Lecture 3-4: Peptides

BIOC 460, Summer 2011
Membrane Transport
Reading: Berg, Tymoczko & Stryer, Chapter 13, pp. 351-376
Problems in textbook: chapter 13, pp. 379-380: #1, 3, 14a, 20
If a picture is worth a thousand words, an animation is worth a million
Cool aquoporin animation:
http://www.youtube.com/watch?v=XxadMJ9zqpA
An interesting animated tour of the K+ channel:
http://www.youtube.com/watch?v=lnKIBZYarzM
Active transport general:
http://www.youtube.com/watch?v=STzOiRqzzL4&feature=related
Very cool sarcoplasmic reticulum and SERCA ATPase animations:
http://www.youtube.com/watch?v=EpH_FUWBu2M&feature=related (SR)
http://www.youtube.com/watch?v=UCJWp1ntpm4 (SERCA ATPase)
Key Concepts
•
Free energy of transporting material across membrane depends on
concentration gradient across membrane:
•
For uncharged solutes,
•
Solutes move spontaneously (ΔGt < 0) from compartment of higher
concentration to compartment of lower concentration.
Equilibrium: ΔG = 0 when C1 = C2
•
Charged solutes: presence of a membrane potential as well as the
chemical concentration gradient influences the distribution of ions:
Passive transport: spontaneous passage of solute "down" concentration
and/or electrical potential gradient -- no input of free energy required
• Simple diffusion (no assistance)
• Facilitated diffusion (rate enhanced by carrier or channel, generally
an integral membrane protein (transporter or permease)
–rapid diffusion, "down" a concentration gradient
–saturable (max. velocity depends on transporter concentration)
–specific (depends on interaction of solute with transporter)
–Example: GLUT1 glucose transporter in erythrocytes
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Key Concepts, continued
Passive transport, continued:
•
Bacterial K+ channel: passive diffusion with very high specificity for K+
ions
•
GLUT1 transporter of erythrocytes
–
Example of characteristics of a transporter proteins:
• works by conformational changes linked to ligand binding
• Rapid transport “down” concentration gradient
• Saturable (shows a maximum velocity; can measure Kt analogous
to Km)
• Specific
•
Gated ion channels (ligand-gated or voltage-gated)
–
VERY rapid, ~107-108 ions/sec, "down" a concentration gradient
–
not saturable
–
degree of specificity/ion selectivity varies
–
Examples:
• Acetylcholine receptor of motor neurons
•
•
Terminology applying to all transporter proteins, passive or active:
Uniport (system in which one solute transported)
Cotransport (system in which transport of one solute is coupled to
transport of another solute)
– Symport (different solutes transported in same direction)
– Antiport (different solutes transported in opposite directions)
Key Concepts, continued
Active transport (transport of solute against its concentration gradient)
– requires an exergonic process to drive the “uphill” transport
•
Primary active transport (transport of solute against its concentration
gradient, coupled directly to an exergonic chemical reaction, e.g.,
ATP hydrolysis)
–
Examples:
• P-type ATPases
– Ca2+ ATPase of muscle cell sarcoplasmic reticulum
– Na+-K+ ATPase of animal cell plasma membranes
•
Secondary active transport (transport/"flow" of one solute "down" its
concentration gradient is used to drive transport of a different solute
against its concentration gradient energy
–
Concentration gradient of solute that “drives” the unfavorable process
comes from ATP hydrolysis (via Na+-K+ ATPase activity)
–
Example
• Na+-glucose symporter (in some animal cells
•
Membrane Transport
Mechanisms of transport processes involving membrane proteins
usually involve protein conformational changes.
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Thermodynamics of Transport Processes
•
equilibrium for a transport process (as for any process): conditions
under which ΔGt = 0.
uncharged solutes: free energy change for transporting an uncharged solute
across a membrane depends only on concentration gradient across the
membrane:
• For uncharged solute:
•
•
where C1 = concentration at "origin"and C2 = concentration at "destination"
Uncharged solutes move from region of higher concentration to region
of lower concentration (direction in which C1 > C2) so
ΔGt = 0, when C1 = C2
(Equal concentrations defines equilibrium for a transport process involving
uncharged solutes.)
