Membrane Permeability Flashcards
What is a semi permeable membrane?
A layer through which only allowed substances can pass
Which molecules pass scraight through the membrane?
Hydrophobic molecules e.g. O2, CO2, N2, benzene
Small uncharged polar molecules pass through easily - H2O, Urea, Glycerol
What molecules can go through by passive diffusion?
Large uncharged polar molecules such as glucose and sucrose, but this is so slow that other mechanisms are needed (you wold die
What cannot pass through the membrane?
Ions unless the membrane is broken
What is passive transport dependent on?
Permeability and concentration gradient
Rate of passive transport increases linearly with increasing concentration gradient
What are the roles of transport processes?
- Maintenance of ionic composition
- Maintenance of intracellular pH
- Regulation of cell volume
- Concentration of metabolic fuels and building blocks
- The expulsion of metabolic waste products and toxic substances • The generation of ion gradients necessary for the electrical excitability of nerve and muscle*
What are models of membrane transport proteins (not actually what happens)
Reciprocating carrier - flip flop Rotating carrier Ping pong (Binds in middle Causes a conformational change Proteins opens on in terms substance Substance released Another conformational change Goes back to original position Takes a long time)
Describe facilitated diffusion via channels
Gate which opens
Lots of material can go through at the same time
Briefly describe LGICs using an example
E.g. NAchR Ligand Act binds to its receptor on channel Causes conformational change Causes Na to flow into cell From high to low conc But channels are bi directional
Briefly describe VOCCs using an example
E.g. Na+ channel AA in channel +ve charged Potential charge across all membranes - -ve on inside In unstimulaed VG ion channels associated with inner When memb depolarised Repulsion bc +ve now on inside Pushes AA away Conformational change Barrier preventing movement is now gone Na can flow into the cell
How can you tell if transport will be active or passive
Delta G will be positive if its active
Passive - diffusion across the membrane
Do not need to memorise equation but need to use it
-ve G = does not require energy
+ve G = put energy in
What is transport dependant on?
Concentration ratio and membrane potential
Potential across membrane
-ve on inside bc of proteins which are anions
Protens do not leave cell
On outside cations are attracted towards membrane
On inside cations are repelled
Double electric potential double delta g
What is active transport?
Active transport allows the transport of ions or molecules against an unfavourable concentration and/or electrical gradient
• Energy directly or indirectly from ATP hydrolysis
• Some cells spend up to 30 – 50% of their ATP on active transport
– e.g. nerves, muscle cells, erythrocytes, enterocytes, hepatocytes
Give examples of active and passive transport
Passive - simple diffusion, facilitated diffusion (e.g. channels and carriers)
Active transport - via carriers, against conc gradt
What is a uniport?
Only one molecule transported at a time
What are the 2 types of co transporter?
Symport - molecules transported same direction
Antiport - molecules transported in opposite directions
Where is there more sodium/potassium relative to the cell membrane
More potassium inside cell
More sodium outside cell
What does the NKX do?
Na/K ATPase
3NA out or every 2 K in - important for generating ion gradients to allow an AP
Plasma membrane associated pump
• Uses ATP (energy source) to pump ions and so ACTIVE TRANSPORT
• 25% of Basal Metabolic Rate is used to keep this pump working
• Called a P-type ATPase
– (ATP phosphorylates aspartate, producing a phosphoenzyme intermediate)
What are the subunits of the NKX?
P-type ATPase Phosphorylated on Aspartate
Alpha subunit K+, Na+, ATP, ouabain binding sites
Beta Subunit glycoprotein directs pump to cell surface
What creates the high intracellular K+?
Sodium pump creates the high intracellular [K+ ]
• K+ diffusion through channels is mainly responsible for resting membrane potential (-70 mV)
• Sodium pump generates ONLY ~ 5-10 mV through electrogenic pump activity
What are the primary and secondary Ca2+ transporters?
Primary
Ca2+/Mg2+-ATPase - high affinity, low capacity
UNIPORT
Secondary (no ATP)
Na+/Ca2+-exchanger - low affinity, high capacity
Uses Na+ from NKX
ANTIPORT
Give examples of some co transport systems
Na+-K+-ATPase (Na Pump)
– Maintains cellular concentrations of Na+ and K+ (antiport)
• Na+-Ca2+-exchange (NCX)
– Inwards flow of Na+ ions down the Na+ concentration gradient drives the `outward flow of Ca2+ up its concentration gradient (antiport)
Na+-H+-exchange
– Inward flow of Na down its concentration gradient leads to cell
alkalinisation by removing H+ (antiport)
• Na+-glucose co-transport (small intestine and kidney)
– Entry of 2Na+ provides the energy for the entry of 1 glucose
molecule against concentration gradient (symport)
Explain the action of Fluoxetine
Outward flow of K+ down its concentration gradient leads to uptake of serotonin in the presynaptic cleft and by platelets with Na+ ion (symport)
• Fluoxetine inhibits SERT and increases 5- HT (serotonin) action; elation and ‘sticky blood’
Which transporter is not present in cystic fibrosis?
CFTR not in memb
Cl- ions do not pass out. Of cells
Water doesn’t follow
Mucus thick
