Module 3b- Membrane function
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Module 3b- Membrane function
difference in electrical charge across a membrane
created when there's a difference in ion concentrations across a membrane
resting membrane potential
where there is
no net flow occurring across the membrane
how do cells establish a membrane potential?
what is the significance of membrane transport? (2)
important in setting up different environments inside versus outside the cell
or within organism
optimizes the environment for function
sets up membrane potentials that store energy
what is the difference between solute and solvent?
solvent is a fluid in which another substance, the solute, can dissolve
water is an excellent solvent
why is water able to dissolve a large variety of substances?
hydrogen has a positive electric charge and oxygen has a negative electric charge
which allows is to attract different kinds of molecules
Describe the key feature that makes water a universal solvent.
: has a partial positive charge and a complementary negative charge
most of the molecules in cells are also polar and can form
hydrogen or ionic bonds with water
what type of chemical bonds are found in these molecules?
affinity for water
dissolve in it easily
polar molecules or ions
, such as sugars, organic acids, & some amino acids
polar covalent or ionic bonds
what type of chemical bonds are found in these molecules?
not easily soluble in water
large and non-polar
lipids and proteins in membranes
nonpolar covalent bonds
what type of chemical bond exists between a cation and an anion? Is this a strong or weak interaction?
what needs to happen in order for a hydrophilic solute (NaCl) to dissolve in a liquid?
to dissolve in a liquid, the
attraction between anions and cations in the salt must be overcome
what do polar water molecules do to ions?
form spheres of hydration around ions
how does water interact with proteins?
ionic and polar regions on the protein's surface attract water molecules
release of ordered water is necessary for the formation of an enzyme-substrate complex
ordered water interacts through hydrogen bonding
how is enzyme-substrate interaction stabilized in desolvation?
stabilized by hydrogen bonding, ionic and hydrophobic interactions
Fatty acids hydrophilic or hydrophobic?
tend to be hydrophobic
some fatty acids with no net charge are polar and thus hydrophilic
if they have a region that is positively charge and another that is negatively charged
how is the movement of a molecule that has no net charged determined?
determined by its
difference between simple or facilitated diffusion and active transport.
simple or facilitated diffusion involves
exergonic movement "down" the concentration gradient
active transport involves
endergonic movement "up" the concentration gradient
what is ion movement influenced by?
influenced by the combined effect of its
concentration gradient and it's electrochemical potential
(which is impacted by the charge gradient across the membrane)
what does active transport of ions across a membrane create?
what is solute movement affected by?
a charge gradient or membrane potential across the membrane
affected by electrochemical potential
The 3 permeability guidelines for a typical lipid bilayer are?
small nonpolar molecule readily dissolve in lipid bilayer
diffuse across the membrane
uncharged polar molecules may also diffuse rapidly depending on size
(only small, water etc.)
Ions and charged molecules do not cross without assistance
(ions, sugars, amino acids, nucleotides, and wastes)
The three methods through which solutes cross the membrane are:
(osmosis) [passive transport]
simple diffusion (passive transport)
down the concentration gradient
direct unaided movement is dictated by difference in the concentration of the solute on the two sides of the membrane (higher to lower concentration)
ex. gases, nonpolar molecules, and small polar molecules such as water, glycerol, ethanol
facilitated diffusion (passive transport)
movement down the gradient that requires transport proteins
transport proteins(integral membrane proteins)
assist most solute across membranes
move solutes to regions of lower concentration, this does not use energy
transport proteins move solutes against the concentration gradient
requires energy like the hydrolysis of ATP or by the simultaneous transport of another solute down an energy gradient
since diffusion always moves solutes toward equilibrium:(2)
free energy is minimized, as molecules move down their gradient
solutes always move toward regions of lower concentration until concentrations are equal
describe the three factors affecting diffusion.
smaller molecules are able to diffuse across the membrane
more permeable to nonpolar substances
than polar ones; polar molecules will repel the fatty acids and NOT cross membrane; small polar molecule will diffuse very slowly
for charged solutes to move into a membrane, the water molecules (that form a shell of hydration around polar substance) must be removed
; requires energy
measure of solute polarity
measured by the
ratio of its solubility in an organic solvent to its solubility in water
more nonpolar (hydrophobic) a substance is, the higher the partition coefficient
(solute can pass through membrane)
rate of simple diffusion
rate of simple diffusion is directly proportional to the concentration gradient (steepness of the concentration gradient)
is an exergonic process
permeability coefficient and diffusion through membrane.
