Membrane Transport Flashcards
Describe endocytosis
Membrane wraps around the molecule and brings it into the cells
Why is clathrin important in endocytosis
Aggravates aand binds to membrane
Forces it to bend inwards and pinch off
Describe exocytosis
Vesicles fuse with the membrane and releases its contents
Why is clathrin important in exocytosis
Hen it binds it opens up the vesicle
Describe the permeability of the phospholipid bilayer
High permeability to gases and steroids
Low permeability to ions
What determines the resting membrane potential
The distribution of ions across the membrane
What can oppose the movement of ins across the membrane
A charge difference
What is the importance of the Nernst equation
Predicts the equilibrium potential of conc gradient across membrane
What is the Nernst potential
No net movement of ions as net charge and conc are in equilibrium
Why is the Goldman-Hodgkin-Katz equation better than the Nernst equation
It takes into account all the ions and their respective permeabilities
How is resting membrane potential generated
The asymmetrical distribution of ion especially Na+ and K+
The differential permeability of the membrane to the ions
Why is the resting potential closer to the Nernst value for K+
Membrane more permeable to K+ due to having more K+ leaky channels
Whats the permeability ratio of K+:Na+ in non excitable cells
2:1
What is the voltage clamp used for
To allow for detailed measurements and analysis of electrical activity across the tissue
How does the voltage clamp work
Apply and change the voltage in increments
Record the effect on ion channel currents
What does measuring the current in a voltage clamp show
The total ionic current flowing through the membrane
Describe how a patch clamp works
Recording pipettes isolate a patch of membrane on the cell surface
Measures current flow through a single ion channel
What is a gigaseal
High resistance seal between membrane and micropipette created by suction
Why is a gigaseal important
Minimises background noise that can swamp the single channel currents
What are the 2 types of secondary transporters
Co transporters i.e symporters
Counter transporter i.e antiporters
How does a symporter work
Couples movement of a solute w movement of another’s down its conc gradient
How do antiporters work
couples movement of 2 or more solutes in opposite directions
What is the equation for flux
Flux = permeability x conc difference
Describe the flux at equilibrium
Inward flux = outward flux
Net flux = 0
What is the equation for permeability
Permeability = diffusion coefficient x partition coefficient /distance
What does the diffusion coefficient measure
The size of the diffusion area at a give time point
What does the partition coefficient measure
How easily a substance crosses the membrane
What are the 3 types of transporters that facilitate diffusion
Channels
Gated channels
Uniporters
Why’s is there a greater rate of flux at lower [solute]
Less solute flowing through so channels are not fully saturated
How do P-type ATPases work
Use phosphorylation of ATP to move substrate from low to high conc
Describe the structure of Na+/K+ pumps
Has 10 transmembrane domains and 3 cation binding domains
What domains are involved in regulation in Na+/K+ pumps
Nucleotide, phosphorylation and actuator
Why is there a 3:2 stoichiometry for Na+/K+ pumps
Na+ fits into the domain easier as K+ is larger
What can inhibit Na/K+ pumps
Ouabain and cardiac glycosides
What is the Gibbs-Donnan effect
Describes the unequal distribution of ions on either side of the membrane in the presence of other ions that can’t cross the membrane
What is the impact of negatively charged molecules on the electrical activity of the cell
Largely try and hold onto the positive ions that try and leave the cell
How does the Na+/K+ pump raise the water potential in the cell
Removes some of the dissolved solutes
Less driving force for the water to come in
What are ATP binding cassette (ABC) transporters
Channels that transport small molecules
What increases the probability of ABC transporters opening
Presence of ATP in the NBD domain
Phosphorylation of R domain
What are the 2 different membrane spanning domains that ABC transporters have
Nucleotide binding domain NBD
Regulatory domain R
How is the CFTR an atypical ABC transporter
ATP regulates the channels
It binds to the R domain not just to the NBDs
What is osmolarity
The total conc of dissolved particles in a litre solution
What is osmolality
The number of dissolved particles per unit mass
What is osmotic pressure
Pressure exerted by flow of water across membrane determined by solute conc
What is tonicity
Measure of the effect of a solution has on cells placed in it
Driven by osmolarity
What is the equation for water flow
Water flow = hydraulic water permeability x change in pressure
What is the movement of a solute dependent on
The change in concentration
Concentration difference and the water flow
What is osmotic pressure
The pressure applied to a system to perfectly balance the flow of water across the membrane
What does it eat if the solute coefficient equals 1
Membrane is impermeable to the solute
What does it eat if the solute coefficient equals 0
Membrane is fully permeable
What does it eat if the solute coefficient equals 0.5
The membrane is partially permeable
What hormone increase transcription of aquaporins
Vasopressin
Why is there a high proliferation rate in epithelial cells
Resit a lot of damage and therefore are prone to cancer
Describe the structure of epithelial cells
Sheet like structures that line external and internal structures
What are the protective epithelial cells
Simple columnar and stratified squamous
What epithelial cells are involved in secretion and absorption
Simple squamous and simple cuboidal
What epithelial cell is stretchy
psudeostratified squamous
What are tight junctions
Membrane proteins that bind adjacent epithelial cells
What is the function of tight junctions
Form a physical barrier to separate the baso lateral and apical sides
How are tight junctions useful for absorption and secretion
