Topic 4: Membranes and Transport Flashcards
What is the function of the biological membrane?
- Define boundaries; compartmentalize organelles
- Compartmentalization allows unique functions for organelles
- Regulate movement of molecules
- Cell-cell communication
What is the structure of the biological membrane?
composed of amphipathic molecules, contain both hydrophobic and hydrophilic domains
phospholipids are major constituent of the biological membrane
2 fatty acids attached to a glycerol backbone with a phosphate head
fatty acids, hydrocarbon chains 12-20 carbons long, can be saturated (all C-C) or unsaturated (C=C)
What is the basic structure of a phospholipid?
polar head group (phosphate head (-) charged)
C=C causes kink in the chain
two fatty acids attached to 3C glycerol backbone
What are three important properties of the biological membrane?
stable and self healing
provides sealed and closed compartments
hydrophobic core = major permeability barrier
What is the lipid composition of the biological membrane?
phosphoglycerides/phospholipids
sphingolipids
cholesterol
membrane glycolipids
What are phosphoglycerides and phospholipids?
derivatives of glycerol-3-phosphate with 2 fatty acyl chains esterified to glycerol backbone and polar head attached to a phosphate group
What are the four different head groups that can attach to the phosphate?
PE: small, NH3 containing head group
PS: amino acid head group
PC: largest head group, most abundant
sphingomyelin
What are sphingolipids?
polar head group is derived from sphingosine (amino alcohol with large HC tail)
amphipathic but not as cylindrical as phospholipids therefore don’t form bilayers
larger than phospholipids
most common type: sphingomyelin (forms myelin sheath)
sphingolipids form signaling centers called lipid rafts
What is cholesterol?
cholesterol derivatives make up the steroids
basic structure is at 4 ring hydrocarbon
has a single OH giving it amphipathicity
abundant in mammalian cells that acts to modulate membrane fluidity
up to 50% of the total membrane lipid in animal cells is cholesterol
usually found in both membranes of the bilayer
orientates itself in the layer using its single hydroxyl group (polar end) to interact with the polar head group of phospholipid (H-bond)
cholesterol is amphipathic but cannot form bilayers
modulates fluidity at high and low temperatures
What are membrane glycolipids?
formed by adding carbohydrate (sugar groups) to the lipids
some are glycerol based, but most are derivatives of sphingosine
glycosphingolipids: glycosylated in ER and Golgi, function in cell attachment and communication
prominent components of brain and nerve cell membranes
blood group antigens within the RBC membrane are glycosphingolipids called A antigen and B antigen
membrane glycolipids face the extracellular space (not cytoplasm)
functions: involved in cell recognition in nervous system, different sugars, different functions
Why is there a difference between fluidity and flexibility between membranes?
degree of fluidity and flexibility depends on lipid composition, temperature, structure of tails
What is membrane fluidity?
membrane fluidity is crucial for proper permeability and cell function
phospholipids are only held together by hydrophobic interactions therefore they are mobile within the bilayers
3D membrane mobility
lateral movement is rapid and frequent
phospholipids are held together by hydrophobic association
flip flop movement is rare without flippases
How is FRAP used to study membrane fluidity?
fluorescence recovery after photobleaching
laser can photobleach the fluor if exposed too long
measure degree of recovery, indicates mobility of components
measure time for recovery, indicates degree of fluidity
How does lipid composition influence fluidity?
sphingolipid rich membranes are less fluid therefore larger recovery time
PC is less fluid than PS and PE
cholesterol generally restricts phospholipid movement
How does the structure of phospholipid tails influence fluidity?
longer HC tails –> less fluid because more hydrophobic
more C=C –> more fluid because less tightly packed
How does temperature influence fluidity?
high temperature = more fluid, shorter recovery
low temperature = less fluid, longer recovery
What do the FRAP curves look like for biological and artificial membranes?
biological: slow and incomplete
artificial: fast and complete
What do the FRAP curves look like for membranes with cholesterol and without cholesterol?
with cholesterol: rapid
without cholesterol: slower but equally complete
What do the FRAP curves look like for membranes in high and low temperatures?
high temperature: fast and incomplete
low temperature: slow and incomplete
What is membrane lipid asymmetry?
all biological membranes exhibit an asymmetry across the bilayers
all types of phospholipids are present in both leaflets but they are more abundant on one side (leaflet) compared to the other
asymmetrical distribution of phospholipids
outer leaflet: contains sphingomyelin, PC, and glycosylation
inner leaflet: contains PS and PE
asymmetry may affect membrane curvature because exoplasmic face is less fluid
How is membrane asymmetry accomplished?
SER is site of phospholipid synthesis (inserted into bilayer at lumenal face)
flippases flip PE and PS to cytoplasmic side
cytoplasmic face always faces the cytoplasm
What are other forms of asymmetry?
lipid rafts: microdomains within the bilayers called lipid rafts
rich in cholesterol and sphingolipids
thicker membrane
less fluid
likely signaling centers
What are membrane proteins?
proteins are the functional unit of cell, therefore the functions of the membrane like cell-cell recognition, attachment and communication are mediated by membrane-associated proteins
What are the three ways proteins can associate with the membrane?
integral membrane proteins
transmembrane proteins
peripheral membrane
What are integral membrane proteins?
must be amphipathic in order to span the hydrophobic core of the membrane
cytosolic and exoplasmic space must contain hydrophilic surfaces in order to interact with the water in these domains
the membrane spanning domain must have hydrophobic amino acids exposed on the surface of the protein to span the hydrophobic core of the membrane
alpha helical membrane segments can be inserted: co-translationally at ER and post-translationally at the nucleus and mitochondria
What are alpha-helix containing membrane spanning segments?