larger permeability coefficient, diffusion through a membrane is easier
and vice versa
membrane characteristics also influence permeability
what is the name of the interaction between water molecules? Are these weak or strong interactions?
weak initially but will become a force to contend with when together
what happens to water molecules when solute molecules are dissolved in water?
solute molecule that are dissolved in water
disrupt the interactions that normally occur between water molecules
decreases free energy of the solution, allows water to move from regions of low to high solute 
for most cells water tends to move inward
diffusion of water across a selectively permeable membrane
water will move toward region of
higher solute concentration
the pressure required to stop osmosis
solutes exert an osmotic pressure
measure of the tendency of a solution to take in water by osmosis
no net movement of water
water moves into the cell and may burst
water leaves the cell and shrivels
how do cells use osmotic pressure to regulate water balance? (Na, Cl, and K)
the  of Na+ and Cl- are higher outside the cell than inside the cell, whereas the  K+ is higher inside than out
the low  of Na+ and Cl- inside the cell balance the high intracellular  of organic compounds. this equalizes the osmotic pressure and prevents the net influx of water
can regulate water using aquaporins
specific to water molecules
single file movement through channels
plant cells prefer a __________ internal environment (ex. central vacuole)
cell wall has poor osmotic pressure which moves water through the cell wall from an area of low  to are of high  which is the central vacuole
guard cells in plant tissue
regulate water loss by controlling stoma opening and closing
guard cells take up K+ by active transport; causes water to enter the cell my osmosis
: guard cell walls are unevenly thickened and have radially oriented cellulose microfibrils; causes cells to bow as they become turgid
: when K+ ions are pumped out of the cell, water follows my osmosis and they stomata closes
water regulation in protozoans
some protozoans use
to remove water and prevent
what are the transport proteins in facilitated diffusion? describe the 2 types.
large, integral membrane proteins with multiple transmembrane segments
bind solute molecules on one side of the membrane, undergo a conformation change, and release the solute of the other side of the membrane
; high specificity, not energizing
form hydrophilic transmembrane channels through the membrane to provide a passage route for solutes
; creates pore in membrane, very specific
what are the 3 types of channels?
what are the characteristics?
tiny pores lined with hydrophilic atoms
passive transport occurs based on electrochemical gradients
: very specific ions
: closed when not in use
: move millions of ions/ sec across membrane
Selectivity of the K+ channel (check diagram)
pore is just wide enough to allow passage of dehydrated K+
from which all associated water molecules have been displaced as a result of interactions between K+ and carbonyl oxygen
Na+ is too small to interact with the carbonyl oxygen
of the selectivity filter, so it
remains bound to water in a complex that is too large to pass through the channel pore
what does it mean that channels are gated?
means that they open and close in response to stimulus
: flow of ions blocked by gate
: allows ions to flow rapidly through the channel
describe the 3 types of gate channels.
open and close in response to changes in membrane potential
(depends on resting membrane potential)
are triggered when specific substances bind to the channel protein
respond to mechanical forces acting on the membrane
(ex. pressure on hand)
importance of Gated Channels
because there is a  difference across a membrane, a
transient opening of one of these channels is able to locally alter the membrane potential
this can lead to
Functions of ion channels?
establish a resting membrane potential
(muscle contraction & electrical signaling in nerve cells)
maintaining salt balance
in cells and airways linking the lungs
what are porins?
what is the function of
multipass transmembrane proteins that form membrane pores
rapid but non-specific passage of solutes
describe the structure of porins and how the embed in the plasma membrane.
3 Beta barrel in a ring is quaternary structure
Beta barrel has
hydrophilic and hydrophobic residue
, facing towards outside of barrel,
attract lipids in plasma membrane which allows it to be embedded
aquaporin structure (3)
tetrameric integral proteins
4 identical monomers associate with their 24 transmembrane segments in an orientation that
forms 4 central channels
channels lined with hydrophilic side chains
that are just large enough to
let water pass through single file
function of aquaporin.
found in animal cells
rapid passage of water by interrupting a minimum number of hydrogen bonds
describe structure of gap junction and function.
