Understanding:
• 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
Osmosis
- 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
Bulk Transport
Endocytosis
- 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
