USA Biology Olympiad (USABO) Practice Exam

What nutrient is absorbed by passive diffusion across the intestinal epithelium?

1. Glucose

2. Protein

3. Fructose

4. Fatty acids

The correct answer is: Fructose

Passive diffusion across the intestinal epithelium occurs when molecules move from an area of higher concentration to an area of lower concentration without the need for energy input or transport proteins. In this context, fatty acids are absorbed by passive diffusion because they are relatively nonpolar and can easily cross cellular membranes, which are composed of lipid bilayers. This nonpolar nature allows fatty acids to diffuse effectively into intestinal epithelial cells where they can be further metabolized or packaged into chylomicrons. Glucose and protein, on the other hand, require specific transport mechanisms to cross the intestinal epithelium. Glucose uses sodium-glucose transport proteins (SGLT) for absorption, involving active transport mechanisms that depend on sodium gradients. Proteins or their amino acids are typically absorbed through facilitated diffusion or active transport mechanisms as well, which also do not allow for passive diffusion. Fructose, like glucose, is absorbed by facilitated diffusion via specific transport proteins (GLUT5), meaning that it does not utilize the passive diffusion mechanism that fatty acids do. Thus, the nutrient absorbed by passive diffusion across the intestinal epithelium is fatty acids, due to their ability to cross lipid membranes without assistance.