6 transmembrane proteins called connexins come together and create a channel in the center
called the gap junction
resting potential change is what allows cells to communicate
describe what the alternating conformation model state about carrier proteins.
states that a carrier protein is an
alternates between two conformational proteins
in one state, the
solute binding site
of the protein is
accessible on one side of the membrane
the protein shifts
to its alternate conformation,
solute binding site on the other side of the membrane
, triggering the solute's release
how are carrier proteins analogous to enzymes?
how are carrier proteins regulated?
facilitated diffusion involves
binding the substrate on a specific solute binding site
carrier protein and solute
form an intermediate after conformational change, the "product" is released
(the transported solute)
carrier protein specificity
highly specific for a single compound
small group of closely related compounds
when does competitive inhibition of carrier proteins occur?
can occur in the
presence of molecules or ions that are structurally related to the correct substrate
Active transport performs 3 important cellular functions:
uptake of essential nutrients
removal of wastes
maintenance of non-equilibrium concentrations of certain ions
how does active transport differ from diffusion in regards to the direction of transport .
with respect to the membrane (direction of movement driven by concentration gradient)
and is therefore
how is the energy for direct active transport provided?
how is indirect active transport driven?
driven by ion gradients
ATP does not fuel pump directly (takes energy from direct active transport pump)
what are the 2 types of
direct active transport
compare uniport and coupled transport.
when a carrier protein (uniporter) transports a single solute across a membrane
when two solutes are transported simultaneously and their transport is coupled
what does coupled transport depend on?
In couple transport, the
transport of one solute depends on the transport of another solute
energy stored in the electrochemical gradient of one solute is used to move the other
compare symport (cotransport) and antiport (countertransport).
: if the 2 solutes are
moved across a membrane in the same direction
: if the 2 solutes are
moved in opposite directions
proton pump (ex. interior of lysosome within animal cells)
(acts as the proton pump)
binds to the proton in the cytoplasm while in conformation A
energy from ATP then drives the transport of the proton to the interior of the lysosome through the formation of conformation B
release of the proton results in the protein reassuming conformation A
describe the structure and function of Ca2+ pump. (generally in cell)
transmembrane protein that has
9 alpha helixes
Ca2+ from cytoplasm and transports it to ER membrane which stores it until needed
ATP hydrolyzes to ADP and collects energy to move Ca2+
Ca2+ a second messenger, helps neurotransmitters and enzyme function
moves from high to low  because charged
what is the glucose transporter and describe the structure?
that is uniquely active
erythrocyte capable of glucose uptake by facilitated diffusion
because level of blood glucose is much higher outside than the inside of the cell
glucose transported inward by
GLUT1 is integral membrane protein (single polypeptide with 12 passes)
doesn't use ATP
describe the 4 steps in the transport of glucose by GLUT1.
1. Glucose binds to GLUT1 transporter protein that has its binding site open to the outside of the cell
2. glucose binding causes the GLUT1 transporter to shift to its
with the binding site open to the inside of the cell
3. glucose is released to the interior of the cell, initiating a
second conformational change in GLUT1
4. Loss of bound glucose causes GLUT1 to
return to its original T1 conformation
ready for the next cycle
what is the benefit of phosphorylation of glucose upon entry into a cell?
the immediate phosphorylation of glucose upon entry into the cell
keeps the concentration of glucose low
glucose cannot bind the carrier protein any longer and is stuck in the cell
highly polar (negative) can't diffuse back
what is the Na+/K+ pump?
maintains electrochemical ion gradients
typical animal cell,
K+ inside cell
Na+ outside cell
electrochemical potentials for sodium and potassium are essential as a driving force for coupled transport and for transmission of nerve impulses
what's the energy requirement for Na+/K+ pump?
the pump uses the
exergonic hydrolysis of ATP to drive the transport of both ions
responsible for the asymmetric distribution of ions
across the membrane of animal cells
structure of Na+/K+ ATPase
4 polypeptides transmembrane protein
2 alpha subunits that connect to the ion
2 Beta subunits, function unclear, for structure?
the Na+/K+ pump conformational states.
