Ch.12, Part 1 - Membrane Transport

Question 1

How does the rate of diffusion across the lipid bilayer change with the size and hydrophobicity of a molecule?

Answer 1

• 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.

Question 2

Rank the following in order of ↑ rate of diffusion across lipid bilayer:

• Ions
• Small, uncharged polar molecules
• Hydrophobic molecules
• Large, uncharged polar molecules

Answer 2

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.

Question 3

T/F: charged molecules (ions) are basically impermeable to the lipid bilayer, no matter how small.

Answer 3

True

Charged molecules (ions) are basically impermeable to the lipid bilayer, no matter how small.

Question 4

The two main classes of membrane transport proteins are _______ and ______.

Answer 4

The two main classes of membrane transport proteins are transporters and channels.

Question 5

All known mem xprt proteins are multipass. How does this affect their mechanism of transport?

Answer 5

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.

Question 6

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?

Answer 6

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.

Question 7

Channels form much ______ (stronger/weaker) interactions w the specific solutes they transport, and, as a result, transport is much _____ (slower/faster).

Answer 7

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.

Question 8

For individual uncharged molecules, its ______ drives passive transport.

Answer 8

For individual uncharged molecules, its concentration gradient drives passive transport.

Question 9

For individual solutes w a net charge, what forces drive passive transport?

Answer 9

Solute w net charge → both c-grad AND e-pot diff across mem (mem pot) influence xprt; i.e. echem grad.

Question 10

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.

Answer 10

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.

Question 11

Active transport is movement against the echem grad (“uphill”) and is directly coupled to a source of metabolic energy. Name two possible sources.

Answer 11

Active transport - movement against echem grad (“uphill”); directly coupled to source of metabolic energy, e.g. an ion grad or ATP hydrolysis.

Question 12

T/F: Xmem movement of small molecules mediated by xprtrs can be either active or passive, whereas passage via channels is always passive.

Answer 12

True

Xmem movement of small molecules mediated by xprtrs can be either active or passive, whereas passage via channels is always passive.

Question 13

In regard to transporter proteins, what does V_max measure?

Answer 13

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.

Question 14

Ea type of xprtr protein has 1+ specific binding sites for its solute → xfrs solute by undergoing ________ (reversible/irreversible) conform changes.

Answer 14

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.

Question 15

What are the three main modes of active xprt?

Answer 15

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).

Question 16

T/F: Active and passive xprt proteins share similar AA seq/3D struc.

Answer 16

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.

Question 17

Compare uniporters w coupled transporters.

Answer 17

• 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.

Question 18

What is the key diff b/w primary and secondary active xprt?

Answer 18

• 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.

Question 19

Why is Na+ the typical co-xprtd ion in secondary active xprt?

Answer 19

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.

Question 20

Intestinal/kidney epithelial cells contain a variety of symporters driven by Na+ echem grad. Briefly describe the common glucose xprt mechanism.

Answer 20

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.

Question 21

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?

Answer 21

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.

Question 22

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?

Answer 22

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.

Question 23

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?

Answer 23

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.

Question 24

Describe the typ structure of transporter proteins, i.e. #/type of mem-spanning protein and location of solute/ion binding sites.

Answer 24

Structure of transporter proteins (typ):

• Typ built fr bundles of 10+ α helices spanning mem.
• Solute/ion-binding sites located midway thru mem: some helices are broken/distorted → AA side chains and polyp backbone atoms form binding sites.
  
  • In/outward-open conforms → binding sites accessible by passageways fr only one side of mem.
  • Occluded conform → both passageways closed → prevents ion/solute fr crossing mem alone.
  • Recall: cooperative binding → tight coupling b/w ion/solute xprt assured.

Question 25

In a transporter protein's occluded conformation, binding sites are closed to ea side of the mem. Why is this imp?

Answer 25

Occluded conform → both passageways closed → **prevents ion/solute fr crossing mem alone**. * In-/outward-open conforms → binding sites accessible by passageways fr only one side of mem.

Question 26

T/F: In bac/yeasts/plants/many mem-enclosed organelles of animals, most ion-driven active xprtrs dep on Na+ grads.

Answer 26

False In bac/yeasts/plants/many mem-enclosed organelles of animals, most ion-driven active xprtrs dep on H+ rather than Na+ grads → reflects predominance of H+ pumps in mems.

Question 27

H+ leaks into cell or is produced in acid-forming rxns w/i the cell → ↓ cytosolic pH. How do cells maintain their pH?

