Ch.12, Part 1 - Membrane Transport Flashcards
- Principles of Mem Xprt - Transporters and Active Mem Xprt
How does the rate of diffusion across the lipid bilayer change with the size and hydrophobicity of a molecule?
- Typ*, as size ↓ and hydrophobicity ↑ (solubility in oil, i.e. more nonpolar) → rate/ease of diffusion across lipid bilayer ↑.
- Given enough time, virtually any molecule will diffuse across a protein-free lipid bilayer down its c-grad.
Rank the following in order of ↑ rate of diffusion across lipid bilayer:
- Ions
- Small, uncharged polar molecules
- Hydrophobic molecules
- Large, uncharged polar molecules
Typ, as size ↓ and hydrophobicity ↑ (solubility in oil, i.e. more nonpolar) → rate/ease of diffusion across lipid bilayer ↑.
Slowest diffusion - Ions; larger, uncharged polar; small, uncharged polar; hphobic molecues - Fastest diffusion.
T/F: charged molecules (ions) are basically impermeable to the lipid bilayer, no matter how small.
True
Charged molecules (ions) are basically impermeable to the lipid bilayer, no matter how small.
The two main classes of membrane transport proteins are _______ and ______.
The two main classes of membrane transport proteins are transporters and channels.
All known mem xprt proteins are multipass. How does this affect their mechanism of transport?
All known mem xprt proteins are multipass, i.e. polyp chains traverse bilayer mult times → enable specific hphilic solutes to cross mem w/o coming into direct contact w hphobic interior.
- Mem xprt proteins typ have high specifity.
Which major class of transport proteins are also known as carriers or permeases and undergo a series of conformational changes in moving solutes across the mem?
Transporters (aka carriers/permeases) - bind specific solute → undergo series of conform changes → alternately exposes solute-binding sites on one side of mem and then on other → transfers solute.
Channels form much ______ (stronger/weaker) interactions w the specific solutes they transport, and, as a result, transport is much _____ (slower/faster).
Channels form much weaker interactions w the specific solutes they transport, and, as a result, transport is much faster.
- form continuous pores across bilayer → when open, allow specific solutes to pass thru.
For individual uncharged molecules, its ______ drives passive transport.
For individual uncharged molecules, its concentration gradient drives passive transport.
For individual solutes w a net charge, what forces drive passive transport?
Solute w net charge → both c-grad AND e-pot diff across mem (mem pot) influence xprt; i.e. echem grad.
Almost all pmems have an e-pot (voltage) across them, w inside typ ________ (positive/negative) wrt outside → favors entry of ___ (pos/neg) charged ions & opposes entry of ____ (pos/neg) charged ions; also opposes efflux of ____ (pos/neg) charged ions.
Almost all pmems have an e-pot (voltage) across them, w inside typ negative wrt outside → favors entry of pos charged ions & opposes entry of neg charged ions; also opposes efflux of pos charged ions.
Active transport is movement against the echem grad (“uphill”) and is directly coupled to a source of metabolic energy. Name two possible sources.
Active transport - movement against echem grad (“uphill”); directly coupled to source of metabolic energy, e.g. an ion grad or ATP hydrolysis.
T/F: Xmem movement of small molecules mediated by xprtrs can be either active or passive, whereas passage via channels is always passive.
True
Xmem movement of small molecules mediated by xprtrs can be either active or passive, whereas passage via channels is always passive.
In regard to transporter proteins, what does Vmax measure?
Max rate of xprt (Vmax) when xprtr is saturated (solute-binding sites occupied).
- Vmax measures rate at wh carrier (xprtr protein) can flip b/w its conform states.
- Recall: ea xprtr has a characteristic affinity for its solute → reflected in Km of rxn → equals concen of solute when xprt rate is half its max value.
Ea type of xprtr protein has 1+ specific binding sites for its solute → xfrs solute by undergoing ________ (reversible/irreversible) conform changes.
