Epithelial Transport of Glucose Flashcards
What effects transepithelial transport
Combining different types of transport proteins
Epithelial tissues
- Consist of cells arranged in ______ sheets in either ___ or ____ layers
- Cells sit on a _____ membrane
- Form the _____ between the body’s organs or between the ___ and _____ environment
- Subject to physical breakdown and ____ … therefore undergo ____ and rapid renewal process
Epithelial tissues
- Consist of cells arranged in CONTINUOUS sheets in either SINGLE or MULTIPLE layers
- Cells sit on a BASEMENT membrane
- Form the BOUNDARY between the body’s organs or between the BODY and EXTERNAL environment
- Subject to physical breakdown and INJURY… therefore undergo CONSTANT and rapid renewal process
What are epithelial cells separated from the neighbours by
Lateral intercellular space
How are epithelial cell held together
Their luminal edges by tight junctions
What type of junction is composed of thin bands that encircle the cell and make contact with thin bands from adjacent cells
Tight junctions
Appearance of freeze fracture tight junctions
Interlocking network of ridges in the PM
Why tight junctions act as a barrier and a fence
To separate the epithelial cells into 2 distinct membrane domains
Name the 2 distinct membrane domains
- Apical (luminal or mucosal)
- Basolateral
Tight junctions act as a barrier or a fence when “they restrict the movement of subst. through the intercellular space between cells”
Tight junctions act as a BARRIER
Tight junctions act as a barrier or a fence when “they prevent membrane proteins from diffusing in the plane of the lipid bilayer”
Tight junctions act as a FENCE
What membrane domain faces the lumen of the organ/body cavity
Apical (luminal or mucosal)
The basolateral membrane…
Adheres to the adjacent basement membrane and interfaces with the blood
Why do we need the distinct membrane domains
So different transport proteins can be inserted into either the apical or basolateral membrane
True or false, “transport can occur via the paracellular OR transcellular pathway OR both”
TRUE
What laws is paracellular transport governed by
Laws of diffusion and tightness of junctions
True or false, “the electrical resistance to ion flow through tight junctions can’t be measured”
FALSE… electrical resistance to ion flow through tight junctions CAN be measured
What happens to number of tight junction strands holding the cell together if there’s higher electrical resistance to ion flow
Greater number of tight junction strands holding the cell together
Epithelial tissues functional classifications
- Leaky epithelium = paracellular transport dominates
- Tight epithelium = transcellular transport dominates
What changes in proximal to distal direction in GI tract and kidney
Tight junction resistance
Features of the proximal direction
- Leaky epithelium
- Low electrical resistance
- Low # of strands
- Bulk transport (paracellular)
e.g duodenum/proximal tubule
Features of the distal direction
- Tight epithelium
- High electrical resistance
- High # of strands
- Hormonally controlled (transcellular)
e.g. colon/collect duct
What is transcellular transport
Epithelial cells use 1o and 2o active transport often in combination with passive diffusion through ion channels to produce transport across the epithelial tissues
Transcellular transport can either be
- Absorption = transport from lumen to blood
- Secretion = transport from blood to lumen
4 areas transepithelial transport is broken down into
- Entry and exit steps
- Electrochemical gradient
- Electroneutrality
- Osmosis
What area of transepithelial transport “the entry step for absorption is the apical but for secretion is the basolateral membrane”
Entry and exit steps
The electrochemical gradient area of transepithelial transport does…
Entry or exit step passive or active
What area of transepithelial transport “movement of a positive or negative ion will attract a counter ion”
Electroneutrality
Osmosis area of transepithelial transport does…
The net movement of ions will establish a difference in osmolarity that will cause water to flow by osmosis
What do epithelial cells use to mediate either secretion or absorption
Different collections of transporters and channels
Step 1 of glucose absorption
Tight junctions divide cells into apical and basolateral membrane domains
Step 2 of glucose absorption
Na-pump sets up ion gradients
Step 3 of glucose absorption
The sodium glucose symporter (SGLT) uses the energy of the Na gradient to actively accumulate glucose above its conc. gradient
Step 4 of glucose absorption
Facilitative glucose transporter (GLUT) mediates glucose exit across the basolateral membrane via passive diffusion down its gradient
Step 5 of glucose absorption
Na take up via the glucose exits via the basolateral Na-pump
Step 6 of glucose absorption
The transport of Na and glucose across the epithelium induces paracellular Cl and water influxes
The ability of glucose to enhance the absorption of Na+ and Cl- and water is exploited in…
Oral rehydration therapy
What causes glucose-galactose malabsorption syndrome
A mutation to the glucose symporter in the small intestine means that sugar is retained in the intestine lumen
What is induced due to the associated increase in lumen osmolarity
Induces a water efflux
The increased water flow produces
Pronounced diarrhea
What is the treatment for glucose-galactose malabsorption
- Remove glucose and galactose from diet
- Use fructose as source of carbohydrate
What utilises a facilitative transporter (GLUT5) that is specific for fructose
Therapy for glucose-galactose malabsorption
What fructose uses to exit across the basolateral membrane
GLUT2
In the kidney ____ in the ____ is filtered and needs to be _____ or it will appear in the ___
In the kidney GLUCOSE in the PLASMA is filtered and needs to be REABSORBED or it will appear in the URINE
Name for when glucose is in the urine
Glucosuria
What is the most common cause in diabetes mellitis as insulin activity is deficient and blood sugar too high (over 200mg/mL)
Glucosuria - glucose in the urine
What happens to glucose symporter in diabetes
Can not absorb glucose fast enough and glucose appears in the urine
What happens to urine if glucose absorption is impaired or the transporter is saturated
Glucose will appear in the urine
Glucose in urine - transporter kinetics
- All filtered ____ is reabsorbed until the ___ threshold is reached
- Once the renal ____ is reached ____ appears in the ____
Glucose in urine - transporter kinetics
- All filtered GLUCOSE is reabsorbed until the RENAL threshold is reached
- Once the renal THRESHOLD is reached GLUCOSE appears in the URINE
What does renal threshold reflect
Transport maximum of SGLT (sodium-glucose linked transporters)