Answer 27

Most cells have 1+ types of Na+-driven antiporters in pmem that help maintain cytosolic pH at ~7.2. * H+ leaks into cell or produced in acid-forming rxns → **xprtrs use energy stored in Na+ grad to pump out excess H+**. * Either H+ is directly xprtd out or HCO3- (bicarbonate) is xprtd in to neutralize H+. * Recall: HCO3– + H+→ H2O + CO2

Question 28

Na+/H+ exchangers and Na+-driven Cl-HCO3- exchangers are two types of Na+-driven antiporters that help maintain cytosolic pH. Why is the latter considered more imp?

Answer 28

* **Na⁺–H⁺ exchanger** - antiporter; couples influx of Na+ to efflux of H+. * **Na⁺-driven Cl^-—HCO3^–** **exchanger** - couples influx of Na⁺ and HCO3^– to efflux of Cl^– and H⁺. * Results in influx of NaHCO3 and efflux of HCl. * Pumps out one H+ and neutralizes another for ea Na+ that enters cell → **twice as effective as Na/H exchanger**. * If HCO3– is available (normal) → **most imp xprtr in regulating cytosolic pH**.

Question 29

Na+-indep Cl–HCO3– exchangers adjust cytsolic pH __ (↑/↓).

Answer 29

**Na+-indep Cl–HCO3– exchangers adjust cytsolic pH** ↓. * Activity ↑ as cytosol becomes too *alkaline*. * **Operates in reverse** of Na+-driven Cl-HCO3 exchangers. * HCO3– passively moves down echem grade out of cell (coupled to uphill import of Cl-) → ↓ cytosolic pH. * I.e. less H+ neutralized → pH ↓. * E.g. RBCs: "band 3 protein" (Na+-indep Cl-HCO3- exchanger) facilitates quick discharge of CO2 (as HCO3–) as cells pass thru capillaries in lung.

Question 30

In addition to Na+-indep Cl–HCO3– exchangers, \_\_\_-driven H+ pumps are also used to control pH of many IC compartments, e.g. lysosomes, endosomes, secretory vesicles.

Answer 30

In addition to Na+-indep Cl–HCO3– exchangers, **ATP**-driven H+ pumps are also used to control pH of many IC compartments, e.g. lysosomes, endosomes, secretory vesicles. * Use energy of ATP hydrolysis to **pump H+ into organelles fr cytosol**.

Question 31

An asymmetric distribution of transporters in epithelial cells underlies the transcellular transport of solutes. Explain how this asymmetry enables such transport.

Answer 31

* 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.

Question 32

Describe the transcellular transport mechanism for glucose b/w the intestinal (gut) lumen into the intestinal epithelium (lining) and into ECF/blood.

Answer 32

**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.

Question 33

In the transcellular transport of glucose, what powers the Na+-driven symporters that drive glucose fr the gut lumen into the epithelial lining?

Answer 33

**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.

Question 34

In the transcellular transport of glucose, what role do tight junctions serve?

Answer 34

**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**.

Question 35

What are the three principal classes of ATP-driven pumps (transport ATPases)?

Answer 35

**Three principal classes of ATP-driven pumps (transport ATPases)**: P-type pumps, ABC xprtrs, and V-type pumps. * **P-type pumps** - related to multipass xmem proteins; “P-type” bc phosphorylate themselves during pumping cycle. * Incl many ion pumps wh set up/maintain grads of Na+, K+, H+, and Ca2+. * **ABC xprtrs** (**ATP-Binding Cassette xprtrs**) - struc diff fr P-type ATPases; primarily pump small molecules across cell mems. * **V-type pumps** - turbine-like; mult diff subunits; V-type proton pump xfrs H+ *into* organelles, e.g. lysosomes, synaptic vesicles, and plant/yeast vacuoles (hence, V = vacuolar), to acidify interior.

Question 36

ATP-driven pumps (xprt ATPases) hydrolyze ATP to ___ and ___ and use energy released to pump ________ across mem in ______ (in/outward/either) direction.

Answer 36

**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.

Question 37

**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?

Answer 37

**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.

Question 38

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?

Answer 38

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.

Question 39

Ca2+ pumps (Ca2+ ATPases) are a **\_\_\_\_\_** xprt ATPase; abundant in **\_\_\_\_\_\_** of skeletal muscle cells.

Answer 39

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;

Question 40

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?

Answer 40

All P-type xprt ATPases contain 10 xmem α helices mechanically coupled to 3 cytosolic domains: **ntide****-binding**(N),**phosphorylation**(P), and**activator** (A) domains

Question 41

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.

