L17 - Traffic across cells: Epithelial transport of glucose Flashcards
(20 cards)
What is the function of tight junctions
They restrict movement of substances through the intercellular space between cells
They prevent membrane proteins from diffusing in the plane of the lipid bilayer
They separate the epithelial cells into two distinct membrane domains
What are the two classifications of epithelial tissues
Leaky epithelium and tight epithelium
Describe ‘proximal’ tight junction resistance
Leaky epithelium
Low electrical resistance
Low number of strands
Bulk transport (paracellular)
e.g duodenum, proximal tubule
Describe ‘distal’ tight junction resistance
Tight epithelium
HIgh electrical resistance
High number of strands
Hormonally controlled (transcellular)
e.g colon, collect duct
What is absorption
Transport from lumen to blood
What is secretion
Transport from blood to lumen
What are the 4 transepithelial transport rules (factors to consider)
Entry and exit steps
Electrochemical gradient
Electroneutrality
Osmosis
What is the entry steps for absorption and secretion
Entry step for absorption is the apical membrane
Entry step for secretion is the basolateral membrane
What is electroneutrality
Movement of a positive or negative ion will attract a counter ion
What are the steps of glucose absorption
Na/K ATPase pump sets up ion gradients
SGLT uses energy of the Na+ gradient to actively accumulate glucose above concentration gradient
GLUT mediates glucose exit across basolateral membrane via passive diffusion
Na+ taken up exits via basolateral Na/K pump
Na+ and glucose across epithelium induces paracellular Cl- and H2O flux
What is the net result of absorbing a molecule of glucose
Absorbs Na+, Cl- and H2O
What is oral rehydration therapy
Sugar, salt and water
- utilises glucose and sodium chloride to stimulate fluid uptake
What is glucose-galactose malabsorption syndrome
A mutation to the glucose symporter in the small intestine
Means that sugar is retained in the intestine lumen
How does glucose-galactose malabsorption syndrome lead to diarrhea
The intestinal lumen has an increase in osmolarity, so water will move into the lumen, resulting in watery chyme
How can glucose-galactose malabsorption be treated
Eating fructose-based sugars instead of glucose
What are the transporters for fructose
The faciliatative transporter on the apical membrane is GLUT5
Fructose exists basolateral membrane via GLUT2
Where is glucose absorbed in the kidney
Proximal tubule
How does diabetes lead to glucose in the urine
The Glucose symporter (SGLT) cannot absorb glucose fast enough
What is glucosuria
Glucose in the urine, most commonly caused by diabetes mellitus
What is the renal threshold
The threshold at which the transport rate of SGLT is at a maximum
