Transport across Membranes

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- 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

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

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)

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

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

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 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

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

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

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

lots of different types can be covalently bonded to lipid may be embedded in a membrane non covalently attached -used for transport -receptors

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

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

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)

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

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- protein that moves two molecules in the same direction across the membrane antiporters- move molecules in opposite directions across the bilayer

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

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

-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