Digestion: Glucose And Amino Acid Absorption

The active transport of sodium is of particular importance because the transport of several other substances (such as glucose and some amino acids) occurs mainly in conjunction with it. Glucose and amino acids are believed to be transported across the brush border by sodium dependent carriers, building high intracellular concentrations of these substances, which permit them to pass across the basal membranes by diffusion or facilitated diffusion. If the sodium pump is inhibited glucose and amino acid transport are diminished because the intracellular sodium concentration rises preventing the diffusion of sodium from the lumen. Several amino acids are initially taken up at the brush border as dipeptides. At this time or during mucosal transport various dipeptidase enzymes present in the brush border attack these peptides, releasing free amino acids which either diffuse or are actively transported

Fat Absorption

Products of fat digestion are monoglycerides, glycerol and fatty acids. Fatty acids are either short chained or long chained. Short chain fatty acids pass across the mucosa by diffusion and being fairly water-soluble enter the blood capillaries along with other water-soluble nutrients. However, long chain fatty acids and other fatty nutrients including cholesterol receive special treatment during absorption. These fatty products diffuse across the brush border. Within the mucosal cells the triglycerides are resynthesised on the smooth endoplasmic reticulum and packed with cholesterol and other fatty substances within certain lipoprotein particles called chylomicrons. The packaging process occurs within the Golgi apparatus. Chylomicrons like other members of the family of lipoprotein particles contain a coat of protein and a core of fat allowing large amounts of fat to float in the bloodstream without coalescing. After formation, chylomicrons are extruded by exocytosis from the mucosal cells into the lacteals, move into the larger lymph vessels and finally into the veins in the upper trunk near the neck.

Vitamin Absorption

Vitamins are divided into two categories. Water-soluble and fat soluble. The water soluble vitamins, such as the B family and C, pass across the mucosa by diffusion and also by association with specialized membrane carriers. Vitamin B12 (cyanocobalamine) is the largest of the vitamins and its transport utilises yet another mechanism. The secretory cells in the stomach wall normally produce a specific transport mucoprotein (proteins containing special polysaccharides) called intrinsic factor. In the chyme, intrinsic factor binds with vitamin B12; the absorptive cells use endocytosis to take up this complex. Vesicles are released at the basal surface by exocytosis. Diseases of the stomach (eg., gastritis) reduce vitamin B12 absorption because they deplete intrinsic factor, causing pernicious anemia. Fat-soluble vitamins such as vitamins A, D and K are absorbed in the chylomicrons along with the fatty nutrients.