M&R 2.1 Membranes as permeability barriers Flashcards
What method is used to study the permeability of substances through a bilayer? How does it work?
Black Film method
Container with a septum in the middle, which has a pinhole in it. Pain phospholipids over the pinhole so a bilayer forms over it - can then look at permeability of substances from one side of the septum to the other
Which types of molecules can passively diffuse through the bilayer?
Gases (O2, CO2, N2 etc) - diffuse through easily because small, uncharged & non-polar Hydrophobic molecules (e.g. steroid hormones, benzene) because lipophilic Small uncharged polar molecules (e.g. H20, urea, glycerol)
Which types of molecules cannot passively diffuse across the bilayer?
Large uncharged polar molecules (e.g. glucose, sucrose) - have 6 carbons so are too large
Ions (e.g. H+, Na+, K+, Ca2+, Cl- etc) because charged
How do gases cross the bilayer?
Passive diffusion, because they are small, uncharged and non-polar
How do steroid hormones cross the bilayer?
Passive diffusion, because they are lipophilic
How do urea, water and glycerol cross the bilayer?
By passive diffusion, because they are small and uncharged
Can ions cross the bilayer by passive diffusion?
No, because they are charged
Why can’t glucose and sucrose cross the bilayer by passive diffusion?
Because they are too large (6C)
How does the rate of passive diffusion relate to the concentration gradient of a substance?
Rate of passive diffusion increases linearly with increasing concentration gradient
Membranes act as permeability barriers to what molecules with what properties?
Charged
Large
Hydrophilic
What is facilitated diffusion?
Movement of a solute down its concentration gradient by the action of membrane proteins
Is facilitated diffusion an example of active transport or passive transport?
Passive transport - because despite involving the action of membrane proteins, the solute is still moving DOWN its concentration gradient (so the process does not directly require chemical energy)
Name two main types of membrane proteins that assist with facilitated diffusion
Gated pores (protein binds the substrate and then undergoes a conformational change, releasing the substrate on the other side of the membrane) - AKA ‘ping-pong’ transport
Ion channels (closed channels will respond to a stimulus (ligand or voltage) and open, creating a channel through which the substance can move)
Which type of membrane transport protein transports substances at a greater rate, gated pores or ion channels?
Ion channels (once the channel is open the substances can be transported through quickly, whereas gated pores have to individually bind each molecule of substrate, which takes longer)
Name the two main categories of ion channel
Ligand gated ion channels
Voltage-gated ion channels
On which side does the ligand bind on a ligand-gated ion channel?
Can be inside or outside
Outside - e.g. nAChR binding ACh
Inside - e.g. ATP-sensitive K+ channel (normally open, when ATP from inside the cell binds the channel closes)
How do voltage-gated ion channels work?
At rest the cell is more negative inside and more positive outside When depolarised (e.g. during an AP) this changes the other way around The channel detects this change and opens
How does the rate of facilitated diffusion relate to the concentration gradient of a substance?
As the concentration gradient increases, the rate of diffusion increases, up until a maximum rate of transport (when all the membrane transporters are busy)
Therefore facilitated diffusion is a saturable transport process
What is the difference between active and passive transport?
Passive transport - transport of the ion/molecule can occur spontaneously
Active transport - transport of the ion/molecule requires energy
What is the purpose of active transport?
It allows transport against an unfavourable concentration gradient and/or electrical gradient
What is primary active transport and how does it work?
A molecule is transported against its concentration/electrical gradient using free energy, usually chemical energy from hydrolysis of ATP (but also from electron transport, light etc)
What is secondary active transport and how does it work?
When the transport for one molecule is linked to the concentration gradient of another molecule, via a co-transporter. The primary energy source (e.g. ATP hydrolysis) is used indirectly to generate a gradient of something else (often Na+). This gradient is then used to drive the membrane transporters.
What is co-transport?
When the transport of one molecule depends on the simultaneous or sequential transport of another molecule, either in the same direction or the opposite direction
Co-transport where both molecules move in the same direction is via…
Symporters
Co-transport where molecules move in opposite directions is via…
Antiporters
What are the concentrations of sodium inside and outside a typical cell?
