Final answer:
Lipids are absorbed differently than carbohydrates and proteins due to their lipid-soluble nature, requiring transformation within epithelial cells and transport via chylomicrons into the lacteals of the villi, bypassing the hepatic portal vein, and eventually entering the systemic circulation through the lymphatic system.
Step-by-step explanation:
Lipids are absorbed into a different structure than carbohydrates and proteins because they are lipid-soluble substances, which allows them to diffuse directly through the plasma membranes of epithelial cells. Unlike water-soluble nutrients which typically use active transport that requires ATP, lipids enter the intestinal cells and are transformed into micelles with the aid of bile salts. These micelles then release the lipids into the cells where they are re-esterified into triglycerides and assembled into chylomicrons. Subsequently, they bypass the hepatic portal vein that transports water-soluble nutrients directly to the liver, instead entering the lacteals—special lymphatic vessels in the villi of the small intestine—and into the lymphatic system to eventually reach the systemic circulation via the thoracic duct.
This unique absorption pathway is also critical for the absorption of fat-soluble vitamins, which require the presence of dietary lipids to be effectively absorbed. The small intestine is the primary site where about 95% of lipids are absorbed with the help of bile salts. Short-chain fatty acids are more water-soluble and can be absorbed by simple diffusion and follow a similar route as monosaccharides and amino acids into the blood capillaries of the villus.