IB Biology SL

Revision Notes

6.1.3 Absorption

Methods of Absorption

  • Digestion breaks down food into smaller, soluble molecules
  • These products of digestion then pass from the lumen through the cell membrane and into the epithelium cells. Then they move through the next cell membrane (the outer membrane of the mucosa) and are absorbed into the blood capillaries and lacteal
  • 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 a co-transport protein 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 (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

Amino Acid Cotransporter, downloadable AS & A Level Biology revision notes

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

  • Glucose is polar so cannot pass through 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 other end of the cell
  • Finally, the glucose molecules diffuse across the epithelial cell and enter the capillary at the other end of the cell by facilitated diffusion through a glucose channel protein

Glucose Cotransporter, downloadable AS & A Level Biology revision notes

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.

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
  • Triglycerides are digested into 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 simple diffusion
    • They are non-polar molecules so they can diffuse through the phospholipid bilayer of the cell membrane
  • Fatty acids also move by facilitated diffusion through fatty acid carrier proteins
  • Inside the epithelial cell, fatty acid chains recombine with monoglycerides and glycerol to form triglycerides, which cannot diffuse back into the lumen
  • The triglycerides are packaged up with cholesterol and encased in phospholipids and proteins to form lipoproteins
  • These droplets are transported to a lacteal (a lymph vessel within the villus) via exocytosis

Lipid Absorption, downloadable AS & A Level Biology revision notes

Image showing the absorption of monoglycerides, fatty acids and glycerol.

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