Cholesterol is absorbed without being digested into smaller pieces. How does this compare to absorption of carbohydrates and proteins? What characteristic of cholesterol suggests it would be transported by simple diffusion? What is the evidence that transport proteins are involved in cholesterol absorption? Does this discovery rule out transport by simple diffusion? Explain.

Question

What will be an ideal response?

Answer 1

Carbohydrates are digested to monosaccharides before they are absorbed. Proteins are absorbed as amino acids, di- or tripeptides, or even oligopeptides. As a lipid, cholesterol can dissolve in the phospholipid bilayer and therefore cross membranes by simple diffusion. The drug ezetimibe inhibits cholesterol absorption, suggesting there must be a membrane transporter. This discovery does not rule out additional transport of cholesterol by simple diffusion, but does suggest there is a significant amount of transport dependent upon a transport molecule.