Transport across Membranes Flashcards
membrane structure
thin film of lipid and protein held together my non covalent attractions
approx 5nm thick
double lipid layer- bilayer
impermeable barrier to water soluble (polar) molecules
fatty acids and lipids
fatty acids- long hydrocarbon chains with carboxyl end (COOH) and methyl end (CH3)
can be free or covalently bonded to glycerol via ester linkage
many have double bonds in hydrocarbon chain- unsaturated
phospholipids
main lipid constituent of a typical bilayer 2 fatty acids covalently bonded to glycerol (1 unsaturated)
3rd hydroxyl group on glycerol is bonded to a phosphate which is also bonded to choline, ethanol amine or serine
membrane lipids
amphipathic
polar head- hydrophilic
non polar tail- hydrophobic
polarity- how equally two atoms share electrons (C-H) share atoms equally
fatty acids are non polar and uncharged (hydrophobic)
phosphate is polar and charged (hydrophillic)
thermodynamics + lipid bilayers
2nd law- entropy increases over time
polar molecules dissolve- energetically favourable, more disordered/increased entropy
non polar molecules do not dissolve- energetically unfavourable, more ordered/ decreased entropy
formation of bilayer
amphipathic nature of phospholipids causes bilayers to form
hydrophobic tails cluster together leaving hydrophillic head out to water
edges of bilayer meet to form continuous spheroid
preventing hydrophobic FA being exposed to water (energetically favourable)
damaged membranes ca heal using this mechanism
PLB spontaneously form sealed compartments
membrane fluidity
viscosity of membrane
membranes are dynamic structures due to ability of phospholipids to move
move laterally- not across the membrane
at low temps, lipid bilayer can undergo a phase transition and become rigid as the phospholipids pack closely together
prevented by cis double bonds on FA chains
cholesterol
cis double bonds + cholesteral
cis double bond- H on same side of chain (upper side) - as opposed to trans
makes membrane thinner and more difficult to pack together
cholesterol- polar region + rigid steroid ring region
orientation means steroid region of cholesterol stiffens upper region of FA chain in phospholipid
-immobilises phospholipid, less able to move laterally, less fluid
high levels of cholesterol prevent PL compacting together and being too rigid
lipid rafts
lipids are randomly distributed throughout the membrane
van der waals attractions not strong enough to hold molecules together ( fluid)
sphingolipids have long saturated fatty acid chains, attractive forces are strong enough to hold adjacent molecules together in lipid rafts
raft domain- longer FA chains, stronger attractions, proteins + vesicles congregate in these areas
Independent monolayers can interact with each other in lipid rafts.
Proteins often congregate to lipid raft regions in preparation for vesicular budding and transport
glycocalyx
glycoproteins- proteins glycosylated in RER ad golgi
glycolipids - lipids from SER glycosylated in goligi
always found on non cytoplasmic side of membrane
cho coating to membrane is known as glyocalyx- protective
helps in cell binding + recognition
glycocalyx
glycoproteins- proteins glycosylated in RER ad golgi
glycolipids - lipids from SER glycosylated in goligi
always found on non cytoplasmic side of membrane
cho coating to membrane is known as glyocalyx- protective
helps in cell binding + recognition
membrane proteins
lots of different types
can be covalently bonded to lipid
may be embedded in a membrane
non covalently attached
-used for transport
-receptors
transport across membranes
lipid bilayer is impermeable to polar (hydrophillic) molecules due to tails
intracellular compartment maintains a different environment to the outside
specialised proteins required for transport across membrane
molecules being transported
hydrophobic molecules eg o2, co2 etc = non polar, can dissolve and move across
small uncharged polar molecules- eg h2o, urea etc= polar, very slow to move across
large uncharged polar molecules- eg glucose + sucrose= polar so may need transporter
ions= charged so need special mechanism
membrane transport proteins
transport proteins are transmembrane, multi pass proteins- no contact with hydrophobic core
carrier proteins- bind to solute and undergo conformational change to transfer across membrane
channel proteins- interact weakly with solute, form aqueous pore that solutes can pass through quickly
enable facilitated diffusion, conc gradient determines direction of flow
for ions- conc and charge determine direction of flow (electrochemical gradiet)
active transport
pumping solutes across membrane against conc gradient
using carriers
required energy- light energy (bacteria), energy release from electron transfer, atp hydrolysis
mechanisms used- coupled carriers, atp driven pumps
coupled carriers
electrochemical gradient= stored energy
this energy can be used to transport another ion across a membrane against its conc gradient
carriers can be symporters or antiporters
symporters and antiporters
symporters- protein that moves two molecules in the same direction across the membrane
antiporters- move molecules in opposite directions across the bilayer
Na+ K+ pump
Na+K+ ATPase pump uses free energy released by ATP hydrolysis to actively pump 3Na+ out of the cell and 2K+ in the cell
primary active transport
coupled carriers participate in secondary active transport
channel proteins
form pores across a membrane
channels on membrane are very narrow
highly selective pores that open and close = gated
specifically concerned with transport of ions
facilitated diffusion
voltage gated, ligand gated, mechanically fated
resting membrane potential
-70mV
maintained by Na+-K+ atpase pump- Na+ out K+ in
K+ leak channels- K+ leaves the cell following its conc gradient
neg charge in cell - conc gradient and electrical gradient even out and charge is balanced = equilibirum= resting potential