Create a polarity across the cell
Cell can express different transporters on different sides of the cell
What molecules form a tight junction
Claudin and occludin
What happens if there is low resistance in tight junctions
Leaky junctions - daily secrets and absorbs
What molecule causes leaky junctions
Claudin 2
What are anchoring junctions
Proteins that anchor to the basal lamina and neighbouring cells
Forms cohesive unit for stability
Why are gap junctions important
Allow neighbouring cells the share the same intracellular environment
Important for cell signalling
What os the basement membrane
Basal lamina + reticular laminate
What is the importance of basal lamina
Strong flexible foundation
Allows cell to root to something
What is the first step in NaCl absorption
Passive facilitated entry of Na+ across apical lumen membrane
Through Na+ channels
What is the send step in NaCl absorption
Active exit of Na+ across the basolateral membrane
What is the third step of NaCl absorption
Paracellular diffusion of Cl- through tight junctions
What is the last step in NaCl- absorption
Absorption of water that is osmotically driven
What is ENaC
Major sodium epithelial channel
What is the function of ENaC in the lungs
To control the amount of airway surface fluid and alveolar lining fluid
What is the function of ENaC in the kidneys
Controls whole body [Na+] and blood pressure therefore blood volume
What is the function of ENaC in the sweat glands
Na+ retention/reabsorption by sweat ducts to produce hypotonic sweat
What is the structure of ENaC
3 functional subunits; alpha, beta and gamma
2 transmembrane domains per subunit
Each subunit has a large loops that stick out of the luminal side
What is necessary for a functional ENaC
All 3 subunits - constituency active but regulated
What forms the pore of ENaC
The trans membrane domains of the subunits
What inhibits EnaC
Amiloride
What is the importance of the loops on hte subunits
Site of proteases (CAP) that crave the loops to increase activity
What is the importance of SPLUNC1 when bound to ENaC
Binds to beta ENaC to protect regulation
What is the ENaC expressed by in the nephron
Aldosterone sensitive distal nephron (ASDN)
What is the importance of ENaCs in ASDN
Determine final urine salt composition through aldosterone
What happens if the salt intake is low
Low blood vol -> aldosterone release -> increase Na+ reabsorption -> increases blood vol
How does aldosterone stimulate Na+ absorption through ENaC
Via principle cells in the ASDN
What does aldosterone bind to in principle cells
Mineralcorticoid receptor R
What does aldosterone increase
Surface ENaC levels
Na/K ATPase density
ATP supply
K+ secretion across apical membrane
How does aldosterone lead to an increase in surface ENaC levels
Bind to mineralocorticoid receptor -> regulates the production of aldosterone regulatory proteins
Decreased rate of retrieval
What regulates the retrieval of channels
Ubiquitination
How is a ubiquitin group added to a lysine residue in ENaC
Nedd4-2 binds to P-Y motif in the C terminus of an ENaC subunit
Covalently adds a ubiquitin group
How does aldosterone inhibit ubiquitination of ENaC
Stimulates serum and glucocorticoid regulated kinases (SGK1) which phosphorylates Nedd4-2
This allows 14-3-3 to bind to it which prevents Nedd4-2 binding to ENaC inhibiting ubiquitination
What is hypertension
High blood pressure
Too much aldosterone which stimulates the creation of ENaC channels -> too much Na+ present
What are the 2 types of Cl- channels
CTFR and calcium activated Cl- channel (CaCC)
What regulates CTFR channel activity
Regulated by PKA and ATP
What drives the accumulation of chloride in the cell
NKCC transporter on the baso lateral membrane
Utilise the pre existing Na+ gradient
What happens if another ATP binds to site 2 on the CFTR after the pore closes
The channel will reopen a s ling as the RD is phosphorylated
Where are CaCCs located
Apical membrane of most epithelial cells that express CFTR
Not intestinal cells
What activates CaCCs
A rise in cytosolic Ca2+ and the activation is usually transient
What are the 2 members of the TMEM16 family
TMEM16A and TMEM16B
Where is TMEM16A found
Epithelial cells
What allows the pore to open in TMEM16A
Ca2+ bind to glutamate residues in the alpha helices forcing them to move apart i.e opening the pore
What are the mechanism of HCO3- secretion
Directly through the Cl- channel
Indirectly via coupling the Cl- channel (apical Cl/HCO3 exchanger)
How does CFTR regulate the activity of SLC26A anion exchangers in epithelial cells
.cAMP/PKA activation of CFTR switches on AE
Requires interaction of 2 proteins added by CAP70 and CFTR RD phosphorylation
What is the function of acinar cells
Produce a small amount of NaCl fluid fluid
Regulated through TMEM16A channels
What is the function of duct cells
Produce a high volume of NaHCO3 rich secretion using both CFTR and SLC26A6
Why do CF patients produce mucus in the digestive system
CFTR exchanges aren’t working
No secretion from ductal cells so digestive juices don’t travel = mucus build up
Why are some CF patients pancreatic insufficient
Digestive enzymes begin digesting the exocrine pancreas due to the mucus build up
Where does ENaC mediated fluid absorption happen in the digestive system
Only in the colon - other mechanisms in other part of the GI tract
Describe the sodium linked absorptive ion transporter
Utilise PAT1 and DRA and couple to Na+/H+ exchanger on apical membrane
Combine H+ and HCO3- in the gut
Describe how nutrient absorptive transporters work
Use glucose and other osmotically active solutes that are brung into the cell
Brings water into the cell
What is the main cause of secretory diarrhoea
Dysregulation of fluid homeostasis due to dysregulation of cell signalling
What is the mechanism that cholera inhibits absorption and stimulation of secretion
Causes ADP-ribosylation that blocks GTP hydrolysis making adenylyl cyclase permanently active
= over stimulation of CFTR and inhibition of NaCl absorption
Describe oral rehydration therapy
Utilises nutrient absorptive transporters that bring in solutes allowing water to follow