The Chemical Potential Term
Since C1 >> C2
DG < 0 for flow of species
from C1 to C2.
Flow down concentration
gradient.
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Thermodynamics of Transport Processes, continued
charged solutes
• Electrical potential (charge gradient across membrane) influences distribution
of ions.
• For ion of electrical charge Z:
• where F = Faraday constant [96.5 kJ/(V•mol)]
- DV = membrane electrical potential (charge gradient across membrane), in
Volts.
• ZFΔV (charge gradient, electrical potential gradient)
– can result from concentration AND charge differences across membrane
for OTHER ions than the one you're looking at
– typically inside of membrane is negatively charged, relative to outside.
- DV term can work either “with” or “against” concentration gradient.
You should always draw a picture of the transport process to decide on the
sign of the ZFDV term taking into account the charge, Z, of the ion.
The Electrical Potential Term
•Positively charged ions will flow
from right to left (DG<0).
•Negatively charged ions will flow
from left to right (DG <0)
•In order for ions to move against the
electrical potential gradient, energy
must be put into the system, i.e. they
must be “pumped”
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
•
Free Energy Change for Transport of Charged Solutes
DGt depends on signs and relative magnitudes of concentration gradient
term and charge gradient term.
Sum of chemical potential term + electrical potential term =
electrochemical potential ( = DGt )
Does the concentration gradient of Cl– ion across this cell membrane
favor transport of the ion into or out of the cell?
Without doing a calculation, what would be the sign on the first term of
the equation for free energy of transport of Cl– out of the cell?
Cout/Cin > 1, so transport out would have this term > 0, i.e. positive;
concentration gradient (C2/C1 term) unfavorable to transport out of cell.
Does the charge gradient across this cell membrane favor transport of
Cl– ion into or out of the cell?
Without doing a calculation, what would be the sign on the charge
gradient term for free energy of transport of Cl– out of the cell?
If you’ve drawn a picture like the one above, it’s clear that there’s more
neg. charge inside cell than outside, so last term favors transport of a
negative ion like Cl– out of cell (term would have a – sign).
Another example: Transport of Charged Solutes
•
•
•
Suppose intracellular [K+] = 157 mM and extracellular [K+] = 4 mM.
For transport of K+ ions into the cell (298K), is chemical potential term
(concentration gradient) favorable or unfavorable? Its sign?
Is electrical potential term (ZFDV term) favorable or unfavorable? Its
sign?
Draw picture:
• Chemical potential term: C1= 4 mM, C2= 157 mM, so C2/C1 > 1.
• Thus sign of concentration term is positive (chemical potential
unfavorable for transport in).
• Electrical potential term: Inside of cell is negative relative to outside
(info given, drawn on diagram), so charge gradient means transporting a
+ ion (K+) in would be favorable from a charge point of view.
• ZFDV term (2nd term in equation) must be negative (electrical potential
favors transport of a + ion into cell), since Z = +1 for K+.
• Calculation is on posted sample problems PDF file.
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Thermodynamics of Transport Processes, Summary
1) Thermodynamically favorable processes: net DGt' < 0
• Uncharged solute: moving down concentration gradient (C1 > C2)
equalizing concentration distribution
• Charged solute (ion): DGt' depends on 2 terms:
•chemical potential
•
•electrical potential: moving down charge gradient.
