Absorption (DP IB Biology: SL)

• 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

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 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

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