Lecture 35: Epithelia 2 Flashcards
(22 cards)
What is absorption driven by?
Na+ transport
What is secretion driven by?
Cl- transport
What is simple diffusion?
Ions and molecules move through a membrane, down their gradient, if it is permeable for these ions or molecules
What does facilitated diffusion require?
Specific membrane protein e.g. AQPs, GLUTs
What does active transport require?
Specific membrane protein and energy
* Primary active: ATP e.g. Na+/K+-ATPase
* Secondary active: Na+ e.g. SGLT1, SGLT2
Describe the rate and saturation of diffusion:
Occurs at a constant rate and can’t be saturated
Describe the saturation of mediated transport:
Can be saturated, dependent on the capacity of the transporter
Describe the transport of sodium across aborptive epithelia:
- Na+/K+-ATPase pumps 3Na+ out of cell and 2K+ into cell, generating a low [Na+] inside the cell (15mM) and a higher [Na+] (145mM) in the interstitium
- Na+/K+-ATPase requires ATP, to move Na+ against its gradient
- Na+ flows down its chemical gradient through the sodium channel in the apical membrane into the cell, with no energy required
- Typical in tight epithelia
How does Na+/K+-ATPase generate a negative charge inside the cell?
Pumps 3Na+ out of cell and 2K+ into cell
What is the role of K+ channels in the basolateral membrane?
Makes it permeable for K+ and the K+ gradient drives more K+ out of the cell making it more negative (-70mV)
What is the role of the negative charge inside the cell?
Attracts more positive ions such as Na+ into the cell from the lumen
How is a solute transported across a cell with sodium?
- Apical: Solute X is cotransported (secondary active) with Na+ into the cell up its concentration gradient
- Basolateral: Solute X is transported by facilitated diffusion/ transporter to blood side down in concentration gradient
- Typical in leaky epithelia
Where are channels and transporters mostly found?
Channels: tight epithelia
Transporters: leaky epithelia
Describe how glucose is transported into a cell?
- Sodium is the driving force
- Glucose is absorbed if there is a sodium-dependent glucose transporter (SGLT1)
- Once the glucose inside the cell reaches a certain conc. it is released into the interstitium via sodium-independent glucose transport (GLUTs)
What is primary active transport?
Directly uses ATP to move substances across a membrane e.g. Na+/K+-ATPase
What is secondary active transport?
Indirectly uses ATP to move substances across a membrane by cotransport
-> One substance moves down its gradient and is coupled with another substance to transport it against its gradient
What drives the absorption of Cl- and what type of absorption is it?
Para-cellular absorption
* In leaky epithelia increase in Na+ through channels/transporters causes lumen to become more -ve and interstitium to become more +ve
* Cl- can move from lumen to interstitium
Describe the transport of sodium through the para-cellular pathway:
- Leaky absorptive epithelium
- No hormonal control to allow bulk absoprtion
What drives the absorption of water in leaky epithelia and what type of absorption is this?
Trans- and para- cellular absorption
* In leaky epithelia Na+ absorption increases water gradient over the epithelium
What type of epithelium is in the proximal tubule and what is its permeability to water?
- Leaky epithelium
- High water permeability: transcellular through AQP1 and paracellular through tight junction
What type of epithelium is in the collecting tubule and what is its permeability to water?
- Tight epithelium
- Low water permeability: transcellular through AQP2
Is tight epithelia controlled by hormones?
Yes - fine regulation
