Membrane Transport

Front 1

An aqueous pore in a lipid membrane, with walls made of protein, thourgh which selected ions or molecules can pass

Back 1

channel

Front 2

the movement of small molecule or ion across a membrane due to a difference in concentration or electrical charge

Back 2

passive transport

Front 3

general term for membrane embedded protein that serves as a carrier of ions or small molecules from one side of the membrane to another

Back 3

membrane transport protein

Front 4

movement of a molecule across a membrane taht is driven by ATP hydrolysis or other form of metabolic energy

Back 4

active transport

Front 5

driving force for on movement that is due to differences in ion concentrations and electrical charge on either side of the memrbane

Back 5

electrochemical gradient

Front 6

T/F the plasma membrane is highly impermable to all charged molecules

Back 6

false

Front 7

transport by transporters can be either active or passive whereas transport by channels is always passive

Back 7

true

Front 8

Order the molecules n the following list according to their ability to diffuse through a lipid bilayer, beginning with the one that crosses the bilayer most readily

Back 8

CO2, ethanol, H2O, glucose, Ca2+, RNA. Smaller molecules diffuse better than larger ones. nonpolar > polar > charged

Front 9

Why are the maximum rates of transport by transporters and channels thought to be so differentSolu

Back 9

Solutes move more slowly vi transporters because they must bind to the solute and undergo a series of conformational changes to transfer the solute across the membrane. Transport through channels is much faster because they are ion-specific and dont need to bind or undergo any conformational changes

Front 10

a symporter would function as an antiporter if its orientation in the membrane were reversed T/F?

Back 10

false

Front 11

the co-transport of Na+ and a solute into a cell, which harnesses the energy in the Na+ gradient, is an example of primary active transport

Back 11

false

Front 12

in response to depolarization of the nucleus muscle cell plasma membrane, the Ca2+ pumps in the SR use the enry blah blah

Back 12

false

Front 13

what are the 3 main ways in which cells carry out active transport?

Back 13

1)Coupled transporters- link uphil transport of one solute across membrane with downhill transport of another
2)ATP-driven pumps- couple the uphill transport of a solute to hydrolysis of ATP
3) Light driven pumps- couple uphill transport to input of energy from light

Front 14

T/F transporters saturate at high concentrations of the transported molecule when all their binding sites are occupied, channels on the otherhand, do not bind the ions they transport and thus do not saturate

Back 14

false, both saturate

Front 15

the resting membrane potential of a typical animal cell arises predominantly through the action of Na+=K+ pump, which in each cycle transfers 3 na+ out and 2K+ in the cell

Back 15

false

Front 16

the membrane potential arises form movements of charge that leave ion concentrations practically unaffected, causing only a very slight discrepency in the number of positive and negative ions on the 2 sides of the membrane

Back 16

true

Front 17

upon stimulation of a nerve cell, 2 processes limit the entry of Na ions, the membrane potential reaches Na equilibrium and NA channeles are inactivated

Back 17

true

Front 18

the aggregate current crossing the membrane of an entire cell indicates to the degree to which individual channels are open

Back 18

false

Front 19

transmitter gated ion channels open in response to specific neurotransmitters in their environment but are insensitive to the membrane potential therefore they cannot by themseleves generate an action potential

Back 19

true

Front 20

when an action potential depolarizes the muscle cell membrane, the ca2+ pump is responsible for pumping ca2+ from the sarcoplasmic reticulum into the cytosol

Back 20

false

Front 21

In water, an ion moves at a constant velocity in an electric field. Why do you suppose that is?

Back 21

Just as a falling body in air reaches a terminal velocity due to friction, an ion
in water also reaches a terminal velocity due to friction with water mol-
ecules. An ion in water will accelerate for less than 10 nanoseconds before it
reaches terminal velocity.

Front 22

what 2 properties distinguis an ion channel from a simple aqeuous pore?

Back 22

ion channels are ion selective and they are gated, whereas simple aqueous
pores allow movement of many different ions and they are open all the time.

Front 23

name 3 ways in which an ion channel can be gated

Back 23

by a change in voltage across a membrane (volt-
age-gated channels), by a change in mechanical stress (mechanically gated
channels), and by the binding of a ligand (ligand-gated channels).

Front 24

Aquaporins allow water to move across a membrane but prevent the passage of ions, How does the structure of the pore thorugh which the water molecules move prevent passage of ions such as K, Na, Ca, and Cl? H ions present a different problem because they move by relay along a chain of hydrogen bonded water molecules. How does the pore prevent the relay of H ions across the membrane

Back 24

The narrow pore in aquaporins is lined with hydrophobic amino acids on
one side and a string of carbonyl oxygens on the other, which forms a path
that water molecules follow. The narrowness of the pore does not allow pas-
sage of hydrated ions of K+, Na+, Ca+, and Cl–, nor does the channel provide
enough polar groups to balance the charge on the ions.
H+ ions, which are present in cells as H3O+, offer a special challenge
because they normally ‘move’ through solution by relay along a chain of
hydrogen-bonded water molecules. If such a chain of water molecules
existed in the aquaporin pore, then H+ions would whiz through membranes
unobstructed. Aquaporins prevent this eventuality by positioning two
asparagines in the middle of the pore, thereby tying up both free valences of
the central water molecule in the string (Figure 11–22). Without a free
valence for hydrogen binding, the central water molecule cannot participate
in the relay of the H+ion.