“Passive Transport”
2) Thermodynamically unfavorable processes: net ΔG't > 0
•
•
•
Moving a solute against concentration and/or charge
gradient,
Requires free energy coupling to another process with a
greater negative free energy change, so overall ΔG't will be
negative
“Active transport”
Kinds of Passive Tranport
1) Simple diffusion
–solute moves freely across lipid bilayer; no membrane protein
needed to assist process
–lipophilic solutes, passing freely across membrane -- don't need help
2) Facilitated diffusion
• specific protein required to "assist" solute across bilayer
• moves "down" concentration or charge gradient
• high activation barrier (kinetic) to get across membrane
• polar or charged solutes must lose solvating H2O molecules
• requires Protein Transporter
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Passive Transport, continued
•
Protein transporter: reduces activation energy barrier to get polar solute
across membrane:
– provides an opening (a channel) for solute to cross membrane
– binds solute and transports across membrane, ONLY in direction dictated by
concentration or charge gradient
•
•
Transporters and channels both display substrate specificity.
Some transporters and channels working/"open" all the time; others "gated"
(opening regulated by some signal that opens or closes)
Energy changes as hydrophilic solute crosses membrane
Transporter protein:
a) Forms noncovalent
interactions with solute
replacing interactions
with “lost” H2O of
hydration
b) Provides hydrophilic
passageway across
membrane
Nelson & Cox, Lehninger Principles
of Biochemistry, 4th ed., Fig. 11-28
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Passive Protein Transporters: DGt' < 0
1)The Streptomyces lividans K+ channel
• first channel to have high resolution structure determined (1998).
• allows passive transport of K+ across membrane
• effectively decreases distance from 30 A to 12 A.
• highly selective for K+ over Na+
K+
Note: this is NOT
the voltage gated
K+ ion channel
found in
vertebrates, but is
very similar in
many respects!
http://www.youtube.com/watch?v=lnKIBZYarzM (K+ channel)
http://www.youtube.com/watch?v=XxadMJ9zqpA (aquoporin)
Selection occurs due to
ionic diameter of ions:
• Channel has 3 A diameter.
• K+ : 2.66 A
;
Na+ : 1.90 A
• “Price” of desolvating ions
made up by interaction with
channel.
• K+: 230 kJ/mol
• Na+: 301 kJ/mol
• K+ interacts with channel
better than Na+
• Movement of K+ thru channel
by electrostatic repulsion with
other K+ in cytosol
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Passive Protein Transporters cont’d
2. Gated eukaryotic ion channels: Closed  Open
• Background: Animal cells maintain steep gradient of Na+ and K+ ions across
their plasma membranes:
[Na+]OUT >>> [Na+]IN (143 mM >>> 14 mM)
[K+]IN >> [K+]OUT .
(157 mM >>> 4 mM)
• Membranes with this large ion concentration/charge gradient are in a state of
polarization – they have a difference in electrical potential across their
membrane (DV ≠ 0; typically – 60 mV outside to inside).
• Gated channels have 2 conformational states: open and closed
– Closed  Open transition regulated by some signal:
• ligand binding (“ligand-gated channel”)
or
• electrical potential change (“voltage-gated channel”)
• OPEN conformation permit ions to dissipate gradient, flowing DOWN
concentration gradient.
• Open states often spontaneously convert back to closed, a kind of built-in
"timer" that determines duration of ion flow.
Acetylcholine Receptor, a ligand-gated channel
Acetylcholine binding is the signal that opens the channel.
• Acetylcholine (ACh)
– neurotransmitter
– released from synaptic
vesicles into synaptic
cleft
– binds to ACh receptors
on postsynaptic
cell membrane.
• ACh binding to receptor
triggers an action potential
by opening ion channel
portion of receptor protein so
– Na+ ions rush into cell (down
their concentration gradient),
and
– K+ ions rush out of cell (down
their concentration gradient).
(Where ACh
receptors are
located)
http://www.blackwellpublishing.com/matthews/neurotrans.h
Berg et al., Fig. 13-26
tml
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Acetylcholine Receptor, a pentameric ligand-gated channel
A, structure of single
subunit of ACh receptor
Berg et al., Fig. 13.27
B, model of open form of
pentameric ACh receptor
looking down channel from
outside cell
Acetylcholine Receptor, proposed gating mechanism
•Structural basis for opening of channel due to acetylcholine binding:
– Helices lining channel (white in figure below) change conformation
when ACh binds, rotating along their long axis to change what kind
of residues are exposed to surface of channel.
