Question

Describe absorption of nutrients in the small intestine. What is the nature of the absorbed compounds (how do they differ from the starting materials), how are they absorbed and what is their destination?

Answer 1

Carbohydrates are polysaccharides and they are digested into monosaccharides like glucose, galactose and fructose. Absorption takes place in small intestine. Glucose and galactose is co-transported with sodium ions into epithelial cells and by facilitated diffusion enters the blood in capillary in villi and reaches liver through hepatic portal vein. Fructose is absorbed through facilitated diffusion and enters the blood in capillary in villi and then goes to liver by hepatic portal vein.

Proteins are macromolecules of long chain of amino acids which are digested into amino acids in small intestine and are then absorbed in small intestine. The amino acids are transported to capillary blood of villi by co- transportation with sodium ions and then goes to liver via hepatic portal vein.

Lipids are digested into fatty acids(long chain and short chain), monoacylglycerides and glycerol and are absorbed in small intestine. Short-chain fatty acids are water soluble and enter enterocytes directly by simple diffusion due to small size even though it is hydrophobic and then enters the blood capillary of a villus from where it goes to liver via hepatic portal vein. Glycerol is also absorbed through simple diffusion and enters capillary blood of villi from where it goes to liver via hepatic portal vein. Long chain fatty acids and monoacylglycerides diffuses into intestinal cells where they combine with proteins to form chylomicrons and then enters lacteals of villi from where it goes to systemic circulation via lymph which enters thoracic duct.

Nucliec acids are absorbed through active transport by membrane carriers and enter capillary blood of villi in small intestine and then goes to liver via hepatic portal vein.

About 90 percent of water is absorbed in small intestine and 10 percent is absorbed in colon. The absorption takes place by concentration gradient i.e from region of high concentration to low concentration.

Vitamins are absorbed in small intestine. Fat soluble vitamins (A, D, E, and K) are absorbed along with lipids via simple diffusion. Water soluble vitamins (B and C) also are absorbed by simple diffusion. Vitamin B12 binds to Intrinsic factor and forms a complex that binds to mucosal receptors in the terminal ileum, where it is absorbed by endocytosis and enters systemic circulation.

Electrolytes dissociates into ions in water and then they are absorbed into small intestine by active transport and enters systemic circulation.