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•  Different methods of membrane transport are required to absorb different nutrients

During absorption, digested food monomers must pass from the lumen into the epithelial lining of the small intestine

  • Tight junctions between epithelial cells occlude any gaps between cells – all monomers must cross the membrane
  • Different monomers undertake different methods for crossing the apical and basolateral membranes

Membrane Transport Mechanisms

Secondary Active Transport

  • A transport protein couples the active translocation of one molecule to the passive movement of another (co-transport)
  • Glucose and amino acids are co-transported across the epithelial membrane by the active translocation of sodium ions (Na+)

Facilitated Diffusion

  • Channel proteins help hydrophilic food molecules pass through the hydrophobic portion of the plasma membrane
  • Channel proteins are often situated near specific membrane-bound enzymes (creates a localised concentration gradient)
  • Certain monosaccharides (e.g. fructose), vitamins and some minerals are transported by facilitated diffusion


  • Water molecules will diffuse across the membrane in response to the movement of ions and hydrophilic monomers (solutes)
  • The absorption of water and dissolved ions occurs in both the small and large intestine

Simple Diffusion

  • Hydrophobic materials (e.g. lipids) may freely pass through the hydrophobic portion of the plasma membrane
  • Once absorbed, lipids will often pass first into the lacteals rather than being transported via the blood

Methods of Membrane Transport

absorption methods

Bulk Transport


  • Endocytosis involves the invagination of the plasma membrane to create an internal vesicle containing extracellular material
  • Vesicle formation requires the breaking and reforming of the phospholipid bilayer and hence is an energy-dependent process
  • In the intestines, vesicles commonly form around fluid containing dissolved materials (pinocytosis – cell ‘drinking’)
  • Pinocytosis allows materials to be ingested en masse and hence takes less time than shuttling via membrane proteins

Mechanism of Endocytosis


Summary of Absorption Methods

absorption summary