Traffic across cells and glucose absorption Flashcards
What is the structure of epithelial tissues?
Consist of cell arrange din continuous sheets in either single or multiple layers
What are epithelial tissue on top of?
A basement membrane
What are epithelial tissues the boundaries between?
The body organs or between the body and the external environment
What is the rate of epithelial cell renewal? Why is this?
Very high Subject to physical breakdown and injury constantly
What are the two types of epithelium? Where are they found?
Covering and lining - epidermis of skin, lining of blood vessels and ducts Glandular - endocrine glands, sweat glands etc.
What are the classifications of their arrangements? Describe them
Simple - one cell layers stratified - many cell layers pseudo stratified - single layer of cells which look striated but is single layer
What are the classifications of the shape? Describe them
Squamous - flat Cuboidal - cube shape Columnar - tall Transitional - shape changes due to stretching
How are epithelial cell separated from each other? What keeps them connected then?
Separated by lateral intercellular space Held together by tight junctions
What are tight junctions composed of? What how do they allow adjacent cells to maintain cellular contact?
Thin bands that encircle the cell make contact with thin bands from adjacent cells
How does the number of contact points between neighbouring cells impact the tight junctions functions?
More contact points makes it harder for substances to move between adjacent cells so becomes ‘tighter’ junction
What are the functions of tight junctions? Explain what this function does?
A barrier - restrict the movement of substance throughout intercellular space between cells A fence - prevent membrane protein from moving all over the cell membrane as it controls their free movement
What kind of materials are epithelial cells a barrier to? How does it restrict these?
Larger molecules/cells their movement is stopped as they can’t physically fit through gaps
What does the fencing function of tight junctions create? What are they?
Apical membrane - membrane that is on top facing the external enviro (e.g. lumen, top layer of skin FYI) Basolateral membrane - membrane that adheres to the basement membrane and interfaces with blood
What are the two transport methods created by the apical and basolateral membrane?
Paracellular transport and transcellular transport
Explain where paracellular transport takes place
It occurs between the tight junctions down the intercellular space
What controls paracellular transport?
The laws of diffusion and the tightness of the tight junctions
How does more tight junctions affect paracellular transport?
More tight junctions/tighter junctions creates a stronger resistance to ion flow
How does tight junction resistance vary in the GI (gastrointestinal) tract and kidney?
It is leakiest at the proximal end (near the stomach) and tightest at the distal end At the start of the digestion are large molecules which need to be absorbed therefore leaky, at the end it is mainly just ions and non-digestible food therefore need to be more selective = tighter junctions
Explain where transcellular transport takes place
Through the apical membrane and basolateral membrane
What controls transcellular transport?
Passive diffusion combined with ion channels
What kind of processes can transcellular transport be involved in? What are the relative directions of each process?
Absorption (lumen to blood) and secretion (blood to lumen)
How can epithelial transport occur?
Either via paracellular, transcellular or both
What is the relationship between paracellular and transcellular transport? Explain the cause of this relationship?
They are interdependent The movement of ions through the transcellular pathway almost always results in the movement of water and other molecules through the paracellular pathway in order to maintain an osmotic equilibrium
How does the electrical resistance of epithelial material correspond and its ability to function as an impenetrable barrier correspond with the tight junctions? Give examples of a strong vs weak barrier.
Epithelia which has more tight junctions will have more electrical resistance and be a better barrier Eg skin = high electrical resistance as it is a protective barrier, small intestine = low electrical resistance as its primary function is to absorb
How can epithelial tissues be functionally classified in terms of their function as ion barrier? What kind of transport mechanism dominates for each?
Leaky epithelium: paracellular transport dominates (tight junctions are not protective) tight epithelium: transcellular transport (relying on protein channels)
What are the structural difference between tight junctions at the proximal (start) and distal (end) parts of the GI? What properties does this give?
proximal has a lower number of tight junction strands, distal is more involved in hormonally controlled transcellular transport
What kind of methods of transport does trancellular transport use?
Use primary and secondary active transport in combination with passive diffusion through ion channels
What are the functions of transcellular transport?
Absorption, lumen to blood Secretion, blood to lumen
What are the rules of transepithelial transport?
-There is an entry and exit point on the cell: either apical or basolateral membrane depending if secreting or absorbing) -there is always energy being transferred during transport: either from the electrochemical gradient bring in material, by harnessing the gradient in secondary transport or from hydrolysis of ATP -when an ion moves it attracts another oppositely charged one: the cell needs to maintain electroneutrality -when an ion moves it will affects the osmolarity of the cell
What is the epithelial structure of glands?
Two different types of epithelial cells connected in series: Acinar ells and Duct cells
What is the function of Acinar cells?
