Why can the human digestive system break down starch, but not fiber?

• A. Fiber is a complex polysaccharide, whereas starch is a simple carbohydrate.
• B. The bonds between saccharides in fiber are different to the bonds between saccharides in starch.
• C. Fiber has branched structure & starch does not, making it harder for enzymes to bind to it.
• D. Starch always has a branched structure, while fiber always consists of straight chains of saccharides.

Answer: (B)

Verified step by step guidance

1. Understand the composition of starch and fiber: Both starch and fiber are polysaccharides, meaning they are made up of long chains of glucose molecules. However, the type of bonds between the glucose units differs between the two.
2. Examine the bonds in starch: Starch is composed of glucose molecules linked by α-glycosidic bonds. These bonds can be broken down by enzymes like amylase in the human digestive system.
3. Examine the bonds in fiber: Fiber, such as cellulose, is composed of glucose molecules linked by β-glycosidic bonds. Human digestive enzymes, including amylase, cannot break β-glycosidic bonds, making fiber indigestible.
4. Clarify the structural differences: Starch can have both branched (amylopectin) and unbranched (amylose) structures, while fiber like cellulose consists of straight chains. However, the key factor is the type of glycosidic bond, not the branching.
5. Conclude why starch is digestible and fiber is not: The human digestive system has enzymes specifically designed to break α-glycosidic bonds in starch, but it lacks enzymes to break β-glycosidic bonds in fiber, which is why fiber passes through the digestive system largely intact.