Answer 41

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.

Question 42

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).

Answer 42

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.

Question 43

IC [K+] is typ 10–30x __ (\>/\<) EC IC [Na+] is typ 10-30x __ (\>/\<) EC.

Answer 43

IC [K+] is typ 10–30x **\>** EC IC [Na+] is typ 10-30x **\<** EC. Both maintained by Na/K Pump (Na/K ATPase; P-type)

Question 44

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.

Answer 44

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.

Question 45

Na/K pumps (Na/K ATPases) are said to be electrogenic. What does this mean?

Answer 45

**Electrogenic** - drives a net electric current across mem → creates an elec pot (voltage), typ neg (inside relative to outside). * Note, h/e, electrogenic effect of Na/K pump typ only contribs ~10% of mem pot; remainer only deps on Na/K pump indirectly.

Question 46

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?

Answer 46

All P-type ATPases phos/dephosphorylate **aspartate** during the pumping cycle. * Aspartate is a salt/ester of aspartic acid (Asp, D)

Question 47

T/F: ATP-binding cassettes (ABC xprtrs) constitute the largest family of mem xprt proteins.

Answer 47

True ATP-binding cassettes (ABC xprtrs) constitute the largest family of mem xprt proteins.

Question 48

ATP-Binding Cassettes (ABC xprtrs) - contain two highly conserved ATPase domains ("Cassettes") on the ________ (cytosolic/non) side of mem.

Answer 48

ATP-Binding Cassettes (ABC xprtrs ) - contain two highly conserved ATPase domains ("Cassettes") on the **cytosolic** side of mem.

Question 49

T/F: the two ATPase domains ("cassettes") of ABC xprtrs are always formed fr diff polypeptide chains.

Answer 49

False The two ATPase domains ("cassettes") of ABC xprtrs can be part of same/diff polyp chains.

Question 50

wrt ABC xprtrs: ## Footnote ATP ______ (binding/hydrolysis) → ATPase domains come t/g ATP ______ (binding/hydrolysis) → dissoc

Answer 50

wrt ABC xprtrs: ## Footnote ATP **binding** → ATPase domains come t/g ATP **hydrolysis** → dissoc

Question 51

Describe the functional mechanism of ABC xprtrs.

Answer 51

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.

Question 52

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?

Answer 52

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**.

Question 53

T/F: bacterial ABC xprtrs can be bidirectional, while most euk ABC xprtrs export substances.

Answer 53

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.

Question 54

Multidrug resistance (MDR) protein (P-glycoprotein), wh pump ______ (hphobic/philic/amphi) drugs out of cytosol, was one of the first euk ABC xprtrs discovered.

Answer 54

**Multidrug resistance** (**MDR**) **protein** (**P-glycoprotein**), wh pump **hphobic** drugs out of cytosol, was one of the first euk ABC xprtrs discovered.

Question 55

Peptides are produced by protein degradation in proteasomes → pumped fr cytosol into ER via ABC xprtrs → carried fr ER to cell surface → displayed for scrutiny by cytotoxic T lymphocytes → kill cell if peptides are derived fr virus/other microorg lurking in cytosol of an infected cell. What specific type of ABC xprtr actively pumps a wide variety of peptides fr the cytosol into ER?

Answer 55

**Transporter assoc w antigen processing** (**TAP** **xprtr**) - ABC xprtr in ER mem of most vertebrate cells; actively pumps wide variety of peptides fr cytosol into ER lumen.

Question 56

T/F: Channels have specific binding pockets for the solute molecules they allow to pass.

Answer 56

False Channels do not have binding pockets for the solute that passes through them. **Selectivity of a channel is achieved by the *size* of the internal pore and by *charged regions at the entrance* of the pore that attract or repel ions of the appropriate charge**.

Question 57

List the following compounds in order of ↓ lipid bilayer permeability: RNA, ca2+, glucose, ethanol, N2, water.

Answer 57

The permeabilities are N₂ (small and nonpolar) \> ethanol (small and slightly polar) \> water (small and polar) \> glucose (large and polar) \> Ca2+ (small and charged) \> RNA (v large and charged).

Question 58

Amino acids are taken up by animal cells using a symport in the plasma membrane. What is the most likely ion whose electrochemical gradient drives the import? is ATp consumed in the process? if so, how?

Answer 58

Animal cells drive most transport processes across the plasma membrane with the electrochemical gradient of Na+. ATP is needed to fuel the Na+ pump to maintain the Na+ gradient.