How are amino acids absorbed in the small intestine and what transporter is involved?

• A. Amino acids are absorbed via sodium-dependent and independent transporters on the apical membrane and exit via basolateral transporters into portal circulation.
• B. Amino acids are absorbed exclusively by passive diffusion.
• C. Amino acids are transported only through endocytosis.
• D. Amino acids exit via apical transporters into the lumen.

Answer: (C)

Amino acids are absorbed by transporter proteins on the enterocyte surfaces, not by simple diffusion or endocytosis. On the brush-border (apical) membrane, specific transporters move amino acids from the intestinal lumen into the cell, and these include both sodium-dependent and sodium-independent systems that use ion gradients to drive uptake. Once inside the cell, amino acids exit across the basolateral membrane into the portal circulation via basolateral amino acid transporters, delivering them to the liver and systemic circulation. Passive diffusion is not the main route because amino acids are charged and rely on these carrier systems. Endocytosis isn’t the primary mechanism for free amino acids (it’s more relevant for larger peptides or whole proteins), though dipeptides and tripeptides can be transported by a separate transporter (PEPT1) and then hydrolyzed to amino acids inside the enterocyte.