3.3.4 Mechanisms of Absorption

• Digestion breaks down food into smaller, soluble molecules
• The products of digestion are absorbed through the intestinal lining
• The absorption of amino acids and monosaccharides uses a similar mechanism - co-transport
• The absorption of lipids takes place in a different way

Co-transport Mechanisms for the Absorption of Amino acids and Monosaccharides

Absorption of Amino Acids

• Specific amino acid co-transport proteins (carrier molecules) are found within the cell-surface membrane of the epithelial cells in the ileum
• They transport amino acids only when there are sodium ions present
• For every sodium ion that is transported into the cell, an amino acid is transported in
  • This occurs via facilitated diffusion, which requires the movement of molecules down their concentration gradient (from high concentration to low concentration)

• Amino acids diffuse across the epithelial cell and then pass into the capillaries via facilitated diffusion
• The concentration gradient of sodium ions from the lumen of the ileum into the epithelial cell is maintained by the active transport of sodium ions out of the cell and into the blood via a sodium-potassium pump at the other end of the cell

Image showing the co-transport of sodium ions and amino acids in the ileum. Both facilitated diffusion and active transport are involved in the process.

Absorption of Monosaccharides

• The glucose carrier proteins in the cell-surface membrane of the small intestine work in a similar way to the amino acid carrier proteins
• Sodium ions and glucose molecules are co-transported into the epithelial cells via facilitated diffusion
• The glucose molecules diffuse across the epithelial cell and enter the capillary at the other end of the cell by facilitated diffusion
• The concentration gradient of sodium ions is maintained by actively transporting sodium ions out of the epithelial cells into the blood

Image showing the co-transport of sodium ions and glucose in the small intestine. Both facilitated diffusion and active transport are involved in the process.

The Role of Micelles in the Absorption of Lipids

• The products of lipid digestion are fatty acids, monoglycerides and glycerol
• Absorption of these products is significantly different from the absorption of carbohydrates and proteins
• The monoglycerides and fatty acids associate with phospholipids and bile salts to form micelles (very small droplets)
• Monoglycerides and fatty acids are not very soluble so the micelles aid the transport of these molecules to the surface of the epithelial cells
• The micelles break down and add to a pool of fatty acids and monoglycerides that are dissolved in the small intestine solution surrounding the epithelial cells. These freely dissolved molecules enter the epithelial cell by diffusion
  • They are non-polar molecules so they can diffuse through the phospholipid bilayer of the cell membrane

• The short fatty acid chains within the epithelial cells can move directly into the blood via diffusion but the longer chains are unable to do so
• The longer fatty acid chains recombine with monoglycerides and glycerol to form triglycerides in the endoplasmic reticulum
• The triglycerides are packaged into lipoproteins called chylomicrons
  • Chylomicron structure is a spherical 'case' made of phospholipids and lipoproteins that hold the non-polar triglycerides inside, hidden from the aqueous environment of the cytoplasm and blood
  • Cholesterol and phospholipids are also found within chylomicrons

• These droplets are transported to a lacteal (a lymph vessel within the villus) via exocytosis
• Eventually, chylomicrons enter the bloodstream

Image showing the absorption of monoglycerides, fatty acids and glycerol