During absorption, digested food monomers must pass from the lumen into the epithelial lining of the small intestine:
Secondary Active Transport
During absorption nutrients from food must pass from the lumen of the small intestine to the cells in the capillaries or lacteals in the villi.
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+)
- 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
- 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
During absorption nutrients from food must pass from the lumen of the small intestine to the cells in the capillaries or lacteals in the villi.
- Many types of transport are used to move different nutrients into and out of the epithelium cells of the villi
Methods of Membrane Transport
Bulk Transport
Endocytosis
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
These modes of transport will be outlined using two products of digestion.
Glucose
Since glucose has many hydroxyl groups it is a polar molecule and cannot pass through the cell membrane by simple diffusion and therefore relies on different types of facilitated diffusion in order to move into and out of the epithelial cells of the villi.
Glucose
Since glucose has many hydroxyl groups it is a polar molecule and cannot pass through the cell membrane by simple diffusion and therefore relies on different types of facilitated diffusion in order to move into and out of the epithelial cells of the villi.