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BISC 312 > Lecture 14 > Flashcards

Lecture 14 Flashcards

1
Q

What does the ability of a molecule to cross a membrane depend on?

A
    • the membrane permeability to the molecule

- - the presence of appropriate transporter and an energy source

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2
Q

True or False, for uncharge membrane permeable molecules, chemical concentration gradient determines spontaneous movement across membranes

A

True

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3
Q

True or False, for charged molecules, one needs to consider an additional electrical potential term

A

True

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4
Q

For an uncharged molecule what equation determines spontaneity of diffusion?

A

deltaGtrans = RTln(c2/c1) R: gas constant, T: temperature

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5
Q

For charged molecules what equation determines spontaneity of diffusion?

A

deltaGtrans = RTln(c2/c1) + ZFdeltaV

Z: charge of the molecule
F: Faraday constant
deltaV: charge gradient (potential for all ions)

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6
Q

True or False; deltaGtrans indicates if the transport is passive (deltaGtrans <0) or if it requires energy input (e.g. ATP) and is thus active (deltaGtrans > 0)

A

True; basically if deltaGtrans is negative it is passive; and if it is positive or greater than 0 it is active

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7
Q

How are lipid bilayers are barriers to diffusion?

A
    • low permeability to ions (need desolvation to go through the membrane directly)
    • low permeability to polar molecules (except H2O, small and uncharged; however this is slow)
    • High selective permeability barriers to ions, using proteins pumps and channels
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8
Q

True or False, for lipid bilayers, ions need to have H2O molecules removed b4 movement occurs aka desolnation

A

True

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9
Q

T or F, the high selective permeability barriers to ions that lipid bilayers have contributes to the maintenance of different ion conc. on each side of the membrane

A

True; this is important for membrane potential

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10
Q

What are the 3 classes of proteins for transports across

A

– membrane carriers/membrane channels

– membrane pumps

– membrane cotransporters

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11
Q

What is the function of membrane carriers/membrane channels?

A
    • for passive transport or facilitated diffusion
    • provide a selective pore through which ion can flow rapidly
    • when deltaGtrans < 0 (negative)
    • NO USE OF ATP
    • can be sensitive to membrane polarization
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12
Q

What is the function of membrane pumps?

A
    • for primary active transport
    • ATP often source of energy for this process
    • when deltaGtrans > 0 (positive)
    • drive thermodynamic uphill reactions with ATP as free energy source
    • different pumps use different strategy for transport and use of ATP

– pumps have ATPase activity (hydrolyze ATP) to transfer phosphate group to their own residues

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13
Q

What is the function of Membrane cotransporters? aka cariers

A

– for secondary active transport when deltaGtrans > 0 (positive)

– coupling a thermodynamically unfavorable reaction (e.g. transport against conc. gradient) with a favorable one (e.g. transport with conc. gradient)

– No use of ATP –> use concentration gradient instead

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14
Q

T or F, no energy is required for membrane channels but charged molecules are allowed to move across the lipid bilayer

A

True

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15
Q

T or F, membrane channels permit movement more readily across lipid bilayers and molecules will move from high concentration to low

A

True

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16
Q

Describe an aquaporin as a membrane channel.

A
    • they are channels that specialize in H2O transport
    • 6 alpha helices forming hydrophilic channel –> span membrane
    • 10^6 H2O molecule/sec
    • important for rapid H2O transport (e.g. reabsorption of water in kidney, tears) –> also when cells are trying to get flooded with water
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17
Q

What are membrane ion channels?

