Lecture 17: Glucose transport Flashcards
What is a tight junction?
Lateral cell to cell connection near the apical surface formed from transmembrane proteins called claudins and occludins
What is the paracellular space?
The space between the lateral side of adjacent cells formed between cell junctions
What is the function of tight junctions?
Barrier - restrict movement of large substances through the lateral space between cells
Fence - prevent membrane proteins from diffusing in the plane of the lipid bilayer
THEREFORE
the tight junctions separate the layer of epithelial cells into two distinct domains
apical membrane - faces the lumen of the organ (outside)
basolateral membrane - adheres to the adjacent basement membrane and faces the blood
Where are membranes found on epithelial tissue?
Either on the apical membrane facing lumen of organ or the basolateral membrane facing the basement membrane and blood beneath it
What are the two pathways of transport of substances across epithelial tissue?
Paracellular and transcellular pathways
What is paracellular transport?
Passive transport in which water and ions pass through the lateral spaces between adjacent epithelial cells
What determines the amount of transport occurring through the paracellular pathway?
The tightness of junctions - the resistance to flow can be measured
The higher the resistance, the greater the number of junctions, more tight
What are the two types of epithelial tissues with respect to transport?
Leaky epithelium - paracellular transport dominates
Tight epithelium - transcellular transport dominates
How does the resistance of tight junctions change in the proximal to distal direction of the kidney?
Leaky to tight as proximal to distal
What is transcellular transport?
Transport across epithelial tissue by the involvement of primary and secondary active transport often in combination with passive diffusion through ion channels
What are the two types of transcellular transport in terms of function?
Absorption - lumen to blood
Secretion - blood to lumen
What are the rules to consider in transepthelial transport
1) entry and exit steps - where the substance is moving from and to
2) electrochemical gradient - is the entry or exit passive or active
3) electroneutrality - is there movement of a positive or negative ion that attracts a counter ion
4) osmosis - net movement of ions will mean osmolarity changes causing water to move by osmosis
Explain the stages to how glucose absorption occurs in the small intestine?
Glucose will be absorbed from the lumen of small intestine to the blood across epithelial tissue
- A Na+ gradient is set up by Na+ transporters creating high sodium concentration in the blood and low sodium concentration in the lumen of small intestine
- Glucose is moved into the epithelial tissue from the lumen of the intestine through the sodium glucose symporter (SGLT) on the apical membrane and is accumulated in high concentrations
- Facilitative glucose transporter (GLUT) in the basolateral membrane mediates exit of glucose via passive diffusion from the epithelial cells into the blood
- Sodium is pumped out of epithelial cells via Na+ pump in the basolateral membrane into the blood
- The transport of Na and glucose across the epithelium causes paracellular transport of water and Cl ions (water to balance osmolarity, and Cl to balance potential)
What is oral rehydration therapy?
A treatment to dehydration causing diarrhea in babies
A solution of water, salt, and sugar is taken to enhance the absorption of sodium ions -> causing more uptake of Cl- ions with water along the paracellular pathway
What is glucose-galactose malabsorption syndrome?
A mutation to the glucose symporter in the apical membrane lining the small intestine meaning glucose cannot be absorbed.
This means the water potential in the small intestine is low causing a water efflux into the small intestine resulting in diarrheaq