Ea type of xprtr protein has 1+ specific binding sites for its solute → xfrs solute by undergoing reversible conform changes.
- Conform changes alternately expose the solute-binding site first on one side of mem, then on the other—but never on both sides at same time.
- Transition occurs thru an intermediate state in wh the solute is occluded fr either side of mem.
- Vmax - measures rate wh xprtr flips b/w conform states.
What are the three main modes of active xprt?
Three main modes of active xprt: coupled xprtrs, ATP-driven pumps, and light- or redox-driven pumps.
- Coupled xprtrs - harness energy stored in c-grads; couples uphill xprt of one solute to downhill xprt of another.
- ATP-driven pumps - couples uphill xprt to hydrolysis of ATP.
- Light- or redox-driven pumps - common in proks/mito/chloro; couple uphill xprt to input of energy fr light (e.g. b.rhodopsin) or fr a redox rxn (e.g. cytochrome c oxidase).
T/F: Active and passive xprt proteins share similar AA seq/3D struc.
True
Active and passive xprt proteins share similar AA seq/3D struc.
- E.g. bac xprtrs that use H+ grad to drive active uptake of various sugars are structurally similar to passive xprtrs that mediate glucose xprt into most animal cells → suggests evolutionary relationship b/w various xprtrs.
Compare uniporters w coupled transporters.
- Uniporters - passive xprt of a single solute fr one side of mem to other at a rate det by their Vmax and Km.
-
Coupled xprt - xprt of one solute strictly deps on xprt of a second; one method of active xprt.
- Symporters (co-transporters) - simult xprt of second solute in same direction.
- Antiporters (exchangers) - simult xprt of second solute in opp direction, e.g. Na/K pumps.
What is the key diff b/w primary and secondary active xprt?
-
Primary/direct active xprt - uses energy directly to drive xprt of solute against its c-grad.
- Energy typ in form of ATP, but light (photon) or redox energy also used.
- E.g. ATP-driven pumps (ATPases, like Na/K pumps).
-
Secondary active/coupled xprt - uses energy indirectly to establish an echem grad, wh is then used to drive xprt of solute against its cgrad.
- Ion co-xprtd (i.e. downhill; typ Na+ in humans) → establishes ion echem grad → provides large driving force for secondary active xprt of second molecule (typ small molecule, e.g. glucose).
- Na+ enters cell during coupled xprt → pumped out by an Na/K pump/ATPase → maintains Na+ grad → indirectly drives coupled xprt.
- Antiporter - Na/H exchanger.
- Symporter - Na/glucose co-xprtr.
Why is Na+ the typical co-xprtd ion in secondary active xprt?
Secondary active xprt - energy indirectly drives xprt of solute against its c-grad, e.g. ATP-driven Na/K pumps.
- Na+ is the typ co-xprted ion bc its echem grad provides large driving force for active xprt of second molecule.
- Na+ enters cell during coupled xprt → pumped out by an ATP-driven Na/K pump → maintains Na+ grad → indirectly drives coupled xprt.
Intestinal/kidney epithelial cells contain a variety of symporters driven by Na+ echem grad. Briefly describe the common glucose xprt mechanism.
Glucose xprt mechanism:
- Binding of Na+/glucose is cooperative: binding either solute ↑ xprtr protein’s affinity for other solute.
- EC [Na+] >> IC [Na+] → glucose more likely to bind xprtr in outward (EC) facing state.
- Transition to occluded (intermed) state occurs only when both Na/glucose bound → stabilizes occluded state → transition to occluded state is energ fav.
-
Stochastic fluctuations (via thermal energy) drive xprtr randomly into inward-open or outward-open.
- If xprtr opens outward → nothing happens; process repeats.
- If xprtr opens inward → Na+ dissociates quickly into low-concen environ → xprtr affinity for glucose ↓ → glucose dissociates.
Intestinal/kidney epithelial cells contain a variety of symporters driven by Na+ echem grad. In the common glucose xprt mechanism, what is meant by “cooperative binding” of Na+ and glucose?