~145mM Na+ outside cell
~12mM Na+ inside cell
Describe the concentration gradient and electrical gradient of Na+ for a typical cell
Concentration gradient INTO the cell
Electrical gradient INTO the cell
So if you opened an Na+ channel, it would rush in
What are the concentrations of chloride inside and outside a typical cell?
~123mM Cl- outside
~4.2mM Cl- inside
Describe the concentration gradient and electrical gradient for chloride across a typical cell
Concentration gradient INTO the cell
Electrical gradient OUT of the cell (cell at RMP is negative inside and positive outside)
Therefore if open a Cl- channel, Cl- leaves the cell down its conc gradient but not as readily as Na+ because of its electrical gradient
What are the concentrations of potassium inside and outside a typical cell?
~4mM K+ outside
~155mM K+ inside
Describe the concentration gradient and electrical gradient for K+ across a typical cell
Concentration gradient OUT of cell
Electrical gradient INTO cell (because inside is more negative than outside at RMP)
What are the concentrations of calcium inside and outside a typical cell?
~1.5 mM Ca2+ outside cell
~ 10^-4 mM Ca2+ inside cell
Describe the concentration gradient and electrical gradients of calcium across a typical cell
Concentration gradient INTO cell (Very large! Cell is going to great lengths to extrude Ca2+ - means small raises of Ca2+ can act as a signal)
Electrical gradient INTO cell
What are some examples of primary active transporters?
Plasma membrane Ca2+ ATPase (PMCA)
F1F0 ATPase (ATP synthetase)
Na+/K+ ATPase (also a cotransporter)
(Ca2+ - Mg2+ - ATPase)
How does the PMCA transporter work?
= plasma membrane Ca2+ ATPase
Actively transports Ca2+ out of cytosol (either into extracellular fluid or ER, depending which membrane it’s on)
Therefore helps maintain very low Ca2+ inside cytosol
What kind of transporter is the F1F0 ATPase transporter?How does it work in the plasma membrane vs the inner mitochondrial membrane?
In the plasma membrane - if H+ is high inside the cell, the transporter uses energy derived from ATP hydrolysis to extrude H+ and therefore maintain intracellular pH
In the inner mitochondrial membrane - high H+ in the intermembrane space drives the transporter in the reverse direction. Therefore it uses the H+ concentration gradient to generate ATP
The Na+/K+ ATPase is an example of both a __________ and a __________
Cotransporter (antiporter - moves both Na+ & K+ but in opposite directions)
Primary active transporter (uses ATP)
What does the Na+/K+ ATPase do?
Pumps 3Na+ out of the cell and 2K+ into the cell
This maintains the strong concentration gradients of Na+ inwards and K+ outwards
(it DOES NOT set the RMP!!)
Name some secondary active transporters, and what they do
Na+ Ca2+ exchanger (NCX) : uses gradient established by Na+/K+ ATPase. Brings in Na+ down its concentration gradient and in exchange extrudes Ca2+ from the cell against its concentration gradient (antiport)
Na+ H+ exchanger - moves Na+ into the cell and sends H+ out against its concentration gradient (antiporter)
Na+ - glucose co-transporter (symporter - moves Na+ into the cell and uses the energy to drive glucose into the cell also) e.g in enterocytes
Name 2 transporters involved in Ca2+ transport, and their relative affinity/capacity
Ca2+ Mg 2+ ATPase (actively transports Ca2+ out of the cell using energy from ATP [requires Mg2+ as a cofactor]. Has high affinity but low capacity
Na+ Ca2+ exchanger (Na+ in, Ca2+ out using Na+ gradient) - has low affinity but high capacity
What does the CFTR transporter do?
Exports chloride ions so that water will follow
How is CTFR affected in cystic fibrosis?
Mutation of CTFR gene leads to a mutated transporter. Cl- not extruded so water does not follow, leading to thickened mucus in the lungs. Also knock on effects for other transporters due to excess intracellular
How is CFTR affected in cholera?
CFTR gets phosphorylated which increases its activity. Therefore excess Cl- is secreted and water follows, leading to diarrhoea
Also knock on effects on other transporters because of reduced intracellular Cl-
