Chapter 9 Flashcards
what are the membrane functions
-separate cells from external medium to create an intracellular environment of unique
-allows selective transport of substances in and out of the cell
-provides a location for specialized pathways and processed (e.g energy conversion of mitochondria
-rapid changes in electric potential across the membranes of neurons as basis of the nervous system
-localization of receptors to facilitate response to physiological signals (why enzymes are located together)
-mediate cell-cell recognition and interaction
what are the membrane characteristics
- they are sheet-like structures, two molecules thick, that form closed boundaries between compartments
-they consist of mainly lipids and proteins with carbohydrates linked to the molecules
-they are built from amphipathic molecules
-membranes are largely impermeable to polar molecules
-specific membrane proteins mediate particular biological functions
-they are self assembling, non covalent structures - they are fluid and dynamic structures
-membranes and highly specialized in their composition and distribution (asymetric)
what is membrane formation a consequence of
-the amphipathic nature of the membrane lipids
-they are self assembling through hydrophobic effect
what does the structure of a membrane form based off of
on the ratio of the cross sectional areas of the polar head and the hydrophobic tail
what do lipids with to hydrocarbon tails tend to form (glycerophospholipids and sphingolipids)
bilayers
what formation do fatty acids favor
the micelle formation of micelles
exposure of hydrophobic tails at the edge of the bilayer to water is…
energetically unfavourable
what do flat bilayer sheets tend to do
-they are unstable and spontaneously form membrane vesicles with internal volume
-the vesicles are the basis of cells and organelles
what molecules can and cant pass through a lipid bilayer vesicle
-only hydrophobic molecules can pass
-very low permability to ions and most polar molecs
-some smaller non-polar gases (O2 and CO2 for example) and small hydrophobic molecules (like fat soluble hormomes) can pass directly through the membrane
what is the permeability of small molecules correlated with
their relative solubility in water
what challenge can membranes have for drugs
it can represent a critical barrier for polar drugs intended for intercellular targets
how can polar drugs make it across the cell membrane
encapsulation of a drug within a liposome can facilitate transport across the membrane
what thing can liposomes deliver drug molecules to
it will deliver specific molecules to specific cell types but also different organelles within different cell types
what are membranes mostly composed of
lipids and proteins
what do more active membranes have
they have a higher ratio of protein to lipid
can compositions of membranes change?
yes composition of membranes can be dynamic, especially for prokaryotes
why are membranes dynamic structures
due to the nature of the non-covalent interaction
where are lipids and proteins on the membrane
they are freely diffused in the plane of the membrane
how can proteins and lipids move in the membrane
-lateral movement of proteins and lipids within the membrane is very rapid
-movement across the membrane is restricted
what does transbilayer movement require
a polar head group to pass through hydrophobic environment
what is the rate of uncatalyzed lipid molecule crossing from one sheet to another
it is very slow (t 1/2 in days)
what is it called when a uncatalyzed lipid molecule crosses from one sheet to another
flip flop diffusion
what is the translocation of lipids from one side of bilayer to catalyzed by and how fast is it
and enzyme called flippases
and it is fast (t 1/2 in seconds)
what allows for specialization of membrane faces
how lipid composition of inner and outer sheets of the bilayer can be different
what must cells maintain
an appropreate level of membrane fluidity
membranes undergo temperature dependent…
phase transitions
below the phase transition temp the membrane is
too solid
above the phase transition temp the membrane is
too fluid
at the phase transition temp…
they hydrocarbon chains are partially ordered but lateral diffusion is still possible (just right)
how do cells adjust there membrane composition to maintain liquid ordered state
-bacteria vary the length and saturation of the hydrocarbon tails of membrane lipids
-animals use cholesterol to mediate membrane fluidity
while the basic features of the bilayer are simple and consistant what are the mechanisms that enable specification
-composition of membrane components
lipids and proteins
-distribution of membrane components
static and dynamic
-specialized membrane rings
lipid rafts
the lipid composition of membranes varies across…
species and cell types (highly specialized compositions and functions)
in cell membranes what things does highly specialized compositions and functions include
the dynamic changes to composition and/or positioning to regulate biological events
for example the movement of phosphatidylserine to outer leaf functions initiating cell destruction (apoptosis)
what do lipid rafts arise form
the spontaneous association of lipid molecules whose hydrocarbons are simular length
what do sphingopilids (with longer tails) form
clusters that exclude that exclude glycerophospholipids
