Building a house out of French fries seems like a bad idea, but fried potatoes and lumber are, chemically, very similar. Only one bond distinguishes the food we enjoy from the wood with which we build.
Potatoes, made from starches, and lumber, made from cellulose, are both derived from the same sugar molecule: glucose. In order to form carbohydrates, the small glucose molecules bond with each other to form large chains of repeated glucose molecules called polymers in a process known as polymerization. Once these molecules are bound together, they form carbohydrates that can be broken down by our bodies into energy. This polymerization process creates distinct patterns in the glucose chain structure, which is the key difference between the wood we use as lumber, and the food we eat.
When glucose molecules in starch or cellulose bond together to form long chains, they join by either an alpha (α) or beta bond (β), which refers to the configuration of the molecules in the chain. Based on the type of bond formed, the resulting polymers will have an entirely different function.
Starches, known as glycogen in animals and amylopectin in plants, are characterized by alpha linkages between each molecule. This bond ensures that all the glucose molecules in the long chain are positioned in the same direction, facing outwards. Due to this orientation, these atoms can welcome additional bonds in the molecule to form side branches. These side branches grow very large, as more glucose molecules join in; and eventually, the chain forms a giant web of glucose polymers—although not a very strong one.
In contrast, cellulose—the stuff we build houses with—is made up of glucose bound by beta linkages. These bonds, however, alternate the position of each molecule in the polymer, largely blocking any side bonding atoms. Whereas the alpha linked glucose chains in starches welcome side branching, the beta linkages in cellulose are better for packing linear glucose strands side by side. Unlike a starch’s weak glucose web, cellulose is composed of strong linear chains of glucose that pack together like a bundle of pencils.
Thanks to the difference in this single bond, humans can digest French fries rather than wood—even though both of these substances are composed of glucose monomers. The branching caused by alpha linkages results in a starch molecule with weak bonds, meaning it is not difficult for the body to digest starch. Furthermore, humans possess the enzyme amylase (a protein that helps break down starch) to be used in the digestion of glucose. This enzyme only has the capability to digest alpha linkages; meaning humans cannot break down cellulose in wood to use as a source of nutrition.
Therefore, although our appetites appreciate the soft web of glucose in potatoes, our minds and our houses can appreciate the shelter that a strong beta glucose linkage offers in trees.