membranes part 2 Flashcards
what are the stages of intracelluclar memrbane traffic, what does it mediate
stages:
– Budding (fission) of the vesicle from the parent membrane
– Transport of the vesicle, along cytoskeleton
– Tethering/docking at target membrane (recognition)
– Fusion of vesicle and target membranes
Mediates:
– reorganization of membrane-bound compartments
– exchange of membrane and “cargo” between compartments
– internalization/recycling/degradation of material from plasma membrane
what is the role of SNARES
Mediate membrane fusion during key cellulat processes:
– insulin secretion
– up-regulation of glucose transporters
– transport between ER and Golgi
– phagocytosis
– neurotransmitter release
what does SNARE stand for/ what are thy
Soluble N- ethylmaleimide-sensitive factor attachment protein receptor (dont care about the full name)
*Vsnare on vesicle, targt membrane has t snare
-v-SNARE and t-SNARE are single-spanning transmembrane proteins, whole thing is alphahelicle but large parts stick out
They also have an extended helical domains (approx. 60 amino acids)
These helical domains can interact to form a coiled- coil structure with SNAP25
*specific Vsnare recognizes specific tsnare SNAP25 is an ex for a facillitator protein
how do SNAREs mediate neurotransmitter release
- secretory vesile w/ vsnare and t snare interact on plasma membrane and bidn to eachother and “zip up” from amino terimal
*alpha helicies are twisting around each other facilitated by snap 25
- this twisting beinds to membranes in close proximity, get hemi fusion (outer portion of liplayer fuse but inner do not)
- when inner leaflets come into contact get a fusion pore, membrane of vesicle becomes part of plasma membrane and contents of vesible and dumped into extracullular space
explain memrbane permeability
ions and polar molecules cannot cross (essentially impermeable)
e. g. Na+, Cl-, sugars, amino acids
- small, uncharged molecules cross slowly e.g. glycerol, ethanol
- hydrophobic molecules, gases cros squickly e.g. steroid hormones, O2, CO2, N2
explain ion gradients across membranes
The ionic composition of the cytosol is different from the extracellular environment
There are ion concentration gradients across the plasma membrane and across organelle membranes
These ion gradients are actively maintained by cell, at the cost of ATP
These ion gradients can do work
*
explain memrbane transport
-simple diffusion: non polar comp only go down concentration gradeint (steroid hormones, so few and far between will just move in
- facillitated diffusion
goes down an electrochemical gradient (uses a channel that recognizes)
- primary active transport
against electrochemical gradient, driven by atp
- secondary active transport:
against electrochemical gradietn, driven by moving an ion down its gradient, ATP not required uses generated ionic gradient
- membrane transprot via ion channel:
down electrochemical gradient, may be gated by a ligand or ion, closed until signal arrives to open channel
Ionophore mediated ion transport:
down electrochemical gradient, may be protein channel, ionophore can pass thru membrane in both direction but usually doesnt do so until its bound by a particular ion
explain simple diffusion to transport olute across membranes
- free energy of solution (concentration = c)
ΔG=ΔGo +RTlnc
ΔG when molecule moves from c1 (high) to c2 (low)
ΔG = RTlnc2 –RTlnc1 = RTln(c2/c1)
if c1 > c2, ln (c2/c1) is -ve, so ΔG is -ve
- diffusion occurs spontaneously from high to low concentration
- c1 = c2 at equilibrium
explain Movement of a neutral solute across a permeable membrane
- note neutral comp so no charge impolcation
- befor equilibrium there is a net flux bc C1>>>C2 at equilibrium theres no ent flux
explain transport of hydrophilic solutes
- water is surrounding hydrophilic molecule, it takes energy to pull this water off bc it wont pass thru the lipid bilayer
- when comes out other side has to reassociate with water
*decent change in free energy to be able to do this
- mol will probably travel thru transporter so the transport itself doesnt need energy but the stripping off the water itself does
composition of membrane channels
- Doughnut-like pore spans bilayer
- Compared to transporters, solutes flow through rapidly (diffusion)
- Rate of transport α [substrate] – not saturable
- GATED: open and close in response to stimuli
Many types, highly selective e.g. Na+, K+, Cl- and H2O channels
*must be reg bc if it was just open everything would just pass through
what do membrane channels do
*recall they cant be saturated, conc gradient is what determines it
- Channels form pores and transport specific solutes down their concentration gradient
- Solutes flow through very rapidly (diffusion-like rates of ~108 s-1)
- Rate of transport is proportional to substrate concentration and is not saturable (no maximum rate of transport)
- Channels can be gated: i.e. they open and close in response to external stimuli
- There are many types, but all are highly selective – e.g. H2O glycerol urea Na+ K+ Cl-
what are aquaporins
- water channels, also transport glycerol and urea
- key amino acids line the pore the creates mechanisms allowing for specific items (like water) to pass
- there is size restriction, electrostatic repulsion, water dipole reorientation
*only water has atoms/electrons oriented in way to pass fomr key amino acids
how are membrane transporters classified
- uniport (tranports one mol in one direction)
- symport (capable of transporting 2 mol of different identity)
- antiport (two diff mol required
(symport and antiport are types of cotransport)
explain passive transporters
- Transport DOWN a concentration gradient (also known as facilitated diffusion)
- Highly selective (stereospecific)
- Not a continuous pore through membrane
– Transport one set of molecule(s) at a time
- Rate of transport is regulated, saturable number of binding site(s) for substrate
e. g. Passive glucose transporters: GLUT1 in erythrocytes imports glucose GLUT2 in liver, intestine exports glucose
explain GLUT1
passive transporter operon
1) Substrate binds on one side of membrane
2) Conformational change takes place
3) Site opens on other side of membrane & substrate is released
4) Conformational change takes place
*GLUT2 is this guy but inverted
*change back to oringial con occurs very quickly bc dont want glucose to flow back in
explain active transporters
Transport against a concentration gradient
Transporters often called “pumps”
Many are powered by ATP hydrolysis - “ion-pumping ATPase”
Generate ion gradients across membranes
explain ion gradients across cellular membranes
- Na+ concentrations are much higher outside (~150 mM) than inside the cell (~10 mM)
Cl- concentrations are also much higher outside (~110 mM) than inside the cell (~5 mM)
The opposite is true of potassium (~140mM inside the cell, ~5 mM outside)
explain Movement of a charged solute across a permeable membrane
- before membrane is permeable have ammebrane potential, more pos changed on one side and more neg on other
- add voltage geted ion channels, charges equalize to memrbane potential (Vm=0)
- For a charged solute, the energy of moving the solute in the chemical and electrical gradients is additive
ΔGt = RT ln (c2/c1) + zFΔΨ
(may generate chemical potential (conc ignoring charge)or electrical potenital)