what do the longer saturated hydrocarbons of sphingolipids form
stable associations making the rafts thicker and more ordered than the rest of the membrane
what are rafts
they are the docking point in lipid-anchored proteins that contain long chain saturated fatty acid anchors
what do the lipid linked proteins that assoceate with rafts often serve as
signalling functions
how do GPI linked proteins work
-GPI tail is a lipid anchor
-the GPI tail wants to burry itself in a hydrophobic environment
-doing so tethers itself to the membrane
what do proteins with GPI anchors tend to be involved in
signal transduction (involved in how the cell is communicating with the extra cellular environment
what are active roles of membranes often preformed by proteins
receptors and transporters
three categories of membrane proteins are defined based on different mechanisms of association with the membrane. what are they
-peripheral (just on the surface of either side of the membrane
-lipid-anchorred (just on the surface of the membrane attached to a embedded lipid)
-integral membrane proteins (embedded across the bilayer)
what forces do peripheral membrane proteins associate with the membrane through
electrostatic of hydrogen bonding interactions
what can peripheral membrane proteins dock to
either membrane lipids or integral membrane proteins
where is the bulk of the peripheral membrane proteins found
in the cytosol or extracellular space
what things can release the peripheral membrane proteins from the membrane
changes in pH or ionic strength often releases these proteins form the membrane
where can covalently attached lipids anchor proteins
to the membrane
are lipid anchor membrane proteins reversible
sometimes these modifications are allowing fro regulation of cellular location
where are GPI anchored proteins facing
they always at the outer face
how many residues in the membrane do GPI anchored proteins have
two
proteins with single chain hydrocarbon anchored in the membrane proteins are always on the…
inner face
is there one kind of fatty acid chain that acts as lipid anchored membrane proteins
-no there are different types of fatty acids
-different fatty acids are attracted to different points on the protein
where are integral membrane proteins found
they are immersed in, and usually span the membrane
how are proteins positioned in the membrane in integral proteins
positioning within a membrane is specific and directional
what are the 3 varieties that integral membrane proteins tend to be
-single pass a-helical
-a-helical bundles
-B-barrels
how many passes do a-bundles tend to have
they tend to have around 7
what tends to be the context in which you see B barrels
-you have BBs from amphipathic B sheets
-youll get nonpolar residues pointing out and interacting in the membrane
-you have the polar residues on the inside
-through the inside region youll have have water
-represents a pore passageway to get through the membrane
what is the distribution of proteins like for the distribution of amino acids in integral membrane proteins
-charged residues are usually located mostly within the intra and extra cellular portions of the protein
-residues with non-polar side chains dominate inside the hydrophobic slab of the bilayer
-tryptophan and tyrosine cluster at the interface between the hydrocarbon chain and polar head region
how many on the proteins in out body are integral membranes
-30%
-of those only 1% have had there structures determined by procedures like X ray crystallography and NMP (they are very important but very hard to study)
what can membrane spanning regions be predicted from
the amino acid sequence of integral mem proteins
sequences of how many and what kind of amino acids are likely to be membrane spanning
20-24 hydrophobic residues
what technology can be used to look at the hydrophobic characteristics of a protein to predict transmembrane regions for integral proteins
a hydropathy index
amino acids within the teansmembrane region tend to be non-polar how ever something attached is polar what is it
the carbonyl and amide groups of each peptide bond are polar
polar unpaired carbonyl and amide groups in the bilayer core are…
energetically unfavorable
carbonyl and amide groups of the protein backbone within the bilayer have to be…
hydrogen bonded
what are the categories of membrane transport
-simple diffusion
-facilitated diffusion
-active transport
-ion transporters
what are the different kinds of facilitated diffusion
-carriers
-channels
what are the different kinds of active transport
primary and secondary
what molecules can directly cross the membrane for simple diffusion
non-polar gases (O2 and CO2) and hydrophobic molecules
for the things that can cross the membrane through simple diffusion what is the direction and rate of the movement determined by
their concentrations on either side of the membrane
diffusion can only result in the net movement…
down a concentration gradient
how does facilitated diffusion work
-membrane transporters lower the activation energy barrier of crossing bilayer
-activation energy for removing the hydration shell form a polar solute and transferring it into the non-polar environment in the core of the bilayer is very hard
-membrane transporters lower the activation energy for crossing the membrane by replacing the hydration shell with interactions with polar groups along the transfer path in the protein interior
