Transport across Membranes

Question 1

membrane structure

Answer 1

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

Question 2

fatty acids and lipids

Answer 2

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

Question 3

phospholipids

Answer 3

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

Question 4

membrane lipids

Answer 4

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)

Question 5

thermodynamics + lipid bilayers

Answer 5

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

Question 6

formation of bilayer

Answer 6

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

Question 7

membrane fluidity

Answer 7

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

Question 8

cis double bonds + cholesteral

Answer 8

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

Question 9

lipid rafts

Answer 9

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

Question 10

glycocalyx

Answer 10

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

Question 10

glycocalyx

Answer 10

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

Question 11

membrane proteins

Answer 11

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

Question 12

transport across membranes

Answer 12

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

Question 13

molecules being transported

Answer 13

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

Question 14

membrane transport proteins

Answer 14

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)

Question 15

active transport

Answer 15

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

Question 16

coupled carriers

Answer 16

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

Question 17

symporters and antiporters

Answer 17

symporters- protein that moves two molecules in the same direction across the membrane

antiporters- move molecules in opposite directions across the bilayer

Question 18

Na+ K+ pump

Answer 18

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

Question 19

channel proteins

Answer 19

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

Question 20

resting membrane potential

Answer 20

-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