Traffic Across Cells: Epithelial Transport Of Glucose Flashcards
Epithethial tissues consist of cells arranged in what
Continuous sheets in either single or multiple layers
Epithelial tissues are subjectd to physical breakdown and injury therefore …
Undergo constant rapid renewal process
Tight junction
Hold cells together so can restrict movement via paracellular pathway
Intercellular (paracellular) space
Cells sit side by side, so there is space between cells, set up formation of tight junction. Not all can get through.
What is the structure of tight junctions
Composed of thin bands that encircle the cell and make contact with thin bands from adjacent cells.
TIght junctions can act as what
A barrier and a fence
What does it mean when a tight junction can act as a barrier
They restrict the movement of substances through the intercellular space between cells
What does it mean when a tight junction can act as a fence
They prevent membrane proteins from diffusing in the plane of the lipid bilayer.
Due to the barrier and fence function of the tight junction, they can separate the epithelial cells into what
Two distinct membrane domains: apical and basolateral
What is the apical domain
Apical (or luminal or mucosal) membrane faces the lumen of the organ or body cavity
What is the basolateral membrane
Adheres to the adjacent basement membrane and interfaces with the blood.
What do the two distinct membrane domains mean
This means that different transport proteins can be inserted into either the apical or basolateral surface. Tranport can either occur via paracellular, trnascellular or both.
Transport of proteins can occur via what
Paracellular (via tight junctions), transcellular (through the cell) or via both pathways.
What are the two distinct membrane domains that epithelial cells are separed into
Apical and Basolateral membrane
What is paracellular transport
Paracellular transport is goverened by the laws of diffusion and the tightness of the tight junctions.
Electrical resistance if paracelllar transport
heThe electrical resistance to ion flow through tight junctions can be measured.
The higher the electrical resistance to ion flow, the….
The greater the number of tight junction strands holding the cell together.
Epithelial tissues can be functionally classified into what two things
Leaky epithelium - paracellular transport dominates
Tight epithelium - transcellular transport dominates
How does tight junction resistance change
In proximal to distal direction in the GI tract and kidney
What is transcellular transport
When epoithelial cells use primary and secondary active transport often in combination with passive diffusion through ion channels to produce transport across epithelial tissues
Transcellular transport can be either what two things
- absorption: transport from lumne to blood
- secretion: transport from blood to lumen
Transepithelial transport can be broken down into what following areas that need to be considered
- entry and exit steps
- electrochemical gradient
- electroneutrality
- osmosis
Describe what happens in glucose absorption in the small intestine
- tight junctions divide apical and basolateral
- Na pump set up ion gradients
- SGLT uses energy of Na gradient accumulate glucose above its concentration gradient.
- GLUT mediates glucose exit across basolateral membrane via passive diffusion down its gradient
- Na taken up via basolateral Na pump
- transport of Na and glucose across epithelium induces paracellular Cl and water fluxes.
What is oral rehydration therapy
The ability of glucose to enhance the absorption of Na+ and hence Cl- and water is exploited in oral rehydration therapy.
What is glucose-galactose malabsorption syndrome
A mutation in the glucose symporter in the small intestine which means that sugar is retained in the intestine lumen.
In glucose-galactose malabsorption syndrome, what does an associated increase in lumen osomolarity do
Induces a watter efflux, which produces a pronounced diarrhea
What is the treatment for glucose-galactose malabsorption
- Therapy is used to remove glucose and galactose from the diet using fructose as a source of carbohydrate.
- utilises a facilitative transport (GLUT5) tht is specific for fructose.
- Fructose exits across the basolateral membrane and can use GLUT 2.
Glucose reabsorption in the kidney
In the kidney, glucose in the plasma is filtered and needs to be reabsorbed or it will appear in the urine.
Glucosuria - glucose in the urine
The commonest causes is diabetes mellitus beause insulin activity is deficient and blood sugar is too high.
The glucose symporter can not absorb glucose fast enough and glucose appears in the urine.
Glucose in the urine - transporter kinetics
- If glucose absorption is impaired or the transporter is saturated, glucose will appear in the urine.
- All filtered gluocse is reabsorbed until the renal threshold is reached. One reached, the glucose appears in the urine.
What does the renal threshold reflect
The transport maximum of SGLT.