• Closed conformation: large nonpolar residue(s) block channel
(Leu, Ile, Phe), so ions can't pass through.
• Open conformation (with ACh bound): small polar or neutral Aas
face channel (Ser, Thr, Gly), so Na+ and K+ ions can pass through.
A, closed form
B, open form
ACh receptor looking
down channel from
outside cell
Berg et al., Fig. 13.28
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Glucose Transporter GLUT1
2. Glucose transporter (GLUT1) in erythrocyte (red blood cell) membranes:
• NOT an ion channel
• Passive transport of glucose down its concentration gradient
• Other tissues have other isoforms of glucose transporters, homologous but
products of different genes, with different affinities for glucose and rates of
transport:
–e.g., Neurons have GLUT3, with lower Km, so brain gets first "dibs" on
glucose if scarce.
• Red blood cells depend on constant supply of glucose from blood as fuel
(plasma [glucose] ~ 5 mM).
• Erythrocytes use glucose for energy source via glycolysis.
• GLUT1 increases rate of glucose diffusion from blood across plasma
membrane into erythrocyte by a factor of 50,000.
Glucose Transporter GLUT1: proposed structure
• GLUT1 structure not determined, thought to be similar to E. coli lactose
permease.
• 12 transmembrane amphipathic  helices
• 2 “halves” surround binding pocket for glucose
• hydrophobic faces of helices on outer side face lipid core of membrane,
• hydrophilic sides line a polar pocket that's selective for glucose
Berg et al.,
Fig. 13-11
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GLUT1: Kinetics of glucose transport from outside to inside of cells
• Glucose binding is specific using hydrogen bonds to polar residues lining
proposed aqueous channel.
• (other hexoses bind much more weakly.)
• Binds D-glucose with high affinity and transport rate shows saturation
(hyperbolic kinetics as a function of external glucose concentration).
Vo for transport vs.
[extracellular glucose], [S]out
Nelson & Cox,
Lehninger Principles of
Biochemistry, 4th ed.,
Fig. 11-31
Kt analogous to "Km" for an enzyme-catalyzed reaction:
Kt = solute concentration that gives 1/2 the maximal velocity of transport
GLUT1: Proposed Transport Mechanism: 2 alternating conformations,
1 open to outside and the other open to inside of cell
• Binding pocket open on one side of membrane at a time.
• Conformational equilibrium: open or closed conformation analogous to
T R equilibria for Hb or ATCase
• Net direction of transport (binding/release) depends on relative
concentrations inside and out.
• GLUT1 increases rate of
transport toward
equilibrium.
• ΔGt = 0 when Cout =Cin)
• All known passive transport
proteins appear to be
asymmetrically situated
that alternate between 2
conformational states.
Fig. 10-13
Membrane Transport
Voet, Voet & Pratt,
Fundamentals of
Biochemistry, 3rd ed.
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Active Transport: DGt' > 0
•
•
transport of some solute(s) in a thermodynamically unfavorable direction
transport coupled to a favorable process by conformational changes
Berg et al.,
Fig. 13-2
• Source of "driving force" for active transport/conformational change:
potential energy (free energy "stored") in:
1. ATP coupled: “primary active transport”
• Hydrolysis of ATP provides free energy
• Changes in conformation linked to changes in ligand
binding/dissociation and covalent modification/demodification
or
2. Solute/ion gradient: “secondary active transport”
• Gradient can be "tapped" when needed, so "uphill" transport of one
solute is driven by cotransport with some other solute moving
"down" its concentration gradient.
• Ligand binding/dissociation leads to conformational changes
1. ATPase Primary Active Transport
• Control of intracellular ion concentrations very important physiologically, e.g.
