Lecture 14

Question 1

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

Answer 1

• • the membrane permeability to the molecule

- - the presence of appropriate transporter and an energy source

Question 2

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

Answer 2

True

Question 3

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

Answer 3

True

Question 4

For an uncharged molecule what equation determines spontaneity of diffusion?

Answer 4

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

Question 5

For charged molecules what equation determines spontaneity of diffusion?

Answer 5

deltaGtrans = RTln(c2/c1) + ZFdeltaV

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

Question 6

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)

Answer 6

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

Question 7

How are lipid bilayers are barriers to diffusion?

Answer 7

• • 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

Question 8

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

Answer 8

True

Question 9

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

Answer 9

True; this is important for membrane potential

Question 10

What are the 3 classes of proteins for transports across

Answer 10

– membrane carriers/membrane channels

– membrane pumps

– membrane cotransporters

Question 11

What is the function of membrane carriers/membrane channels?

Answer 11

• • 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

Question 12

What is the function of membrane pumps?

Answer 12

• • 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

Question 13

What is the function of Membrane cotransporters? aka cariers

Answer 13

– 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

Question 14

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

Answer 14

True

Question 15

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

Answer 15

True

Question 16

Describe an aquaporin as a membrane channel.

Answer 16

• • 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

Question 17

What are membrane ion channels?

Answer 17

• • 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

Question 18

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

Answer 18

True

Question 19

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

Answer 19

True

Question 20

Describe the structure of a K+ channel

Answer 20

– 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)

Question 20

Describe the structure of a K+ channel

Answer 20

– 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)

Question 21

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

Answer 21

True

Question 22

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

Answer 22

– 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+

Question 23

What is a gated-ion channel?

Answer 23

– 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

Question 24

Describe voltage gating in K+ channels.

Answer 24

– 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

Question 25

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

Answer 25

True; both phosphorylation and dephosphorylation

Question 26

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

Answer 26

True

Question 27

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

Answer 27

True

Question 28

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

Answer 28

• • Regulation of Ca2+: important for cell signaling

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

Question 29

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

Answer 29

• • 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

Question 30

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

Answer 30

True

Question 31

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

Answer 31

• • 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

Question 32

T or F, ATP binding induces strong interaction between ABC

Answer 32

True

Question 33

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

Answer 33

– 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

Question 34

What is an example of ABC in eukaryotes?

Answer 34

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

Question 35

What happens when there are mutations in the CTFR gene?

Answer 35

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

Question 36

What is the difference between an antipoter and symporter?

Answer 36

• • 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 )

Question 37

What is a uniporter?

Answer 37

– transport of single solutes following concentration gradients

Question 38

Describe cotransport systems.

Answer 38

• • 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;

Question 39

What is nerve-impulse transmission?

Answer 39

– 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

Question 40

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

Answer 40

True

Question 41

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

Answer 41

True

Question 42

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

Answer 42

True

Question 43

How does activation of voltage-gated channels work?

Answer 43

– 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

Question 44

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

Answer 44

– 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)

Question 45

What is an action potential and describe how it works.

Answer 45

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

Question 46

What is resting membrane potential?

Answer 46

– 60 mV

Question 47

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

Answer 47

– 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

Question 48

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

Answer 48

• • sudden rise in Na+ ion influx

- - 40 mV

Question 49

Describe action potential propagation in neurons.

Answer 49

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+