Digestion — Humans readily digest starch and many complex carbohydrates, but are unable to digest which structural polysaccharide found in plant cell walls?

Introduction:
Human digestive enzymes are specific for certain glycosidic linkages. This question probes knowledge of why some polysaccharides are digestible and others are not, with a focus on cellulose versus starch/glycogen.

Given Data / Assumptions:

• Salivary and pancreatic amylases hydrolyze alpha-1,4 (and with debranching, alpha-1,6) linkages.
• Cellulose consists of beta-1,4-linked glucose units.
• Humans lack endogenous cellulases that cleave beta-1,4 linkages in cellulose.

Concept / Approach:
Match enzyme specificity to glycosidic linkage type. Because human enzymes target alpha linkages, starch and glycogen are digestible, while cellulose (beta-1,4) passes as dietary fiber and supports gut motility and microbiome fermentation rather than direct caloric extraction.

Step-by-Step Solution:

Verification / Alternative check:
Herbivores rely on symbiotic microbes producing cellulases; humans may ferment a small fraction via colonic bacteria, but host enzymes do not hydrolyze cellulose.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming "complex carbohydrate" means indigestible; complexity refers to polymer length/branching, not necessarily linkage type.

Final Answer:
Cellulose.