3-3-4-mechanisms-of-absorption

Mechanisms of absorption

• The products of digestion are absorbed through the intestinal lining

• Molecules pass into the intestinal epithelial cells, from which they can move into the blood

• Absorption of the major biological molecules occurs by different mechanisms:

  • Amino acids and monosaccharides are absorbed via co-transport

  • Lipid absorption involves micelles

Absorption by co-transport

Amino acids

• Co-transporter proteins are found within the cell-surface membranes of the epithelial cells in the small intestine

• The process of cotransport occurs as follows:

  1. Sodium ions are actively transported from the epithelial cell into the blood via a sodium-potassium pump, decreasing the concentration of sodium ions in the epithelial cell

     • This stage maintains the sodium ion gradient that is essential to the next part of the process

  2. Sodium ions move down their concentration gradient from the intestine into the epithelial cell, carrying an amino acid is transported at the same time by the co-transporter protein

     • This is a form of facilitated diffusion

  3. The concentration of amino acids in the epithelial cell increases, and amino acids diffuse down their concentration gradient into the blood

• While the action of the co-transporter protein is passive, energy is required to create the sodium ion gradient, so the process of co-transport is considered, overall, to be active transport

Monosaccharides

• The co-transport of glucose uses the same mechanism as that of amino acids:

  1. active transport of sodium ions into the blood

  2. facilitated diffusion of sodium and glucose into the epithelial cell, via a glucose co-transporter protein

  3. facilitated diffusion of glucose into the blood

Lipid absorption

• The products of lipid digestion are:

  • fatty acids

  • monoglycerides

• Monoglycerides and fatty acids associate with bile salts to form micelles, which transport these insoluble molecules to the cell surface membranes of the epithelial cells

• Micelles constantly break up and reform; when they break apart their lipid-soluble contents can cross the membrane by diffusion

  • The contents of micelles are non-polar so can diffuse through the phospholipid bilayer of the cell membrane

• Short fatty acid chains within the epithelial cells can move directly into the blood via diffusion

• Longer fatty acid chains recombine with monoglycerides and glycerol to form triglycerides in the endoplasmic reticulum

• The triglycerides are packaged into chylomicrons which eventually enter the bloodstream