Which complex carbohydrate is described as dietary fiber that is not digestible by digestive enzymes and is not an energy source?

Which complex carbohydrate is described as dietary fiber that is not digestible by digestive enzymes and is not an energy source?

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

Answer: (D)

The key idea is identifying a plant polysaccharide that humans cannot break down with our digestive enzymes, so it passes through as fiber and does not provide usable energy. Cellulose fits this exactly: it’s a plant cell wall polymer with beta-1,4 glucose linkages that humans don’t have the enzyme to cleave. Because we can’t digest it, cellulose isn’t converted to glucose and thus isn’t an energy source; it mainly adds bulk to stool and helps with bowel movement. Starch and glycogen are both energy-providing polysaccharides because humans can enzymatically break them down to glucose. Pectin is a soluble fiber that is fermentable by gut bacteria and can yield short-chain fatty acids, giving some energy in a sense, so it’s not purely energy-free like cellulose.

The key idea is identifying a plant polysaccharide that humans cannot break down with our digestive enzymes, so it passes through as fiber and does not provide usable energy. Cellulose fits this exactly: it’s a plant cell wall polymer with beta-1,4 glucose linkages that humans don’t have the enzyme to cleave. Because we can’t digest it, cellulose isn’t converted to glucose and thus isn’t an energy source; it mainly adds bulk to stool and helps with bowel movement.

Starch and glycogen are both energy-providing polysaccharides because humans can enzymatically break them down to glucose. Pectin is a soluble fiber that is fermentable by gut bacteria and can yield short-chain fatty acids, giving some energy in a sense, so it’s not purely energy-free like cellulose.