A
    • fastest transporters: 1000 fold faster than pumps
    • close to diffusion rates of ions
    • different ion channels use similar transport mechanisms yet are highly selective for specific ions
    • channels can work because of ion concentration gradients generated by membrane pumps in live cells
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18
Q

True or False. Na+ and K+ ion channels are particularly important for signal communication in the nervous systems

A

True

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19
Q

True or False; sequential triggering of Na+ and K+ ion channels generates action potential (nerve impulse)

A

True

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20
Q

Describe the structure of a K+ channel

A

– tetramer (4 identical subunits) forming pore through lipid membranes

– peptidic backbone of 5 aa: TVGYG from 2 subunits provides polar interactions with K+ as a replacement for H2O

– polar interaction compensate the energy cost of desolvation for K+ but not for other ions (e.g. Na+)

– size of the pore and position of TVGYG aa optimal for the K+ ion only (tight binding)

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20
Q

Describe the structure of a K+ channel

A

– tetramer (4 identical subunits) forming pore through lipid membranes

– peptidic backbone of 5 aa: TVGYG from 2 subunits provides polar interactions with K+ as a replacement for H2O

    • polar interaction compensate the energy cost of desolvation for K+ but not for other ions (e.g. Na+)
      - —-> Ex: sodium can’t come in; too small on it’s own too big w/ water

– size of the pore and position of TVGYG aa optimal for the K+ ion only (tight binding)

21
Q

T or F, electrostatic repulsions between K+ ion in channel allows for rapid flow

A

True

22
Q

How does the K+ ion interact with K+ channels?

A

– desolvated K+ ion (water must be removed) interacts with a specific aa sequence (binds tightly to carbonyl) along the channel that provides high selectivity for K+

23
Q

What is a gated-ion channel?

A

– passive transport system

– gated pores that respond to highly-specific signals

– most important are those for Na+, Ca2+, and K+

– passage of ions goes down concentration gradient thus they are much faster (1000x) than transport by Na+/K+ ATPase pump

i.e. Nerve impulse signals

24
Q

Describe voltage gating in K+ channels.

A

– voltage gating is provided by a positively charged domain at the bottom of the channel structure (usually filled w/ positively charged amino acids)

– S4++ domain form voltage sensing paddles that can occlude (close) the channel

– close channel: S4++ domain “down position”

– on membrane depolarization inside become +++, electrostatic repulsion of S4++ paddles upward: channel opens

———————–> this is because repulsion occurs by charge inside cell because they are already positive

25
Q

True or False, free energy of ATP induces large conformational changes in pumps

A

True; both phosphorylation and dephosphorylation

26
Q

True or False; the interconversion between 2 conformations (open/close) provides unidirectional pumping of ions

A

True

27
Q

True or False, pumps are important to maintain steady state concentration of ions in cells

A

True

28
Q

What is the importance of the regulation of Calcium concentration and Na+/K+ concentration by membrane channels?

A
    • Regulation of Ca2+: important for cell signaling

- - Regulation of Na+/K+: important for membrane potential

29
Q

Describe the active membrane transport of Na/K+ of ATPase.

A
    • requires work, usually provided by hydrolysis of ATP
    • necessary to move substances across concentration gradient
#1.) Protein (4 subunits) is open to interior of cell and binds Na+ ATP phosphorylates protein
#2.) conformational change. Open to outside, doesn't bind Na+, but does bind K+. P is hydrolyzed, giving Pi
#3.) Goes back to original form, no longer binds K+, which enters the cell. More Na+ can now bind

– pumps Na+ out despite low concentration inside cell; pumping K+ in despite high concentration inside cell

30
Q

T or F, Na+/K+ ATPase pumps against the gradient and there for each cycle uses one ATP that is able to move 3 Na+ out and 2 K+ in

A

True

31
Q

Describe the structure of ATP-Binding Cassette (ABC) transporter pumps

A
    • also uses ATP to move things against their gradient
    • 1 transmembrane domain and 2 ABC
    • 1 molecule transported for 2 ATP hydrolyzed

– binding of specific substrate induces steric fit that enhances affinity for ATP

32
Q

T or F, ATP binding induces strong interaction between ABC

A

True

33
Q

Describe the function of ATP-Binding Cassette proteins (ABC)

A

– multi-domain transport proteins

– have 2 cytosolic ATP-binding sites (cassette)

– small molecules, like drugs, can be transported across the membrane

– drug efflux pumps involved in multidrug resistance of cancer cells

34
Q

What is an example of ABC in eukaryotes?

