Problem 14 - Entrance Test

Which of the following statements accurately describes the primary mechanism of absorption for a specific nutrient in the small intestine, highlighting the energy requirement and specific transporters involved?

A. Glucose is absorbed into epithelial cells by facilitated diffusion via SGLT1, a sodium-independent transporter.B. Short-chain fatty acids are absorbed by simple diffusion directly into the bloodstream.C. Amino acids are absorbed by primary active transport via various specific Na+-dependent cotransporters.D. Fructose is absorbed by secondary active transport via GLUT2, an apical membrane transporter.

Correct: B

This question requires detailed knowledge of nutrient absorption mechanisms. Let's analyze each option: A. Glucose absorption: Glucose is absorbed into intestinal epithelial cells (enterocytes) by secondary active transport via the SGLT1 cotransporter (Sodium-Glucose Linked Transporter 1) on the apical (luminal) membrane. This process requires energy indirectly because it utilizes the electrochemical gradient of Na+ established by the Na+/K+ pump (primary active transport) on the basolateral membrane. SGLT1 is sodium-dependent, not sodium-independent. Glucose then exits the enterocyte into the bloodstream via facilitated diffusion using GLUT2 on the basolateral membrane. So, this option is incorrect. B. Short-chain fatty acids (SCFAs): These are produced by bacterial fermentation of fiber in the colon, but some can also be derived from dietary fats. SCFAs are small and lipophilic. They are primarily absorbed by simple diffusion directly across the enterocyte membrane and then into the capillaries of the villi, entering the bloodstream. This statement is accurate. C. Amino acid absorption: Amino acids are absorbed into enterocytes primarily by secondary active transport (cotransport with Na+) via various specific Na+-dependent cotransporters on the apical membrane. There are also some H+-dependent cotransporters (e.g., PEPT1 for di- and tripeptides). They are not absorbed by primary active transport in this step. Primary active transport refers to ATP-hydrolyzing pumps. So, this option is incorrect. D. Fructose absorption: Fructose is absorbed into enterocytes via facilitated diffusion using the GLUT5 transporter on the apical (luminal) membrane. It then exits the enterocyte into the bloodstream via facilitated diffusion using GLUT2 on the basolateral membrane. It is not absorbed by secondary active transport, and GLUT2 is primarily a basolateral transporter for fructose efflux, not apical intake. So, this option is incorrect. Therefore, the absorption of short-chain fatty acids by simple diffusion into the bloodstream is the only accurate description.