– Ca2+ concentration regulated in cell and in intracellular organelles
(Ca2+ a signal for many cellular processes)
– Na+ and K+: primary active transport sets up ion gradients to drive
other (secondary) active transport processes
• Membrane ATPases: free energy of net ATP hydrolysis to pump ions
across membranes.
• Coupling mechanism of ion movement (against concentration gradient) to
ATP hydrolysis
• Ligand binding/dissociation induces conformational changes that lead to
covalent modification by phosphorylation/dephosphorylation
Examples of primary active transport processes:
• P-type ATPases: large family of homologous ATPases, including
– Sarcoplasmic Reticulum Ca2+ ATPase of muscle cell
– Na+-K+ ATPase (Na+K+ pump) of animal cell plasma membranes
– gastric H+-K+ ATPase (pumps protons into out of parietal cells into
stomach to generate pH < 1 in lumen of stomach)
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Sarcoplasmic Reticulum Ca2+ ATPase (SERCA ATPase, or SR ATPase)
Putting into physiological context:
• Muscle contraction initiated by nerve
impulse (electrochemical potential)
• Signal triggers release of Ca2+ ions from
the SR (~1 mM) through Ca2+ channels
• Ca2+ increases cytosolic concentration to
~10 µM.
• Ca2+ binds to troponin C, inducing other
changes leading to muscle contraction.
• Contraction ends:cytosolic Ca2+ is
removed by being pumped back into
sarcoplasmic reticulum by Ca 2+ ATPase
• Ca2+ ATPase transports 2 Ca2+/ ATP
• ~80-90% of total protein in the
sarcoplasmic reticulum is Ca2+
pump (Ca2+ ATPase)!
Garrett & Grisham, Biochemistry,
3rd ed., Fig. 16-23
•
P-type ATPases catalyze 2 coupled processes: "uphill" ion transport plus
ATP hydrolysis via covalent modification/de-modification of enzyme
– Phosphorylation/dephosphorylation → conformational changes
altering ligand (ion) binding affinities
– Same basic catalytic mechanism: transfer of terminal phosphoryl group
from ATP to Asp side chain on enzyme → phosphoaspartate (baspartyl phosphate, phosphoric-carboxylic anhydride linkage), a
covalent intermediate (covalent catalysis).
– Phosphorylation of Asp triggers conformational change required for ion
transport.
– Hydrolytic cleavage of phosphoryl group off Asp residue triggers another
conformational change, also required for ion transport.
There are numerous Na+K+ ATPase animations either on YouTube or can be
Googled using either the enzyme name or more generically “active transport”.
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Comparison of the H+, the Na+/K+, and Ca2+ ATPases (Pumps)
Notice all three have the same
overall cytoplasmic geometry!
From D.C. Gadsby (2007) Nature
450, 957-959 based on the work from
the Nissen laboratory.
SERCA ATPase Structure: ATPase membrane-spanning domain of 10 
helices, plus a 3-domain (A, P, N) cytosolic "headpiece"
• P domain: Asp-351 gets Phosphorylated by ATP
• N domain: Nucleotide (ATP) binding site
• A (Actuator) domain: links conformational changes in P & N domains to
transmembrane Ca2+ binding domain
• Transmembrane domain: binding site for 2 Ca2+ ions.
Berg et al., 5th
ed., Fig. 13-4
Membrane Transport
Berg et al., current
ed., Fig. 13-3
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SERCA ATPase: Proposed Mechanism
(Mechanism typical of P-type ATPases in general)
Enzyme interconverts between 2 different conformations, E1 and E2.
1) E1 conformation (not phosphorylated) binds 2 Ca2+ ions.
2) E1-(Ca2+)2 binds ATP on cytosolic side of membrane and cytosolic domains
rearrange, trapping the two bound Ca2+ ions in transmembrane domain.
