Sacarid — Carbohydrate Chemistry | sacarid.com
Alpha versus beta linkages, and why the difference changes what a body can digest.
A glycosidic bond is the link between two sugar units. It forms when one hydroxyl group reacts with another and a water molecule leaves. The geometry of that link is described as alpha or beta.
Maltose and starch use an alpha-1,4 bond. Lactose and cellulose use a beta-1,4 bond. The numbers refer to which carbons of the rings are joined; the Greek letter refers to the orientation of the bond at the anomeric carbon.
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Human digestive enzymes are shaped to fit the alpha bond. They can split maltose and starch into glucose units. They cannot split the beta bond in cellulose, which is why cellulose passes through the human gut largely unchanged despite being a glucose polymer.
Lactose is a beta-1,4 disaccharide, but it is small enough that a dedicated enzyme, lactase, can handle it. Many adults stop producing enough lactase, which is why lactose tolerance varies across populations and with age.
The practical rule is compact: alpha bonds are breakable by the standard human enzyme set; beta bonds in polymers are not. The monomer does not decide this. The bond does.
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