M&R S2 - Membrane Permeability and Cell Volume/pH Regulation Flashcards
What properties of solutes determines whether they can pass through a phospholipid bilayer?
Hydrophobic
Small
Uncharged Polar
What types of solutes can’t pass through a phospholipid membrane?
Large, charged, or polar molecules cannot make it through a membrane
Ions can also not pass through a membrane
What is passive diffusion?
What determines rate of PD?
Diffusion that is dependent on:
Permeability
Concentration gradient
Rate increases linearly with increasing concentration gradient
What is a permeability coefficient?
What is the permeability coefficient of water?
A numerical expression of permeability of a solute
Expressed in cm/s
5x10-3 cm/s
What could be said about the permeability of hydrophobic vs hydrophilic solutes in terms of permeability coefficient?
Hydrophobic molecules have a higher PC
Hydrophilic molecules smaller (tend to be <10-10 cm/s)
What is the key function of a phospholipid bilayer in terms of their interaction with hydrophilic molecules?
Act as a permeability barrier to hydrophilic molecules
How is the movement of hydrophilic molecules across a membrane mediated and regulated?
What roles do these processes have?
By specific membrane transport systems
Transport processes are important in:
Maintenance of intracellular pH, ionic composition, metabolic fuels and volume
Extrusion of metabolic waste and toxins
Generation of ion gradients
What is facilitated diffusion?
Name two models of facilitated diffusion
Permeability of a membrane to a substance is ‘facilitated’ by specific proteins in the bilayer
Models include Carrier molecules (ping pong) and channel proteins
Why is facilitated diffusion a saturable process?
Limited number of proteins in the membrane means a maximum rate can be achieved when all transporters are busy
What effect does facilitated diffusion have on equilibrium points of solutes moving across the membrane?
Has no effect, similar to enzymes
What is channel protein gating?
Some channel proteins may be gated, meaning that they open or close in response to stimulus such as:
Ligand binding
Voltage change
ATP binding
What is active transport?
Active transport allows transport of ions or molecules against unfavourable electrical or chemical gradients requiring energy from the hydrolysis of ATP
What determines whether a solute must be transported by active transport?
The free energy change of the transported species
This is in turn determined by:
Concentration gradient
Electrical potential across the membrane bilayer (only when the molecule is charged)
What percentage of cellular energy can be expended on active transport?
Some cells spend 30-50% of ATP on AT
What are co-transporters?
Hint: Secondary active transport
Membrane proteins that will transport more than one ion or molecule per reaction cycle
In this way the transport of one substance can be linked to the concentration of another (secondary active transport)
What are the three types of membrane transport?
Uniport - One molecule/ion transported, one direction
Symport - Two molecules/ions, one direction
Antiport - Two molecule/ions, opposing directions
Give an example of Symport SYSTEM found in the small intestine and kidney
Na+/Glucose co-transport system
Entry of Na+ provides energy for the entry of glucose against the concentration gradient
What is Na+/K+ - ATPase and how does it work?
Hint: Form AND function
What it is:
A plasma membrane associated pump with 2 subunits, Alpha and Beta
Alpha - Contains binding sites for Oubain, K+ extracellularly and Na+ and ATP intracellualrly
Beta - Gycoprotein, Oligosaccharide chains direct pump to the surface
Called a P-type ATPase
(ATP phosphorylated Asp, producing phosphoenzyme intermediate)
What it does:
Uses ATP to transport 2K+ into the cell and 3Na+ out (Active transport/Antiport)
How much of the BMR is used by the Na+/K+ ATPase proteins in a cell?
25% of BMR
How is Na+/K+ ATPase controlled?
Oubain binding inhibits transport
What processes is Na+/K+ ATPase involved in?
Forms Na+ and K+ gradients necessary for electrical excitability
Drives secondary active transport processes such as:
Control of pH Regulation of Cell volume Regulation of Ca2+ concentration Absorption of Na+ in epithelia Nutrient uptake (Eg. Glucose in small intestine)
What is the relevance of the interaction of K+ channels and Na+/K+ ATPase?
K+ diffusion down the concentration gradient set up by the Na+/K+ ATPase through K+ channels is mainly responsible for membrane potential
What is the importance of intracellular Ca2+ concentration control?
High Ca2+ concentration is toxic to cells
What is the concentration difference in intra and extra cellular Ca2+ concentration?
The is a `20,000 fold difference in intra and extracellular concentration
Intra - <100nM
Extra - 2mM
What is the function of Ca2+ ATPases?
Control of resting Ca2+ concentration intracellularly
Uses ATP to pump ions
What are the two major forms of Ca2+ ATPases?
Where are they found?
SERCA and PMCA
a.k.a.
Plasma membrane Calcium ATPase
Sarco(endo)plasmic reticulum calcium ATPase
Names denote where they’re found
Describe the action of PMCA
Removes residual Ca2+ from the cell in exchange for H+
Uses ATP (Active transport/Antiport)
High affinity, Low capacity
Describe the action of SERCA
Accumulated Ca2+ in the SR/ER in exchange for H+
Uses ATP (Active transport/Antiport)
High affinity, low capacity
What is mitochondrial Ca2+ uniport?