Glucose xprt mechanism:
- Binding of Na+/glucose is cooperative: binding either solute ↑ xprtr protein’s affinity for other solute.
- EC [Na+] >> IC [Na+] → glucose more likely to bind xprtr in outward (EC) facing state.
Intestinal/kidney epithelial cells contain a variety of symporters driven by Na+ echem grad. From its occluded (intermed) state—stablized by binding of BOTH Na+ and glucose—what causes the transporter protein to open inward/outward?
Glucose xprt mechanism:
-
Stochastic fluctuations (via thermal energy) drive xprtr randomly into inward-open or outward-open.
- If xprtr opens outward → nothing happens; process repeats.
- If xprtr opens inward → Na+ dissociates quickly into low-concen environ → xprtr affinity for glucose ↓ → glucose dissociates.
Intestinal/kidney epithelial cells contain a variety of symporters driven by Na+ echem grad. In the common glucose xprt mechanism, how does does the Na+ concentration gradient affect the direction of glucose transport?
Glucose xprt mechanism:
- Binding of Na+/glucose is cooperative: binding either solute ↑ xprtr protein’s affinity for other solute.
- EC [Na+] >> IC [Na+] → glucose more likely to bind xprtr in outward (EC) facing state.
An asymmetric distribution of transporters in epithelial cells underlies the transcellular transport of solutes. Explain how this asymmetry enables such transport.
- Solutes are moved across epithelial cell layer → into ECF → into blood.
- Na+-linked symporters located in apical (absorptive) domain of pmem actively xprt nutrients into cell → build up large c-grads for xprtd solutes.
- Uniporters in basal/lateral (basolateral) domains allow nutrients to leave cell passively down c-grads.
Describe the transcellular transport mechanism for glucose b/w the intestinal (gut) lumen into the intestinal epithelium (lining) and into ECF/blood.
Transcellular Transport Mechanism:
-
Na+-powered glucose symporters in apical domain imports glucose (and Na+) fr gut lumen into lining.
- I.e. the passive xprt of Na+ (down its cgrad) pulls glucose along w it (against its cgrad); same direction of xprt but no ATP req’d, therefore “coupled xprt” via a symporter.
-
Glucose uniporters in basal and lateral mem domains export glucose fr gut lining into ECF (down cgrad).
- Recall: uniporters in euks typ export solutes.
-
Na/K pumps in basal/lat mem domains keep IC [Na+] low → powers Na+ grad that drives symports.
- Recall: Na/K ATPases (P-type) pump 3 Na+ out (and 2 K+ in).
- Tight junctions connect adj epithelial (lining) cells → prevents solutes fr crossing b/w cells → allows cgrad of glucose to be maintained across cell sheet.
In the transcellular transport of glucose, what powers the Na+-driven symporters that drive glucose fr the gut lumen into the epithelial lining?
Transcellular Transport Mechanism:
- Na+-powered glucose symporters in apical domain xprts glucose fr gut lumen into lining.
- Glucose uniporters in basal and lateral mem domains xprts glucose fr gut lining into ECF (down cgrad).
- Na/K pumps in basal/lat mem domains keep IC [Na+] low → power Na+ grad that drives symports.
- Tight junctions connect adj epithelial (lining) cells → prevents solutes fr crossing b/w cells → allows cgrad of glucose to be maintained across cell sheet.
In the transcellular transport of glucose, what role do tight junctions serve?
Transcellular Transport Mechanism:
- Na+-powered glucose symporters in apical domain xprts glucose fr gut lumen into lining.
- Glucose uniporters in basal and lateral mem domains xprts glucose fr gut lining into ECF (down cgrad).
- Na/K pumps in basal/lat mem domains keep IC [Na+] low → power Na+ grad that drives symports.
- Tight junctions connect adj epithelial (lining) cells → prevents solutes fr crossing b/w cells → allows cgrad of glucose to be maintained across cell sheet.