Create a primary secretion rich in organic molecules (e.g. enzymes, regulatory molecules etc.)
What is the function of Duct cells?
Modify the composition of the primary secretion by either absorbing or secreting specific ions (e.g. HCO3-, Cl- etc.)
Label the diagram and explain the processes 1-6 that are shown on the diagram
What kind of transporter is used to bring glucose into the epithelial cell from the lumen?
A symporter using Na+
How is glucose transported from the epithelial cell to the blood? What protein is used?
By mediated passive diffusion protein GLUT
How is it that glucose is able to passively diffuse from the epithelial cell into the blood?
There is a higher concentration of glucose in the cell due to the symporter than the blood therefore it moves down its concentration gradient
When glucose is absorbed what other particle does it bring in? What does the cell do about this?
It also brings in a Na+ ion as part of the symporter Na+ is removed from the cell by the primary active transporter
What needs to be conserved when the Na+ is transporter from the epithelial cell to the blood? What is used to conserve this? How and where is this transported to?
Electroneutrality Cl- Paracellular transport into the blood
What has been created once glucose, Na+ and Cl- has been absorbed into the blood? What is used to restored equilibrium? How does it get transported there?
An osmotic gradient H2O Paracellular transport
When glucose is no longer present in the lumen, what is the movement of glucose? What kind of movement is this? Why?
Glucose is diffused from the blood into the epithelial cell Glucose is transported from the epithelial cell by a passive channel during digestion and when there is no glucose in the lumen there is no active transport elevating the concentration of glucose in the cell causing it to drop therefore glucose diffuses back into the cell as the conc is higher in the blood now
How is the rate of water absorption affected by glucose and salt? What is a real life application of this?
Glucose enhances the absorption of Na+ and Cl- and this increases the absorption of water Used as a oral rehydration therapy for dehydration caused by diarrhoea
What is the name of the syndrome caused by an inability to absorb glucose and galactose?
Glucose/galactose malabsorption syndrome
What causes glucose/galactose malabsorption syndrome?
A mutation in the sodium dependent glucose co-transporter protein (SGLT)
How does glucose/galactose malabsorption syndrome affect somebody? What does it cause? What are the consequences of this cause?
It means that glucose and galactose remain in the small intestine It causes an osmotic imbalance as there is a higher concentration in the lumen This causes water to move from the epithelial cells into the lumen resulting in diarrhoea
How is glucose/galactose malabsorption syndrome treated? What about this treats the patient?
By substituting a diet with glucose and galactose with fructose instead Fructose uses a different facilitative transporter specific to fructose (GLUT5) so energy can be extracted from this sugar instead
What is function of the kidney?
Filters out the substances in the blood and returns the nutrients into the blood and removes the waste/toxins into the bladder
What kind of nutrients are reabsorbed into the blood by the kidneys?
Glucose, Na+, Cl-, water
Explain how is glucose reabsorbed from the kidney back into the blood stream? What other particles does it reabsorb in this process?
1 - Na/K pump pumps Na+ out of proximal tubule cells (cells lining the kidney ducts FYI) into the Peritubular capillary 2 - glucose and 2Na+ are then transported by a symporter into the cell 3 - higher concentration of glucose in the cell than blood diffuses via passive facilitated transporter into peritubular capillary 4 - to maintain osmolarity and electroneutrality H2O and Cl- are diffused through the tight junctions and into the capillary Na+, H2O, Cl-
How much water is filtered a day? How much is reabsorbed into the blood stream? What is the name of the capillary that these nutrients are reabsorbed into?
180L a day 70-80% Peritubular capillary
In a healthy person, how much glucose is found in the urine?
None
What kind of condition can causes glucose to accumulate in the urine?
Diabetes
Why do people with diabetes have glucose in their urine?
There is so much excess glucose in their blood that it cannot all be reabsorbed as the transporters become saturated
What are the parameters used to measure how effectively the kidneys are working? What does this allow?
The concentration of glucose in the blood going into the kidney concentration of glucose in the blood in the pertitubular capillary the amount of glucose being removed as urine Can quantify how effective the kidneys are working
What is the concentration of glucose in the blood for someone with diabetes mellitus?
over 200mg/ml
What is the measured value of the renal threshold? What is it a measure of?
200mg/100ml It is the amount of plasma glucose maximum of SGLT uptake (glucose transporter on the lumen side of epithelial cells)
What is the maximum rate of glucose transport?
375mg/min
When is the maximum rate of glucose transport reached?
When the renal threshold is reached
What happens the concentration of glucose in the blood exceed the renal threshold? Why does this happen?
The glucose starts to appear in the urine The glucose transporters can no longer keep up with the concentration of glucose in the blood so it passes through unabsorbed