3) Phosphoryl group transfer: Asp side chain carboxyl (nucleophile) of E1(Ca2+)2 attacks terminal phosphate of bound ATP, becoming
phosphorylated to E1-P-(Ca2+)2 .
4) ADP dissociates from E1-P, triggering conformational change to E2-P
("eversion") because in absence of bound ADP, the E1-E2 conformational
equilibrium favors E2 state. E2-P conformation has lower binding affinity for
Ca2+ ions (membrane domain's Ca2+-binding site disrupted).
In E2 conformation, cytosolic "entry" for Ca2+ binding is "closed" (no longer
accessible for Ca2+ dissociation).
E2 state has "escape route" open for Ca2+ dissociation on other side of
membrane into the SR lumen, so Ca2+ ions go into SR lumen.
Ion transport has been achieved.
ATP has been cleaved but no hydolysis has occurred yet.
5) When Ca2+ ions dissociate, phosphate is hydrolyzed off Asp residue to
release Pi (E2-P → E2).
6) E2 without covalently attached phosphate "everts" again, back to E1 state.
SERCA ATPase: Proposed Mechanism
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SERCA ATPase Structure: ATPase membrane-spanning domain of
10 -helices, plus a 3-domain (A, P, N) cytosolic "headpiece"
Berg et al., 5th
ed., Fig. 13-4
Berg et al., current
ed., Fig. 13-3
SERCA ATPase Structure: ATPase membrane-spanning domain of 10 
helices, plus a 3-domain (A, P, N) cytosolic "headpiece"
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Na+-K+ ATPase (Na+-K+ pump in animal cell plasma membranes)
•
•
•
•
•
•
•
Animal cells: high intracellular [K+] and low intracellular [Na+] (relative to
extracellular medium)
Opposite gradients of Na+ and K+ require free energy input to maintain.
Energy provided by ATP hydrolysis, catalyzed by a specific P-type ATPase:
plasma membrane Na+-K+ ATPase ("Na+-K+ pump")
Essentially same mechanism as Ca2+ ATPase:
E1 state binds 3 Na+ tightly and K+ .
E2 binds 2 K+ tightly and 3 Na+ weakly.
Reaction cycle:
– E1 binds 3 Na+ and then ATP inside cell, trapping Na+ in binding sites
– E1's active site Asp phosphorylated by ATP to give E1-P-(Na+)3
– ADP dissociates, triggering conformational switch: E1-P-(Na+)3 switches
conformations ("everts") to E2-P-(Na+)3
– E2-P releases its 3 Na+ on outside of cells, but then binds 2 K+ from
outside cell (extracellular medium).
– E2-P-(K+)2 is dephosphorylated (hydrolysis) to release Pi (E2-P-(K+)2 →
E2-(K+)2), and E2 reverts ("everts") to E1 state.
– E1-(K+)2 state dissociates the 2 K+ ions on inside of cell, and binds 3
more Na+ ions, ready to do another cycle.
Importance of Eukaryotic Plasma Membrane Na+-K+ ATPase
•
•
•
Na+ and K+ gradients (established by Na+-K+ ATPase) drive other cellular
processes
– control cell volume
– make neurons and muscle cells electrically excitable
– drive active transport of other solutes like some sugars and amino acids
So important that more than 1/3 of the ATP consumed by resting animal is
used to pump these ions!
Digitalis (drug, a mixture of cardiotonic steroids from foxglove plant's
leaves, including the compound ouabain)
– Ouabain (pronounced "wah'-bane"; from waa bayyo, Somali for "arrow
poison") a potent and specific inhibitor of Na+-K+ ATPase
– inhibits DEphosphorylation of E2-P form of plasma membrane Na+-K+
ATPase
– effective drug for treatment of congestive heart failure
– strengthens heart muscle contractions without increasing heart rate, and
thus increases efficiency of the heart
– (Berg et al. p. 357 explains why/how if you're interested)
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Active Transport, continued
2. Secondary Active Transport:
•
Ion gradients set up and maintained by ATP hydrolysis, e.g., Na+ and K+
gradients, require energy input (expenditure of ATP) to set up and
maintain.
