Cell Processes: Traffic across cells: Epithelial transport of glucose Flashcards
What are epithelial tissues made of?
cells arranged in continuous sheets in either single of multiple layers on top of the basement membrane
What do epithelial tissues form and what are they subject too?
the boundary between the body’s organs or between the body and the external environment
are subject to physical breakdown and injury – therefore undergo constant and rapid renewal process
What separates epithelial cells?
lateral intercellular space
What holds the luminal edges of epithelial cells together?
tight junctions
What do tight junctions consist of?
thin bands of proteins that encircle the cell and make contact with thin bands from adjacent cells
What do tight junctions appear to be in freeze fracture
interlocking network of ridges in the plasma membrane
What do increased ridges in cells mean?
the cells are more tightly packed
What is the barrier function of tight junctions
they restrict the movement of substances through the intercellular space between cells
What is the barrier function of tight junctions in the SKIN
they prevent microbes from entering the body
What is the fence function of tight junctions?
they prevent membrane proteins from diffusing in the plane of the lipid bilayer
What do tight junctions separate in epithelial cells
they separate the cell into two distinct membrane domains
What is the apical membrane
membrane that faces the lumen of the organ or body cavity
other names for the apical membrane
luminal or mucosal
What is the basolateral membrane
membrane that adheres to the adjacent basement membrane and interfaces with the blood
What does a cell having distinct membrane domains mean
transport proteins can be inserted into either the apical or basolateral membrane
What two pathways can transport occur
paracellular or transcellular pathway or via both
What is the transcellular pathway?
crossing 2 membranes (create across epithelial by electrochemical or osmotic gradients which pulls the movement of molecules by paracellular pathway)
What is the paracellular pathway?
crossing the lateral intercellular (paracellular) space
What governs Paracellular transport
the laws of diffusion and the tightness of the junctions
What is NOT involved in paracellular transport?
transport protein
What is measured in tight junctions?
electrical resistance to ion flow through tight junctions can be measured
What does a higher electrical resistance mean?
the greater the number of tight junction strands holding the cell together
What does more strands in a tight junction do
More strands = tighter tight junctions = higher electrical resistance = lower current flow
What is leaky epithelium?
epithelial tissue where paracellular transport dominates
What pathway does leaky epithelial tissue allow?
Ability to move molecules through paracellular pathway
What is tight epithelium?
epithelial tissue where transcellular transport dominates
Does tight epithelium allow paracellular transport?
No
Examples of ‘leaky’ epithelium location
Duodenum, proximal tubule
Examples of ‘tight’ epithelium
Colon, collect duct
What type of epithelium is proximal?
Leaky epithelium
What electrical resistance is proximal?
low
What number of strands is proximal
low
What type of transport is proximal
bulk transport (paracellular)
What type of epithelium is distal?
tight epithelium
What electrical resistance is distal?
high
What number of strands is distal
high
What type of transport is distal?
hormonally controlled (transcellular)
What type of transport is absorption
transport from lumen to blood
What type of transport is secretion
transport from blood to lumen
What is transcellular transport?
Epithelial cells using primary and secondary active transport in combination with passive diffusion through ion channels to produce transport across the epithelial tissues
Rules of transepithelial transport (4)
entry and exit steps, electrochemical gradient, electroneutrality, osmosis
What are is the entry step for absorption and secretion
the entry step for absorption is the apical but for secretion is the basolateral membrane
What is the electrochemical gradient for transepithelial transport
is the entry or exit step passive or active
What is electroneutrality in transepithelial transport
movement of a positive or negative ion will attract a counter ion
What is osmosis in transepithelial transport
net movement of ions will establish a difference in osmolarity that will cause water to flow by osmosis
What is osmosis
Net movement of water through a selectively permeable membrane from an area of high water concentration to an area of lower water concentration
What are the 3 properties of permeability through lipid bilayer
Small (not a lot of water flow through)
Mercury insensitive
Temp dependent (lipid fluidity)- high temp higher fluidity
Formula for permeability to water
Pw= Pd + Pf
What does Pd mean
through lipid bilayer
What does Pf mean
through water channel
Is Pd or Pf bigger
Pf > Pd
What are the 3 properties of permeability through water channel
Large
Mercury sensitive (you can block the channel)
Temp independent
What is osmotic pressure?
The pressure applied by a solution to prevent the inward flow of water across a semi-permeable membrane
What regulates the movement of water (Pf)
aquaporins
What are aquaporins
proteins that allow only water to flow through, 9 isoforms
Why do cells have different Pw
Cells have different Pw because they express different aquaporin isoforms
What does sodium glucose symporter (SGLT) do
Uses the energy of the sodium gradient to actively accumulate glucose above its concentration gradient
What sets up the ion gradient in glucose absorption in the small intestine
Na-pump sets up ion gradients by carrier mediated transport (primary active transport)
What ions are brought into and out of a cell by Na pump in glucose absorption in the small intestine
2K+ in and 3Na+ out
What does Facilitative glucose transporter (GLUT) do in glucose absorption in the small intestine
mediates glucose exit across basolateral membrane via passive diffusion down gradient
Why does Na that was taken up move out of cell via the basolateral Na-pump in glucose absorption in the small intestine
to maintain Na at low levels. sodium is maintained at low level in cell creating positive charge on blood side which attracts Cl- and H20
What does the movement of Na and glucose out of the cell do in glucose absorption in the small intestine?
induces paracellular Cl and water influxes
What is oral rehydration therapy
A simple sugar solution when given to dehydrated babies suffering from diarrhea is responsible for saving millions of lives per year
What does oral rehydration therapy use?
The ability of glucose to enhance the absorption of Na+ and hence Cl- and water
What is Glucose-galactose malabsorption syndrome
A mutation to the glucose symporter (SGLT - glucose absorption entry step) in the small intestine means that sugar is retained in the intestine lumen
Treatment for glucose-galactose malabsorption syndrome
Removing glucose and galactose from the diet, using fructose as a source of carbohydrate
What does glucose not being taken up do in Glucose-galactose malabsorption syndrome
Osmolarity of lumen increases (low H2O as glucose isn’t being absorbed/high solute conc) so Water moves from blood into small intestine to balance out osmolality. The associated increase in lumen osmolarity induces a water influx. The increased water flow produces a pronounced diarrhoea
What is GLUT5
Fructose transporter
What is the Renal threshold
200 mg/dl
What happens if glucose isn’t reabsorbed in the kidney
it appears in the urine
How much glucose is in the kidney
5 mmol
What energy does Glucose reabsorption in the kidney
use
Using energy from sodium ATPase
What is blood sugar level in diabetes
over 200mg/mL
What is Glucosuria
glucose in the urine
What happens to the glucose sympoter in diabetes?
can not absorb glucose fast enough and glucose appears in the urine
When does glucose appear in the urine
When renal threshold is reached
What does the renal threshold reflect?
the transport maximum of SGLT
