8: Membranes and Membrane Transport Flashcards
membrane stucture
thin film - 5nm thick
lipid and protein held together by non-covalent attractions
double lipid layer known and lipid bilayer
impermeable barrier to water soluble (polar) molecules
3 main lipid components for membranes = cholesterols, phospholipids, glycolipids
phospholipids
lipids account for 50% of the mass of a typical bilayer - phospholipids are main constituent
phospholipids = 2 fatty acids covalently bonded to glycerol (one is unsaturated meaning it has cis double bond), the third hydroxyl group is bonded to a phosphate which in turn is bonded to choline, ethanol amine or serine
membrane lipids are amphipathic
amphipathic = have both hydrophobic and hydrophilic parts
membrane lipids have hydrophilic head which is polar and hydrophobic tail which is non-polar
polarity = how equally two atoms share their electrons
the fatty acid tail is non-polar and uncharged because it consists of lots of carbon hydrogen bonds which share electrons equally = hydrophobic
phosphate head is polar and charged = hydrophilic
polar molecules dissolve = energetically favourable
non-polar molecules do not dissolve = energetically unfavourable
formation of lipid bilayer
amphipathic nature of phospholipids causes bilayer
to avoid interacting with water, the hydrophobic tails cluster together, having only the heads exposed
edges of bilayer meet as it folds in upon itself forming a continuous spheroid
all phospholipid bilayers spontaneously form sealed compartments
membrane fluidity
fluidity = viscosity of membrane
membrane = dynamic = more fluid due to ability of phospholipids to move
phospholipids can only move laterally via lateral diffusion, rotation or flexion
bilayer = 2 dimensional bilayer
regulators of membrane fluidity
at low temperatures, the lipid bilayer can undergo a phase transition and become rigid (not fluid) as the phospholipids pack closely together
this phase transition is prevented by cis double bonds AND cholesterol
cis double bonds
cis double bond = hydrogens that are bound to the carbons where the double bond exists are found on the same side
trans double bond = the hydrogens are split equally across the double bond
cis double bonds make the phospholipids more difficult to pack together tightly because there are kinks in the fatty acid chains
also makes the membrane thinner as the kink can bring the two layers slightly closer (vertically wise)
cholesterol
important to maintain impermeable bilayer
there is lots of cholesterol in eukaryotic cells and due to their orientation, the steroid ring region stiffens the upper region of the fatty acid chain in the phospholipid
this immobilises the phospholipids, making the membrane more rigid
however, due to high levels of cholesterol, the phospholipids can compact together reducing rigidity
lipid rafts (feature of plasma membranes)
lipids are randomly distributed through the membrane and van der waals attractions are not strong enough to hold molecules together
however, sphingolipids have long, saturated fatty acid chains and the attractive forces are strong enough to hold adjacent molecules together in lipid rafts
sphingolipids don’t have the kink in their chains so sit much more compact
in the raft domain, the width is lipid bilayer is slightly bigger
proteins tend to congregate to lipid raft regions in preparation for vesicular budding and transport
carbohydrate coating on non-cytoplasmic side termed glycocalyx = protective
membrane proteins
lots of different membrane proteins with different functions - approx. 20% of nuclear genome encodes membrane proteins
transmembrane proteins = stand entire width of membrane
peripheral membrane proteins = on the periphery of membrane (not embedded)
single pass proteins = pass through membrane once
multi-pass proteins = bigger proteins that go through and back again (creates large pores)
alpha helix or beta sheet proteins
membrane proteins
lots of different membrane proteins with different functions - approx. 20% of nuclear genome encodes membrane proteins
transmembrane proteins = stand entire width of membrane
peripheral membrane proteins = on the periphery of membrane (not embedded)
single pass proteins = pass through membrane once
multi-pass proteins = bigger proteins that go through and back again (creates large pores)
alpha helix or beta sheet proteins
transport across membrane
polar (hydrophilic) molecules can’t pass through lipid bilayer
non polar molecules = diffuse through easily
small uncharged polar molecules = move across slowly
large uncharged polar molecules = need a transporter
ions = charged = need special mechanism
membrane transport proteins
all transport proteins are transmembrane multi-pass proteins
two types of transport proteins:
1. carrier proteins = bind to solute and undergo conformational change to transfer across membrane
2. channel proteins = form aqueous pores that solutes can pass through quickly, small opening at top of channel for selectivity
carriers and channel proteins allow for facilitated diffusion = passive transport
concentration gradient determines direction of flow
if it is charged ion, electrochemical gradient determines flow
active transport
pumping pf solutes against concentration gradient
only occurs via carriers
requires energy - commonly from ATP hydrolysis
active transport carried out via: coupled carriers or ATP driven pumps
coupled carriers
two types of coupled carriers: symporters and antiporters
symporters = transport an ion alongside the molecule that wants to pass through the membrane, the ion follows its concentration gradient (both molecules travel in the same direction)
antiporters = two molecules pass through membrane in opposite directions, one follows its concentration gradient which generates free energy for other molecules to pass through against its concentration gradient