ATP-driven pumps (xprt ATPases) hydrolyze ATP to ___ and ___ and use energy released to pump ________ across mem in ______ (in/outward/either) direction.
ATP-driven pumps (xprt ATPases) hydrolyze ATP to ADP and Pi and use energy released to pump ions/other solutes across mem in either direction.
- When ATP/ADP ratio ↑ → hydrolyze ATP; ATP/ADP ratio ↓ → synth ATP.
F-type ATPases (ATP synthases) are struc similar to V-type pump ATPases. What differentiates F-type ATPases fr the three principal classes of ATPases?
F-type ATPases (also, ATP synthases) - struc similar to V-type pump ATPases; typ work “in reverse”: use H+ grad to drive synth of ATP fr ADP + Pi.
- Found in pmem of bac, inner mem of mito, and thylakoid mem of chloro.
- H+ grad is generated either during electron-xprt steps of oxid phos (aerobic bac/mito), during psynth (chloro), or by light-driven H+ pump (b.rhodopsin) in Halobac.
Cells must maintain a steep Ca2+ grad across pmem in order to transmit signals rapidly. Wh type(s) of transporter proteins are involved, and in wh direction is the gradient?
Euks maintain v low IC [Ca2+] (~10-7 M) comp to v high EC [Ca2+] (~10–3 M) → small influx of Ca2+ causes signif ↑ relative IC [Ca2+] → used to transmit signals.
- Ca2+ xprtrs actively pump Ca2+ out of cell → maintain steep grad.
- Two main types: P-type Ca2+ ATPases, and an antiporter (Na+–Ca2+ exchanger) driven by Na+ echem grad.
Ca2+ pumps (Ca2+ ATPases) are a _____ xprt ATPase; abundant in ______ of skeletal muscle cells.
Ca2+ pumps (Ca2+ ATPases) are a P-type xprt ATPase; abundant in SR mem of skeletal muscle cells.
- Recall: P-type ATPases phosphorylate themselves during pumping cycle;
All P-type xprt ATPases contain 10 xmem α helices mechanically coupled to 3 cytosolic domains, denoted N, P, and A. What do these letters represent?
All P-type xprt ATPases contain 10 xmem α helices mechanically coupled to 3 cytosolic domains: ntide-binding(N),phosphorylation(P), andactivator (A) domains
In the pumping cycle of the SR Ca2+ pump, the AP ________ (depols/hyperpols) the muscle cell pmem → Ca2+ released into ______ (cytosol/EC space) fr SR thru Ca2+-release channels → stims muscle to _______ (contract/relax) → Ca2+ pump moves Ca2+ fr ______ (cytosol/EC space) back into the SR.
In the pumping cycle of the SR Ca2+ pump, the AP depols the muscle cell pmem → Ca2+ released into cytosol fr SR thru Ca2+-release channels → stims muscle to contract → Ca2+ pump moves Ca2+ fr cytosol back into the SR.
In the pumping cycle of the SR Ca2+ pump (P-type ATPase), the AP depols the muscle cell pmem → Ca2+ released into cytosol fr SR thru Ca2+-release channels → stims muscle to contract → Ca2+ pump moves Ca2+ fr cytosol back into the SR.
Describe the mechanism of the Ca2+ pump (last step above).
SR Ca2+ pump mechanism:
- In ATP-bound non-phosphorylated state, binding sites accessible only fr cytosolic side of SR mem.
- Ca2+ binding triggers a series of conform changes → close passageway to cytosol → activate phosphotransfer rxn: terminal P of ATP xfrd to an Asp.
- ADP dissociates (Pi remains bound) → replaced w fresh ATP → conform change → opens passageway to SR lumen → two Ca2+ ions exit.
- Ca2+ ions are replaced by two H+ ions + H2O → stabilize empty Ca2+-binding sites → close passageway to SR lumen.
- Hydrolysis of the labile phosphoryl-aspartate bond (i.e. the Pi that remained when ADP dissociated) returns pump to initial conform → cycle repeats.
