Lecture 5- Membranes as permeability barriers Flashcards
Small hydrophobic molecules
will pass the bilayer e.g. Benzene, oxygen, carbon dioxide and nitrogen
Small uncharged polar molecules
will pass the bilayer e.g. water, urea and glycerol
Large uncharged polar molecules
will not pass bilayer without helps e.g. glucose and sucrose
Ions
will not pass without help e.g. H+. K+, Na+, Cl
transport processes (6)
- Maintain ionic composition
- Maintenance of intracellular pH
- Regulation of cell volume
- Concentration of metabolic fuels and building blocks
- Extrusion of waste products out of the cell
- Generate ion gradients necessary for the electrical excitability of nerves and muscles
passive transport
Dependent on permeability and conc gradient
• Rate of passive transport increases linearly with increasing concentration gradient
• Always passive going down concentration gradient
• Exergonic (-ve)
different membranes
have different permeability
- diff ion channels
- excitability of membrane
membran proteins will not
flip flop
ping pong transport
- Conformational change
- Maximum rate of transport exist
facilitated fusion via…
Ion channels
- ligand gated
- voltage gated
facilitated diffusion has a much higher
potential transport rate than simple diffusion
active transport
• Where substances are pumped across the membrane against their conc gradient
AT is an …. process
endergonic- more energy put into the reaction the comes out
how much ATP used on AT
30-50%
passive transport
simple diffusion and facilitated diffusion (transporter and channel)
gradient for sodium
much more [sodium] outside the cell than inside
gradient for potassium
much more [potassium] inside the cell than outsides
gradient for calcium
[calcium] is high outside and low in inside
gradient of chloride
[chloride] is high on the outside and low in inside
primary active transporter
involves the movement of a solute against its electrochemical gradient facilitated by coupling to a process that provides the required free energy, e.g., Na+−K+ pump driven by ATP hydrolysis
secondary active transport
involves the use of an electrochemical gradient, created by a primary active transporter e.g. Na+/K+ ATPase
example primary active transporters
- Na+/K+ ATPase
- Plasma membrane calcium ATPase (PMCA) (pumps calcium out of the cell)
example of active transport int he reverse mode
ATP synthase
- flow of protons powers creating of ATOP from ADP and Pi
co transport exampels
uniport
symport
antiport
uniport
transports a single molecule in one direction (not a co-transport)
symport
more than one molecule in one direction
antiport
more than one molecule transported in opposite directions e.g. Na/k+ ATPase (Na pump)
o Swaps 3 sodium (out) for 2 potassium (in)
the sodium pump (Na/K+ ATPase) is an
antiport
- 3 Na+ ions expelled for every 2K+ ions that enter
Na/K+ ATPase important in
generating ion gradient that allows secondary active transport and action potentials
- Note: only small contribution to resting membrane potential
Na/K+ ATPase only has a small contribution to
resting membrane potential
how much of basal metabolic rate used for pump
25%
Na/K+ ATPase is what type fo ATPase
P-type
- ATP phosphorylates asparate, producing phosphoenzyme intermediate
structure of Na/K+ ATPase
alpha and B subunit
Alpha subunit
- K+
- NA+
- ATP
- ouabain binding site
B subunit
- glycoprotein directs pump to surface
how does Na/K+ ATPase effect membrane potential
- Sodium pump creates high intracellular [K+]
- K+ diffusion through channels is mainly responsible for membrane potential (-70mV_
- Sodium pump only generates -5-10m through electrogenic pump activity
sodium glucose cotransport an example of
secondary active transport
sodium glucose cotransport
ATP used to indirectly drive transport
- Entry of Na+ provides energy for the entry of glucose against its concentration gradient
- Sodium transported with glucose via symport
- Without sodium glucose would not be pushed into cell
calcium transport (2)
1) Ca2+-Mg2+ ATPase
2) Na+-Ca2+ exchanger (antiport
Ca2+-Mg2+ ATPase
a. High affinity
b. Low capacity
Na+–Ca2+ exchanger (antiport)
– Inwards flow of Na+ ions down the Na+ concentration gradient drives the outward flow of Ca2+ up its concentration gradient (antiport)
a. Low affinity
b. High capacity
Na+/ H_ exchanger
– Inward flow of Na down its concentration gradient
leads to cell alkalinisation by removing H+ (antiport)
transporters in CF
- Chloride channel mutated- prevents transport of protein after synthesis (CFTR)
- Means water cannot follow chloride into the lumens
- Sticky, thick mucus that will damage organs such as the lungs, GI tract, vas deferens, can plug cervix
transporters in vibrio cholera infection
- Over-active Chloride channel
- Increase in Protein kinase A
o Hyperphosphorylation of chloride channels - Water follows Diarrhea
Overall ion transport must create an electroneutral environment
e.g. as many K+ are being pumped in as out