A uniporter that operates at high Ca2+ concentration to move Ca2+ into the mitochondria and help prevent cell damage
Outline the normal function of the Na+/Ca2+ Exchanger (NCX)
Utilises secondary active transport
Expels 1 Ca2+ in exchange for 3Na+
To do this it uses the Na+ concentration gradient set up by the Na+/K+ ATPase
Electrogenic - Current flows in direction of Na+ gradient
Activity is membrane potential dependent
Antiport
Low affinity, high capacity
Removes MOST Ca2+ - Expels intracellular Ca2+ during cell recovery (after an action potential)
Outline how NCX function changes in Ischaemic conditions
ATP depleted, Na+/K+ ATPase inhibited
Na+ builds up in the cell, causing depolarisation
NCX reverses (Sodium out, Calcium in) in an effort to repolarise the cell
When cellular buffering capacity is exceeded, how is pH controlled/returned to the set point?
Membrane transporters
Acidification opposed by expelling H+ or inward movement of Bicarbonate (activation of NHE and NBC)
Alkalisation opposed by expelling bicarbonate (activation of AE)
What membrane transporters are involved in acid extrusion?
Na+/H+ exchanger (NHE)
Na+/Cl-/HCO3-/H+ co-transporter (NBC)
Both raise intracellular pH
Describe the function of the Na+/H+ Exchanger (NHE)
Hint: Include inhibitors/activators
Moves 1Na+ in, 1H+ out, therefore electroneutral
Uses Na+ Concentration gradient set up by the Na+/K+ ATPase to achieve this
Raises pH intracellularly
Helps regulate cell volume
Activated by growth factors
Inhibited by amiloride
Outline the function of the Na+/Cl-/HCO3-/H+ co-transporter (NBC)
Na+ and HCO3- in (base in)
H+ and Cl- out (acid out)
Uses Na+ concentration gradient set up by the Na+/K+ ATPase
Process can be looked at as coupled Na+/H+ and anion exchange performed by the same exchanger
Raises intracellular pH
Involved in cell volume regulation
What is the major cellular base extruder?
Describe its function
Cl-/HCO3- Exchanger (Anion exchanger/AE)
Cl- in
HCO3- out (base out)
Acidifies cell
Involved in cell volume regulation
How is cell volume regulated?
Osmotically active ions (E.g. Na+, K+, Cl-) or organic osmolytes (amino acids) are transported in or out of a cell and water follows via osmosis causing cell shrinking or swelling
There is no standard regulation method, different cells use different combination of transporters to achieve regulation
If cell is shrinking:
Influx ions (E.g. Na+ or Ca2+ channels) and osmolytes
If cell is swelling:
Extrude ions (E.g. K+ or Cl- channels) and osmolytes
Outline how bicarbonate is reabsorbed from the kidney’s proximal tubule
Na+/K+ ATPase removes Na+ from proximal tubule epithelia
NHE can therefore pump Na+ from the lumen into the cell along the concentration gradient in exchange for H+
H+ goes into the lumen and ‘picks up’ HCO3- to bring it back into the cell
What is the main reason for bicarbonate reabsorption from the proximal tubule?
To retain base for pH buffers
What is the main goal of renal anti-hypertensive therapy?
Reduce the re-uptake of Na+ and other molecules so less water is absorbed via osmosis
With less water absorbed, blood volume, and therefore blood pressure falls
What is the function of Aquaporins?
Describe their involvement in regulation of water reabsorption form the kidney tubules
Allows water to more easily cross the membrane
It’s inclusion in the membrane of Kidney epithelia is stimulated by Anti-Diuretic Hormone (ADH) causing increased water re-absorption.
What are Loop Diuretics?
Block Na+ re-uptake in the thick ascending Limb of the Loop of Henle
What type of drug is amiloride?
How does amiloride affect Na+ re-uptake in the kidney?
Is a potassium sparing diuretic
Acts on both distal (ENaC - Epithelial Na+ Channel) and proximal (NHE) tubules to prevent Na+ re-uptake
What is the result of excess aldosterone and how is this treated?
Aldosterone upregulates ENaC and NHE in the kidney tubules to increase Na+ re-uptake
In excess, this can be a contributing factor to hypertension
Spironolactone (Glucocorticoid receptor agonist) is used to treat
How is glucose moved from the lumen to the blood by intestinal lumen and kidney tubule epithelium?
SGLUT-1 is a sodium and glucose symporter that moves them into the epithelia from the lumen via facilitated diffusion
Na+/K+ ATPase produces the Na+ gradient that allows co-transport of glucose as Na+ moves down its concentration gradient in SGLUT-1
GLUT transporters move glucose from the cell into the bloodstream
What are GLUT transporters?
Transporters that move glucose into the cell
What are the different types of GLUT transporters and where are they found?
By what method do these transporters move glucose into cells?
GLUT 1+3:
- Found throughout the body
- Facilitated diffusion (uniport)
GLUT 2:
- Hepatocytes and pancreatic beta cells
- Facilitated diffusion (uniport)
GLUT 4:
- Striated muscle and adipose
- Facilitated diffusion (uniport), Insulin stimulates
SGLUT-1
- Intestine and kidney
- Secondary active transport (symport)
What is the main function of GLUT 1 and 3?
Maintain a basal glucose level in cells
How does insulin promote glucose uptake into cells?
Up-regulates expression of GLUT4, resulting in more GLUT4 receptors on the cell surface (from storage vesicles)
How does the cell prevent glucose efflux when blood concentration of glucose falls to resting levels following a meal?
Glucose is rapidly metabolised to G-6-P upon entering the cell, so intracellular glucose concentration never rises higher than blood glucose concentration
What other metabolites use the sodium gradient for their uptake into cells?
Amino acids
