Absorption of Amino Acids

Absorption of digested food occurs in the ileum of the small intestine. At the beginning of the small intestine, ions are pumped into the lumen from the epithelial cells. The water potential is lower in the lumen of the small intestine than the epithelial cells and water moves from the epithelial cells lining the small intestine to the lumen by osmosis.Further along the small intestine ions are pumped back by active transport. The water potential is now lower in the epithelial cells. Water moves from the lumen to the epithelial cells by osmosis. Glucose is absorbed by active transport.Sodium ions are actively transported from the epithelial cells into the blood by the sodium-potassium pump.This lowers the sodium ion concentration in the epithelial cell.Sodium ions diffuse from their lumen into the epithelial cell down their concentration gradient through a co-transport protein which carries either glucose or an amino-acid with the sodium ion.The concentration of glucose and amino acids in the epithelium cell is now higher than their concentration in the blood. The glucose or amino acids diffuse into the blood down their concentration gradient by facilitated diffusion.The flow of blood maintains a steep concentration gradient between the blood and the epithelial cell.Amino-acids, glucose and other monosaccharides are absorbed across the epithelium into the capillary network of the villi by diffusion and cotransportThe sodium-potassium pump in the epithelial wall on the inside of the villi actively transports sodium ions out of the epithelial cells into the interstitial fluid.This lowers the concentration of sodium in the epithelial cell and creates a diffusion gradient.Sodium ions enter the cell from the lumen through a cotransport protein down their concentration gradient and are accompanied by glucose, another monosaccharide or an amino acid.The high concentration of glucose or amino acid in the epithelial cell causes these molecules to move into the blood through a carrier protein down their concentration gradient by facilitated diffusion.