Ch.12, Part 1 - Membrane Transport Flashcards Preview

Cell Biology (BIO 301) > Ch.12, Part 1 - Membrane Transport > Flashcards

Flashcards in Ch.12, Part 1 - Membrane Transport Deck (58)
Loading flashcards...
1

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.

2

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.

 

3

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.

4

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

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

5

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.

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?

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.

7

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.

8

For individual uncharged molecules, its ______ drives passive transport.

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

9

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.

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.

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.

11

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.

12

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.

13

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.

14

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.

15

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).

16

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.

17

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.

18

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.

19

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.

20

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.

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?

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.

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?

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.

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?

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.

24

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

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.

25

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

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.

26

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

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.

27

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

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

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?

  • 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.

29

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

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.

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.

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.