Sites of Nutrient Digestion in the Alimentary Canal

Steps:

• Recall that chemical digestion breaks down macromolecules using enzymes at specific sites.
• Identify carbohydrate digestion: starts in mouth with salivary amylase, continues in small intestine with pancreatic amylase, completes in jejunum/ileum.
• Identify protein digestion: begins in stomach with pepsin, continues in small intestine with trypsin and peptidases, completes in ileum with absorption.
• Identify cellulose: indigestible by humans, no specific enzymes secreted.

Why C is correct:

• Protein digestion is completed in the ileum, where final peptidases break peptides into amino acids for absorption, as per the process of enzymatic hydrolysis in the small intestine.

Why the others are wrong:

• A: Carbohydrate digestion completes in the small intestine, not colon, which absorbs water and forms feces.
• B: Humans lack cellulase enzymes; cellulose passes undigested, no breakdown in duodenum.
• D: Stomach secretes pepsin for proteins, not amylase for starch, which is from saliva and pancreas.

Final answer: C