how do channels and carriers for facilitated diffusion differ interns of saturation
channels dont saturate
carriers do
how do channels and carriers for facilitated diffusion differ interms of speed of thing passing through
-channels have high conductance rates because they bind the substrate very weakly
-carriers are slower because they bind the substrate quite strongly
how do channels and carriers for facilitated diffusion differ interms of how they move thing from one side of the membrane to the other
-channels: membrane pores to transport molecules down the conc gradient
-carriers: membrane proteins that undergo substrate induced conformational changes or membrane repositioning to release substrate to the other side of the membrane
what is the rate difference of facilitated diffusion and simple diffusion of glucose
facilitated diffusion of glucose is 50,000x faster the simple
what is Kt for facilitated diffusion of glucose
Kt is about 1/3 the concentration of blood glucose so the transporter is nearly saturated and operates near Vmax
what is the glucose carrier of the RBC
glucose permease (works kinda like an enzyme)
-alway regenerated
-velocity of transport is going to vary as a function of the substrate of the concentration of the substrate out side the cell
why does the glucose permease work at about 75% its max capacity
-enzymes work at around 50% max capacity to make them more responsive at a time of need (if needed they can go faster)
-in RBC theres nothing really going on it has low energy requirements
-cells know there wont be any demands placed on that RBC its ok to run the system a little closer to the red zone
-no situation that the RBC would need something that requires extra energy
what is transport on a single molecule called
uniport
what are antiports
transporters that couple the movement of two molecules. they move the molecules in different directions
what are symporters
transporters that couple the movement of two molecules. they move molecules in the same direction
what does the diffusion co-transport through antiport or symport depend on
the charge of the molecules in order to have a net neutral charge
how does active transport work
input of energy allows movement of molecules against the concentration gradients
what is primary active transport
-its driven by a direct source or energy (ATP)
-includes P-type, V-type, and ABC transporters (p=phosphorolatred v=vaccuole)
what is secondary active transport
it couples the movement of one molecule down it concentration gradient within the movement of another molecule down its gradient
what concentrations of Na+ and K+ does the cell like to maintain. what does this concentration gradient control
-the cells likes to maintain high gradients of Na+ outside the cell and K+ inside the cell
-this controls the cell volume, electrical excitability, and enables uptake of nutrients through secondary active transport systems
how much of your energy does maintaining the Na+ and k+ pump require
about 1/3 of your energy
k+,Na+ ATPase uses the energy of…
ATP hydrolysis to pump three Na+ out of the cell and two K+ into the cell
what is P-type transporter called P type
because it undergoes a phosphorolayted intermediate from aspartate
what is V-type ATPase and what does it do
-used the energy of ATP to move protons against the concentration gradient
-acidification or organelles (lysosomes)
-in chloroplast and mitochondria F-type ATP synthases reverse this reaction to use proton gradients to generate ATP
what are ABC transporters and how do they work
-contain ATP-binding domains (ATP-Binding Cassette)
-transport a variety of biomolecules out of the cell against a concentration graident
-multi-drug resistance protein pums drug (chemotheraputic) out of the cell rendering the drugs ineffective)
what is glucose uptake form the gut driven by? how does it work? and what is it dependent on?
-in intestinal epithelial cells glucose uptake form the gut is driven through symport with Na+
-the movement of glucose up its concentration gradient is enabled by the movement of Na+ ions down there concentration gradient
- it is dependent on the action of the Na+ -K+ ATPase to establish the gradient of Na+ ions
what do ion channels do
-they enable rapid movement of ions across the membrane
what does the action of ion channels cause
-action of ion channels can cause changes in membrane potential (action potential in neurons)
how and what are ion channels regulated by
they are tightly regulated by voltage gated channels and ligand-gated channels
what are the three ways that ion channels differ from ion transporters
-they are faster
-have no saturation limits
-gated/regulated (open and close in response to signal)
ion channels are highly…
selective for the molecules to be transported
how do K+ channels work
-they allow rapid movement of K+ ions out of cell
-although Na+ is smaller the channel is 100-fold more permeable to K
how does selectivity work for specificity of ion channels
-selectivity filter discriminates K and Na based on there ability to shed water molecules to form electrostatic interactions within backbone carbonyls
what helps with the speed of ion channels for K+ channels
-selectivity filter have four equivalent binding sites for K+
–as K+ ions enter the filter the electrostatic repulsion form the other incoming K+ ions helps to push the flow of ions form inside to outside the cell