A

– multi-drug resistance protein (MDR) ejects pharmacological agents, such as those used in cancer treatments

35
Q

What happens when there are mutations in the CTFR gene?

A

CTFR = Cystic Fibrosis Transmembrane Regulator

– mutations in CFTR gene (usually in NBD 1) cause Cystic Fibrosis

– the channel normally transports Cl- across cell membrane

– non-functional, Cl- secretion is decreased, leaders to formation of viscous mucus

36
Q

What is the difference between an antipoter and symporter?

A
    • Antiporter = transport of 2 solutes in opposite directions (1 moving in and 1 moving out)
  • -Symporter = transport of 2 soluetes in same direction (both moving in or both out )
37
Q

What is a uniporter?

A

– transport of single solutes following concentration gradients

38
Q

Describe cotransport systems.

A
    • uses ion gradient that forms from a uniport system
    • the flow of ions down a gradient can be used to perform work

– high concentration of Na+ from outside the cell can flow back inside and bring glucose along with it when BOTH are present in lumen of intestine (concentration is high in lumen so it’s unfavorable); it can transport glucose against concentration gradient by using potential energy of Na+ gradient

Note = bringing in glucose is unfavorable;

39
Q

What is nerve-impulse transmission?

A

– neurons have specialized projections, called dendrites and axons that act as the wires of the system

– signals must be conducted over long distances very quickly

– it accomplished this by waves in the membrane electrical potential on the membrane surface

– basic idea is that we want to move a signal from where it’s received and transmit it along a pathway

40
Q

T or F, ion gradient established across membranes can be used by cells that have gated ion channels in their membranes

A

True

41
Q

T or F, Na+/K+, Na+ only, K+ only, and Ca2+ are used in nerve conduction

A

True

42
Q

T or F, signal is transmitted by a neurotransmitter across the synapse

A

True

43
Q

How does activation of voltage-gated channels work?

A

– influx of Na+ results in a change in voltage and activates voltage-gated channels along the axon

– and finally activate voltage-gated Ca2+ channel to release neurotransmitter at presynaptic terminal

44
Q

What is an example of a ligand gated ion channel? Describe how it works.

A

– acetylcholine receptors

– responsible for electrochemical signal transduction at synapses

– expressed at postsynaptic membrane. Respond to acetylcholine released by massive vesicle fusion at the presynaptic membrane

– binding of acetylcholine triggers opening of this non-selective channel (let both Na+ and K+ ions through)

45
Q

What is an action potential and describe how it works.

A

– generated in neurons by membrane depolarization involving Na+ and K+ ion channels

46
Q

What is resting membrane potential?

A

– 60 mV

47
Q

What are the two types of gates? and describe them,

A

– voltage gating: a channel will open given a specific change in membrane potential

– ligand gating: a channel will open upon binding of a specific ligand

– for both types of gates, stimulus (voltage or ligand) induces a change in channel conformation that leads to opening

48
Q

What leads to the opening of voltage-gated Na+ channels?

A
    • sudden rise in Na+ ion influx

- - 40 mV

49
Q

Describe action potential propagation in neurons.

A

1- Steady state membrane potential (-60 mV)
achieved and maintained by Na+ and K+ ion pumps
(with ATP)

2- Firing of neuron leads to acetylcholine release
in synaptic cleft

3- Binding to acetylcholine receptors and opening
of ligand gated channels

4- Entry of Na+ and exit of K+ ions (chemical
gradient) towards ~ -20 mV (Veq)

5- At ~ -40 mV voltage gated Na+ channels open
and depolarization accelerates with high Na+ influx

6- After a few ms Na+ channels inactivate, and K+
channel open. Action potential reach its max.

7- With efflux of K+ action potential decrease
rapidly, until K+ inactivate.

8- Slight overshoot in K+ (resting potential below
-60mV) is slowly corrected by active pumping of
Na+ and K+

BISC 312 (31 decks)

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