Na+/K+ pump is an
2 alternative conformational states:
: open to he inside of the cell and has high affinity for Na+ ions
: open to the outside of the cell and has a high affinity for K+ ions
what are the six steps in the Na+/K+ pump mechanism?
1. on the inside of the cell,
three Na+ ions bind to the E1 conformation
triggers phosphorylation of the alpha subunit
pump undergoes a shift to the E2 conformation
, which causes the
release of the Na+ ions on the outside of the cell
two K+ ions bind to the E2 conformation
on the outside of the cell
triggers dephosphorylation of the alpha subunits
return to the E1 conformation
6. In the conformational change,
are carried to the inside of the cell and
what are the 3 functions of the Na+/K+ pump?
set up the membrane potential
maintain high [Na] outside the cell and high [K] inside the cell
maintain the osmotic balance of the animal cell
what does direct transport depend on?
what are they?
direct transport depends on
four types of transport ATPases
ATPases are a
class of enzymes that hydrolyzes ATP to ADP and a free phosphate ion.
energy that can be used to drive other chemical reactions
what are transmembrane ATPases? what are the types?
integral membrane proteins that use ATP energy to move solutes across the membrane against their concentration gradient
4 types are:
members of a large family that are reversibly phosphorylated by ATP on a specific aspartic acid residue
8-10 transmembrane segments
sensitive to inhibition by vanadate
what are the 5 subfamilies of p-type ATPases?
Transport heavy metals, like copper, zinc
maintain gradients of ions
pump protons out across the plasma membrane in plants & fungi, which acidifies the external medium
pump hydrophobic molecules such as cholesterol and fatty acids & act as flippases
some are known to transport cations in ER, vacuoles or lysosomes
what is the V-type ATPases?
what is the structure?
what are the 3 functions?
pumps protons into organelles such as vacuoles, vesicle, lysosomes, endosomes, and Golgi complex
have 2 multi-subunit components
V0 is the integral component embedded in the membrane
V1 is a peripheral component that has the ATPase activity (made of 7 subunits)
Three functions are:
generate a proton gradient
act as a pH sensor
energize the membrane
what is the F-type ATPases?
what is the structure?
what are the 3 functions?
transport protons in bacteria, mitochondria and chloroplasts
2 multi-subunit components:
F0 a transmembrane pore
F1 a peripheral membrane component that contains the ATP binding site
function by hydrolyzing ATP to obtain the energy needed to pump protons against the  gradient
what are the ABC-Type ATPases?
transport amino acids, sugars, ions, polypeptides, polysaccharides or proteins
in eukaryotes, they only export. in prokaryotes they import and export
4 protein domains
2 domains are highly hydrophobic and are embedded in the membrane
2 domains are peripheral and are associated with the cytoplasmic side of the membrane
bind ATP and couple its hydrolysis to transport molecules
what are multi-drug resistance proteins? (MDR Proteins)
pump hydrophobic drugs out of cells
reduces the cytoplasmic concentration and hence their effectiveness
can be overcome by inhbitors
small integral membrane protein in the plasma membrane of archaea
uses energy from photons of light to drive the active transport of protons out of the cells
creates an electrochemical proton gradient that powers synthesis of ATP by and ATP synthase
what is the light-absorbing pigment in bacteriorhodospim?
light-absorbing pigment is
retinal, which is related to vitamin A
embedded in plasma membrane
linear in resting state, when it's energized by light it gets bent or cised
function of bacteriorhodopsin
in bacteriorhodopsin present in
retinal absorbs a photon, one of its double bonds isomerases to a cis form
photo activated molecule than transfers protons to the outside of the cell
proton gradient created by pumping protons out of the cell is used to produce ATP
protons flow back into the cell down the concentration gradient and causes ATP synthases to produce ATP
some cells use Na+/glucose symporter called
sodium-dependent glucose transporters or SGLT proteins
to take up glucose. what are the six steps in this mechanism?
2 external Na+ ions bind their sites on the symporter, which is open to the exterior
this allows one molecule of glucose to bind
a conformational change in their protein exposes the glucose and Na+ inside the cell
the 2 Na+ ions dissociate in response to the low internal Na+ 
this locks the symporter in its inward-facing conformation until the glucose dissociates
the loss of glucose frees the symporter to return to the outward-facing conformation