Enterocyte

Overview
Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells found in the small intestines and colon. A glycocalyx surface coat contains digestive enzymes. Microvilli on the apical surface increase surface area for the digestion and transport of molecules from the intestinal lumen. The cells also have a secretory role.

Functions
The major functions of enterocytes include :
 * Ion uptake, including sodium, calcium, magnesium, and iron. This typically occurs through active transport.
 * Water uptake. This follows the osmotic gradient established by Na+/K+ ATPase on the basolateral surface. This can occur transcellularly or paracellularly.
 * Sugar uptake. Polysaccharidases and disaccharidases in the glycocalyx break down large sugar molecules, which are then absorbed. Glucose crosses the apical membrane of the enterocyte using the Na+ dependent glucose transporter. It moves through the cytosol (cytoplasm) and exits the enterocyte via the basolateral membrane (into the blood capillary) using GLUT-2 . Galactose uses the same transport system. Fructose, on the other hand, crosses the apical membrane of the enterocyte, using GLUT-5 . It is thought to cross into the blood capillary using one of the other GLUT transporters.
 * Peptide and amino acid uptake. Peptidases in the glycocalyx cleave proteins to amino acids or small peptides. Enteropeptidase is responsible for activating pancreatic trypsinogen into trypsin, which activates other pancreatic zymogens.
 * Lipid uptake. Lipids are broken down by pancreatic lipase and bile, and then diffuse into the enterocytes. Smaller lipids are transported into intestinal capillaries, while larger lipids are processed by the Golgi and smooth endoplasmic reticulum into lipoprotein chylomicra and exocytozed into lacteals.
 * Vitamin uptake. Receptors bind to the vitamin B-gastric intrinsic factor complex and are taken into the cell.
 * Resorption of unconjugated bile salts. Bile that was released and not used in emulsification of lipids are reabsorbed in the ileum. Also known as the enterohepatic circulation.
 * Secretion of immunoglobulins. IgA from plasma cells in the mucosa are absorbed through receptor mediated endocytosis on the basolateral surface and released as a receptor-IgA complex into the intestinal lumen. The receptor component confers additional stability to the molecule.

Pathology
Dietary fructose intolerance occurs when there is a deficiency in the amount of fructose carrier.

Lactose intolerance is the most common problem of carbohydrate digestion and is created by an insufficient amount of lactase (a disaccharidase) enzyme, which is used to break down the sugar. As a result of this deficiency, undigested lactose cannot be absorbed and is instead passed on to the colonic bacteria, which metabolize the lactose. The bacteria release gas and metabolic products that enhance colonic motility.

Problems with the gastric intrinsic factor or its receptor can result in pernicious anemia.