Which fibrous carbohydrate cannot be digested by humans?

Which fibrous carbohydrate cannot be digested by humans?

• A. Cellulose
• B. Starch
• C. Glycogen
• D. Maltose

Answer: (A)

Digestion depends on the type of bonds between glucose units and whether our enzymes can break them. Starch, glycogen, and maltose all have alpha-linked glucose bonds that our digestive enzymes can cleave to yield usable glucose. Cellulose, however, is made of glucose units connected by beta-1,4 bonds. Humans don’t produce the enzyme cellulase to break these beta bonds, so cellulose can’t be digested for energy. It mostly passes through the digestive system as fiber, though some gut bacteria can ferment small amounts. That’s why cellulose is the fibrous carbohydrate that humans cannot digest.

Digestion depends on the type of bonds between glucose units and whether our enzymes can break them. Starch, glycogen, and maltose all have alpha-linked glucose bonds that our digestive enzymes can cleave to yield usable glucose. Cellulose, however, is made of glucose units connected by beta-1,4 bonds. Humans don’t produce the enzyme cellulase to break these beta bonds, so cellulose can’t be digested for energy. It mostly passes through the digestive system as fiber, though some gut bacteria can ferment small amounts. That’s why cellulose is the fibrous carbohydrate that humans cannot digest.