~22 hydrophobic amino acids are required to span bilayer
some pores formed by alpha helical transmembrane domains alternate polar and non polar amino acids
What are beta-barrels are another type of membrane spanning domain?
contain alternating polar and non polar amino acid create polar pore
e.q. aquaporin beta barrel allows specific passage of H2O
What are peripheral membrane proteins?
do not directly contact the hydrophobic core of the membrane but, rather, associate with integral or lipid-anchored proteins or direct interactions to lipid-head groups
can be cytoplasmic or exoplasmic but must associate with a membrane component
associate by both covalent and non-covalent interactions
interact with integral membrane proteins via non-covalent association (quaternary structures)
What are lipid anchored membrane proteins?
bond covalently to one or more lipid molecules
hydrophobic segment of attached lipid is embedded in one of the leaflets of the membrane
anchors protein in the membrane
protein chain does not enter the bilayers
What are outer leaflet proteins?
proteins that are linked to extracellular surface (GPI anchor, glycosylated inositol)
inositol (PI) is a form of phospholipid head that functions in membrane attachment and cell signalling
What are inner leaflet proteins?
synthesized in cytoplasm
protein is covalently linked to a fatty acid tail
What is trypsin?
trypsin digestion allows the identification of the non-transmembrane segments to discover the topology and orientation of membrane proteins
trypsin is a non-specific protease
How does each transmembrane protein have a specific orientation?
orientation of membrane proteins is established during their synthesis and incorporation into the membrane
transmembrane proteins do not flip flop across the membrane
another level of asymmetry is the glycosylation of membrane proteins by the Golgi
the glyco- part of glycoproteins is only ever found on the exoplasmic face of the membrane
How are transmembrane proteins predicted?
the topology of a particular protein can be predicted using hydropathy plots
hydrophobic amino acids are counted and given particular values in a computer
a strongly hydrophobic domain suggests a transmembrane segment
can predict whether it will be a C-terminal, N-terminal, exoplasmic, or multipass transmembrane segments
What are N-terminal exoplasmic proteins?
N terminal signal sequence is highly hydrophobic and must also have a hydrophobic stop transfer sequence
What are C-terminal exoplasmic proteins?
no N-terminal signal sequence but single strongly hydrophobic peak
What are multipass proteins?
multiple hydrophobic domains
What are hydropathy plots?
maps large stretches of non-polar amino acids to help predict alpha helical transmembrane segments, look for broad peaks
What is the permeability of the bilayer?
permeable to small, uncharged or hydrophobic molecules
relatively impermeable to small uncharged polar molecules
most impermeable to large uncharged and polar molecules
impermeable to charged molecules (ions) because they form favorable interactions with water
How do impermeable molecules cross the bilayer?
simple diffusion: movement across hydrophobic core, even impermeable can leak
facilitated diffusion: use of a transmembrane channel or carrier, can be gated –> open or closed during different conditions
active transport: movement of an impermeable molecule up its gradient, low concentrations to high concentrations, requires input of energy
How are transporters involved in facilitated diffusion?
the role of transport proteins is to provide a path through the lipid bilayer, facilitating the “downhill” diffusion of the polar or charged solutes
What are carrier proteins?
changes shape with transport
bind solutes on one side of the membrane
undergo a “conformational” change
transfers the solute to the other side of the membrane
believed to shield the polar or charged groups of the solute
example: GLUT1 carrier, moves glucose down gradient
What are channel proteins?
do not change shape
form hydrophilic channels through the membrane
passage of solutes without a change in the proteins conformation
some are non-specific
most are highly selective (e.g. ion channels)
rapid transport relative to carrier proteins
can be beta barrels or composed of alpha helices
example: aquaporins, many beta sheet assemble into a barrel, create hydrophilic pore
What is an ion channel?
permeable to specific ion like Ca2+
each formed by integral membrane proteins that surround an aqueous pore
highly effective
net flux depends on electrochemical gradient
What are gated channels?
most ion channels exist in either an open or closed conformation
such channels are termed gated
What are voltage-gated channels?
conformation state depends on charge difference in ionic charge on two sides of membrane
example: K+ channel opens with membrane depolarization
channels are very specific to solute
negative amino acids at opening repels negative ions
gating of this channel is accomplished by a voltage center
How do you get ion selectivity?
by negative charge at surface
selectivity filter
pore size
How does the pore open?
voltage gated
opens when inside of cell becomes more positive (depolarization)
normally inside of cell is more negative at depolarization (+50 mV)
at resting, positive charged S4 domain is attracted to inside of cell
at depolarization, positive charged S4 is attracted to outside of cell
shift = open channel
What are chemical gated channels?
conformational state depends upon binding of a particular substance
example: acetylcholine acts on outer surface of certain cation channels
What is active transport?
movement of a molecule up a gradient
allows passage of substance through membrane against electrochemical gradient for that substance
active transporter acts by undergoing conformational change upon binding substance
movement occurs only in one direction
three ways that energy can be supplied for active carriers: ATP hydrolysis, coupled transport, light driven
example: Na/K pump, establishes resting potential at -70 mV inside, cell constantly pumps 3 Na+ out and 2 K+ against concentration gradient for each ATP consumed
What is coupled active transport of existing ion gradients?
movement of two molecules simultaneously, where one molecule moves using the energy gradient from the other molecule
symport: two solutes are moved in the same direction
antiport: two solutes are moved in opposite directions
can couple the resting potential with active transport of another molecule
example: use Na+ gradient as a source of energy for active transport of amino acids or sugar
