Lecture 23 C3 Flashcards
What is the role of tight junctions
Tight junctions act as a both a barrier to restrict the movement of substances through intercellular space between cells and as a fence to prevent membrane proteins from diffusing in the plane of the lipid bilayer. Therefore separate epithelial cells into two distinct membrane domains
What are the two distinct domains of epithelial cells courtesy of tight junctions
Apical (luminal/mucosal) membrane that faces hte lumen of the organ or body cavity.
Basolateral membrane that adheres to the adjacent basement membrane and interfaces with the blood
Describe the structure of tight junctions
thin bands that encircle the cell and make contact with thin bands from adjacent cells. More proteins= tighter
What are the two pathways transport can occur
Paracellular transport- through lateral intercellular space or Transcellular transport through the cell.
What is paracellular transport governed by
laws of diffusion and the tightness of the junction. greater number of tight junction strands holding the cell together = higher electrical resistance to ion flow so low current.
What is a leaky epithelium vs tight epithelium
in leaky epithelium paracellular transport dominates
in tight epithelium transcellular transport dominates
What is the change in tight junction resistance from proximal (closer to mouth) to distal (closer to anus)
less resistance closer to mouth: leaky epithelium with bulk paracellular transport.
Distal has hormonally controlled transcellular transport: tight epithelium.
What is transcelluar transport governed by
Primary and secondary active transport in combination with passive diffusion through ion channels.
What is the difference between secretion and absorption
secretion is transport from the blood to the lumen but absorption is transport from the lumen to the blood.
What are the rules for transepithelial transport
- The entry point for secretion and absorption is in different domains therefore different transport proteins will be used.
- Electrochemical gradient means that entry & exit step have to be one passive, one active
- Electroneutrality
movement of a positive or negative ion will attact a counter ion (usu. paracellular) - Osmosis: net movement of ions will establish a difference in osmolarity which causes water to flow.
How can you dissociate water flow from ion flow
Decreasing the Pw (membrane permeability to water) by blocking the aquaporins. Water wants to flow but it can’t go too much through lipid bilayer (pd)
What are the types of transporters used for entry and exit for both secretion and absorption usually
There is a primary active transport that set ups an ion gradient at the blood side. the entry step is secondary active transport while the exit step is passive diffusion. Water and counter ions follow whats being transported.
How is glucose transported from lumen to blood through epithelial cells in the small intestine
- Na/K pump sets up a Na+ gradient.
- This gives energy for glucose and Na+ to be co transported across the apical membrane by a SGLT transport protein
- After the glucose has been accumulated above conc grad, Glucose exits through the basolateral membrane through facilitated diffusion carrier mediated by GLUT.
- Na+ taken up by the synporter exits via the basolateral Na/K pump
- Cl- and H2O follow the glucose through the paracellular pathway
how is the glucose gradient maintained in order for facilitated exit of glucose to work?
The facilitated transport letting it exit isn’t as fast as the 2ndary active transport letting it in, so even if some leaves, the conc is still higher in the cell than out.
How does Oral Rehydration therapy work to help babies suffering from dehydration caused by diarrhoea
Glucose can enhance the absoprtion of Na+ and hence Cl- and water, therefore rehydrating the baby
