American Board of Surgery Qualifying Exam (ABS QE) Practice Test

How is glucose and galactose absorbed in the intestines?

1. Passive diffusion

2. Facilitated diffusion

3. Secondary active transport

4. Simple active transport

The correct answer is: Secondary active transport

Glucose and galactose are absorbed in the intestines through secondary active transport, which is an essential and efficient mechanism for the uptake of these monosaccharides. This process utilizes the electrochemical gradient established by sodium ions (Na+), which are actively transported out of the intestinal epithelial cells into the bloodstream by the Na+/K+ ATPase pump. As sodium ions are pumped out of the cell, they create a low intracellular concentration of sodium. This gradient allows sodium to re-enter the cells along with glucose or galactose through co-transport proteins, specifically the sodium-glucose cotransporter (SGLT1). This mechanism does not directly use ATP to transport glucose or galactose; instead, it relies on the energy stored in the sodium gradient, which is established by the active transport of sodium ions. Thus, as sodium ions move back into the cells driven by their concentration gradient, they carry glucose or galactose with them, demonstrating the concept of secondary active transport. This method is critical for efficient nutrient absorption, particularly in the small intestine, where high concentrations of glucose and galactose occur following carbohydrate digestion.