12 H2O(l)} ~~~ \Delta_\textrm{c} H = -2 \times 3138.0 \pu{kJ/mol}\\ \ce{C12H22O11(s) + 12 O2(g) -> 12 CO2(g) + 11 H2O(l)} ~~~ \Delta_\textrm{c} H = -5655.7 \pu{kJ/mol}$

The heat of formation of maltose from glucose is equal to the difference between the heats of combustion in the previous reactions, or -620.2 kJ/mol. You can estimate a similar value for starch. However this number is computed for formation of one solid from the other, not for addition of a monomer to starch in solution (the initial and final states matter).

The two estimates are very different but then again, the real hydrolysis or formation of such a glycosidic bond in starch is bound to be more complicated$^3$.

Note the drawn reaction is the net process and that as is typical of biochemical processes, the actual reaction involves various enzymes, here that activate the glucose monomer by converting it into glucose-ADP by reaction with ATP, and then facilitate encounter of the activated monomer with the growing starch polymer. You can read more about the definition of individual terms elsewhere, such as in the Wikipedia.

References

1. CRC handbook of Chemistry and Physics, 85th ed., D. R. Lide, ed., CRC press, Boca Raton, FL, 2004, p 9-65.

2. Sánchez-Peña, M. J.; Martínez-Navarro, A. G.; Márquez-Sandoval, F.; Gutiérrez-Pulido, H.; Pacheco-Moisés, F. P.; González-Ortiz, L. J. Heats of Combustion of the Main Carbohydrates Contained in Plant-Source Foods. Nutrition Reviews 2020, 78 (5), 382–393. https://doi.org/10.1093/nutrit/nuz063.

3. (1) Sørensen, T. H.; Cruys-Bagger, N.; Borch, K.; Westh, P. Free Energy Diagram for the Heterogeneous Enzymatic Hydrolysis of Glycosidic Bonds in Cellulose. Journal of Biological Chemistry 2015, 290 (36), 22203–22211. https://doi.org/10.1074/jbc.M115.659656.