•
The RESULTING concentration and charge gradients represent
potential energy.
•
That potential energy (electrochemical potential) can be "tapped" to
drive other, unfavorable transport processes.
•
Proteins couple "downhill" flow of one kind of ion (e.g., letting Na + ions flow
back into cell) with "uphill" transport of another ion or solute, using energy
"stored" in concentration/charge gradient (originally “paid for” by ATP) to
drive another otherwise unfavorable process.
• Cotransport in secondary active transport processes:
a) symport: cotransport process in which downhill flow of one species is
used to drive uphill flow of another solute in the same direction
across membrane; e.g., Na+-glucose symporter.
b) antiport: cotransport process in which downhill flow of one species is
used to drive uphill flow of another solute in the opposite direction
across membrane.
2. Secondary Active Transport
Na+-glucose symporter (in some animal cells): uses Na+ gradient
(generated by the Na+-K+ ATPase) to drive import of glucose into some
cells against a concentration gradient, permitting the cell to concentrate
glucose to much higher concentrations than the extracellular glucose
concentration.
Energy transduction by membrane proteins: Na+-glucose symporter
(secondary active transport) is driven by Na+ gradient that was
generated by Na+-K+ ATPase (primary active transport). (See figure
below.)
Berg et al., 5th Ed.
Fig. 13-12
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Learning Objectives
•
Terminology: membrane potential, passive transport, simple diffusion,
facilitated diffusion, ionophore, gated channel, P-type ATPase, active
transport (primary, and secondary), cotransport, symport, antiport,
uniport
What defines equilibrium for a transport process involving an uncharged
solute? (Express it in words, and in terms of concentrations at the
"origin" and at the "destination", C1 and C2, and also in terms of
ΔGtransport.)
Explain free energy changes in biological transport reactions in terms of
whether they are "favorable" (solute is moving in direction to go towards
equilibrium, ΔGtransport < 0) or "unfavorable" (would have to go in other
direction to go towards equilibrium, ΔGtransport > 0).
Given the equation for free energy of transport, be able to calculate
ΔGtransport for an uncharged solute (e.g., glucose), given the
concentrations on both sides of the membrane and the direction of
transport.
Explain the 2 terms involved in calculating ΔGtransport for a charged
solute, under what conditions the first term (concentration gradient term)
would be favorable, and under what conditions the 2nd term (electrical
gradient term) would be favorable.
•
•
•
•
Learning Objectives, continued
•
•
•
•
•
•
Explain whether ion channels mediate passive or active transport, and
use the acetylcholine receptor to explain how one ion channel can
control the flow of ions.
Explain how the bacterial K+ channel selects for K+ ions and against Na+
ions; how that selection is related to the energetic cost of stripping away
solvating H2O molecules AND how that cost is “paid”.
Describe the GLUT1 erythrocyte glucose transporter with respect to
–
its biological function
–
kinetics of transport ("saturation" behavior: plots of Vo vs. [glucose]
and 1/Vo vs. 1/[glucose])
–
proposed mode of action.
Name 2 P-type ATPases and briefly explain their biological
functions. Which one consumes over 30% of the ATP in animal cells in
order to establish/maintain the [Na+] and [K+] concentration gradients
and electrical potential across the plasma membrane? Which one
establishes/maintains the Ca2+ gradient involved in control of muscle
contraction?
Outline the proposed mechanism by which P-type ATPases couple ATP
hydrolysis with "uphill" transport of solutes, with the sarcoplasmic
reticulum Ca2+-ATPase as the example.
How the flow of Na+ ions down its concentration gradient (via the Na+glucose symporter) can assist the flow of glucose into a cell, and
ultimately what molecule provide the free energy for this process.
Membrane Transport
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