Which of the following nutrients is directly absorbed by the small intestine?

• A. Large proteins
• B. Cellulose
• C. Amino acids
• D. Starch

Answer: (C)

Verified step by step guidance

1. Understand the role of the small intestine in nutrient absorption: The small intestine is the primary site for the absorption of nutrients from digested food. It absorbs small, soluble molecules that result from the breakdown of larger macromolecules during digestion.
2. Review the digestion process for proteins: Large proteins are broken down into smaller peptides and eventually into amino acids by enzymes such as pepsin (in the stomach) and trypsin and chymotrypsin (in the small intestine). Amino acids are the absorbable form of proteins.
3. Consider cellulose: Cellulose is a complex carbohydrate found in plant cell walls. Humans lack the enzyme cellulase, which is required to break down cellulose into absorbable molecules. Therefore, cellulose is not absorbed by the small intestine and passes through the digestive system as fiber.
4. Examine starch digestion: Starch is a polysaccharide that is broken down into maltose and eventually into glucose by enzymes like amylase (produced by the salivary glands and pancreas). Glucose, not starch itself, is absorbed by the small intestine.
5. Conclude with the correct answer: Amino acids are directly absorbed by the small intestine after proteins are broken down during digestion. This highlights the importance of enzymatic activity in converting macromolecules into absorbable forms.