- Transient self-phosphorylation of pump is an essential characteristic of all P-type pumps.
Summary: Helix movement opens/closes passageways thru wh Ca2+ enters fr cytosol → binds to two centrally located Ca2+ binding sites → two Ca2+ exit into SR lumen → replaced by two H+, wh are xprtd in opp direction. The Ca2+-dep phosphorylation and H+-dep dephosphorylation of aspartic acid are universally conserved steps in all P-type pumps.
The Na/K pump (Na+- K+ ATPase) is a __-type ATPase; operates as an ATP-driven _________ (uni/sym/antiporter): actively pumps ___ (1/2/3) Na+ ___ (in/out) and ___ (1/2/3) K+ ___ (in/out) for ea ATP hydrolyzed.
The Na/K pump (Na+- K+ ATPase) is a P-type ATPase; operates as an ATP-driven anitporter: actively pumps 3 Na+ out and 2 K+ in for ea ATP hydrolyzed.
Na/K pumps are similar to all other P-type ATPases (e.g. Ca2+ pumps) in that wh AA is phosphorylated and dephosphorylated during the pumping cycle?
All P-type ATPases phos/dephosphorylate aspartate during the pumping cycle.
- Aspartate is a salt/ester of aspartic acid (Asp, D)
T/F: ATP-binding cassettes (ABC xprtrs) constitute the largest family of mem xprt proteins.
True
ATP-binding cassettes (ABC xprtrs) constitute the largest family of mem xprt proteins.
ATP-Binding Cassettes (ABC xprtrs) - contain two highly conserved ATPase domains (“Cassettes”) on the ________ (cytosolic/non) side of mem.
ATP-Binding Cassettes (ABC xprtrs ) - contain two highly conserved ATPase domains (“Cassettes”) on the cytosolic side of mem.
T/F: the two ATPase domains (“cassettes”) of ABC xprtrs are always formed fr diff polypeptide chains.
False
The two ATPase domains (“cassettes”) of ABC xprtrs can be part of same/diff polyp chains.
wrt ABC xprtrs:
ATP ______ (binding/hydrolysis) → ATPase domains come t/g
ATP ______ (binding/hydrolysis) → dissoc
wrt ABC xprtrs:
ATP binding → ATPase domains come t/g
ATP hydrolysis → dissoc
Describe the functional mechanism of ABC xprtrs.
ABC xprtrs:
- ATP binding → ATPase domains come t/g; ATP hydrolysis → dissoc.
- Movements of cytosolic domains are transmitted to xmem segments → drive cycles of conform changes → alternately expose solute-binding sites on either side of mem (like other xprtrs).
- ABC xprtrs harvest energy released upon ATP binding/hydrolysis → drive xprt of solutes across mem.
- Direction of xprt deps on partic conform change in binding site linked to ATP hydrolysis
- Euks: most ABC xprtrs export substances; either fr cytosol to EC, cytosol to IC organelle (e.g. ER), or mito matrix to cytosol.
In small-molecule xprt by ABC xprtrs, the unbound (ATP or solute) xprtr exposes a substrate-binding site on one side of the mem (typ exports; so opens toward cytosolic side first). ATP binding then induces a conform change wh exposes the substrate binding site to the opp side (typ noncytosolic). How do ABC xprtrs return to their original conform fr this point?
Small-molecule xprt by typ ABC xprtrs:
- w/o ATP bound, xprtr exposes a substrate-binding site on one side of mem
- ATP binding induces conform change → exposes substrate-binding site on opp side.
- ATP hydrolysis followed by ADP dissoc returns xprtr to original conform.
T/F: bacterial ABC xprtrs can be bidirectional, while most euk ABC xprtrs export substances.
True
- Bac: contain bidirectional ABC xprtrs.
- Euks: most ABC xprtrs export substances; either fr cytosol to EC, cytosol to IC organelle (e.g. ER), or mito matrix to cytosol.
