6.1.3 Absorption

• Digestion breaks down food into smaller, soluble molecules
• These products of digestion then pass from the lumen of the intestine into the blood
  • They pass through the microvilli of the cell surface membrane and into the epithelium cells
  • Then they move through the cell surface membrane that separates the epithelium cells from the blood; into the blood capillaries and lacteal (lymph vessels within the villus)

• Different mechanisms are required in the process of absorption including diffusion, active transport, exocytosis, and facilitated diffusion
  • Amino acids and monosaccharides both use facilitated diffusion, active transport and co-transport proteins in order to move across the epithelial membrane
  • Lipids are absorbed in a different way using simple diffusion, facilitated diffusion and exocytosis

Absorption of amino acids

• Specific amino acid co-transport proteins (a type of carrier protein) are found within the cell-surface membrane of the epithelial cells lining 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 also transported in
  • This occurs via facilitated diffusion, which requires the movement of molecules down their concentration gradient
• 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 capillary end of the cell

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

• Glucose is polar so cannot pass into the blood by diffusion
• 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
    • This is a passive process but depends on the concentration gradient of sodium ions from the lumen of the ileum into the epithelial cell
    • The gradient is maintained by the active transport of sodium ions out of the cell and into the blood via a sodium-potassium pump at the capillary end of the cell
  • The glucose molecules diffuse across the epithelial cell and enter the capillary by facilitated diffusion through a glucose channel protein

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

Absorption of lipids

• The products of lipid digestion are fatty acids, monoglycerides, and glycerol
• Absorption of these products is different from the absorption of monosaccharides and amino acids
• Fatty acids and monoglycerides can enter the epithelial cell by simple diffusion
  • They are non-polar molecules so they can diffuse through the phospholipid bilayer of the cell surface membrane
• Fatty acids also move by facilitated diffusion through fatty acid transport proteins
• Inside the epithelial cell, fatty acid chains recombine with monoglycerides or glycerol to form triglycerides, which are unable to diffuse back into the lumen
• The triglycerides are packaged up with cholesterol and encased in phospholipids and proteins to form lipoproteins
• These droplets then enter the lacteal or capillaries via exocytosis

Lipids are absorbed in the small intestine through a combination of simple diffusion, facilitated diffusion and exocytosis