Front 25

How is an action potential passed along an axon?

Back 25

When the resting membrane potential of an axon falls below threshold value, voltage gated Na channels open and allow influx of Na, which depolarizs the membrane further causing more distant voltage gated Na channels to open as well. The resulting wave of depolarization, the action potential, spreads along the axon quickly. Because Na channels inactivate soon after opening, the flow of K thorugh the voltage gated K channels and K leak channels quickly restores the original membrane potential after the action potential has passed

Front 26

Excitatory neurotransmitters open Na channels while inhibitory open Cl or K. Rationalize this onservation

Back 26

Opening of Na channels allows for the influx of Na ions which depolarize the membrane for threshold potential for firing an action potential. Opening Cl or K opposes membrane depolarization.

Front 27

Acetylcholine gated cation channels do not discriminate among Na, K, and Ca ions, allowing all to pass through freely. How is it then that whne acetylcholine receptors in muscle cells open there is a large net influx of mostly Na

Back 27

There is little movement of K because it is nearly at equilibrium distribution. However for Na and Ca the membrane does favor them to switch sides. The concentration gradient for Ca is much smaller than the concentration gradient of Na so there is mainly a large influx of Na ions. Thus Na has the greatest overall electrochemical gradient.

Front 28

ion-driven transporters mediate

Back 28

secondary active transport

Front 29

ATP-driven transporters mediate

Back 29

primary active transport

Front 30

3 types of transporter-mediated transport

Back 30

uniports, symports, and antiports are all used for passive and active

Front 31

3 types of ATP-dependent pumps

Back 31

P-type, F-type-differ structurally from P-type and found in plasma membrane of bacteria, ABC-primarily pump small molecules across cell membranes in contrast with the others

Front 32

P-type pumps

Back 32

plasma membrane of plant, fungi, and bacteria(H+ pump). Plasma membrane of higher eucaryotes(Na/K pump)
Apical plasma membrane of mammalian stomach(H/K pump)

Front 33

ABC superfamily transport protein basic structure

Back 33

4 subunites total. It has 2 subunits in the membrane which is wehre a solute will bind to, and 2 ATP-binding cassette subunits taht face the cytosol

Front 34

Describe how the structure of K+ channels results in selectivity for K+ versus the smaller Na+ ions

Back 34

The selectvity pore of the K+ channel has carbonyl oxygens on the side which will bond to K ion because it makes up for energy difference that came about when K ion was dehydrated of water. Even though Na ion is smaller it cannot bind to these carbonyl oxygens to the extent which the K can and so it is energetically unfavorable for Na to go through K channels

Front 35

Describe briefly the ion channels, the ions and the direction of flow of ions in the depolarization and repolarization stages of an action potential in a neuron

Back 35

Action potentials begin when slight depolarization at the dendritic end opens the voltage gated Na channels which allows an influx of Na ions. This results in further depolarization of the cell and openings of other Na channels. After a short time, these Na channels inactivate but before taht the voltage gated K channels open, allowing K ions to leave the cell which starts to repolarize the cel.. Once all the Na channels, close, the K channels close shortly after so part of the neuron is in a refractory period.

Front 36

resting membrane potential

Back 36

equilibrium condition in which there is no net flow of ions across the plasma membrane

Front 37

what equation describes resting membrane potential

Back 37

Nernst equation

Front 38

Which protein functions as a coat recruitment GTPase during formation of vesicles on ER membrane?

Back 38

SAR1

Front 39

Describe breifly how SAR1 recruits protein coat to the membrane of ER during vesicle formation?

Back 39

Sar1 is located in the cytosol in an inactive GDP-bound state. When a COPII vesicle is going to be formed on the ER membrane, a specific GEF binds to SAR1 which makes it exchange its GDP for GTP. This this GTP-bound state, SAR1 exposes its amphillic helix so it can now be inserted into the ER membrane where it can recruit protein coats to ER membrane for budding.

Front 40

2 functions of protein coats

Back 40

1) concentrate specific membrane proteins in a membrane patch that gives rise to the vesicle membrane which helps select appropriate molecules for transport
2)assemble of the protein coats into a curved basketlike structure deforms the membrane and molds the forming vesicle

Front 41

what are the 2 models

Back 41

vesicular transport model and cisternal maturation model

Front 42

vesicular transport model

Back 42

vesicles carry proteins across by budding one cisterna and fusing with the next. Vesicles are also required to maintain the identity of each cisterna by capturing resident proteins that have escaped and bringing them back to the appropriate cisterna. This is also how ER resident proteins that enter the golgo apparatus are returned to the ER

Front 43

Cisternal maturation model

Back 43

Vesicles are not needed to move proteins across the Golgi apparatus. The stacks movement actually accomplishes the forward movement of proteins. Vesicles are still required to maintain the identity of cisterna but they are not returning escaped ones. Instead the vesicles transfer proteins in a retrograde direction to a new location because their old location has changed identities, example from cis to medial cisterna.

Front 44

common in both vesicular transport model and cisternal maturation model

Back 44

vesicles are responsible for returning escaped ER proteins back to the ER

Front 45

difference in vesicular transport model and cisternal maturation model

Back 45

In vesicular, forward movement of proteins is created by vesicles and in cisternal, forward movement of proteins is by the cisternal themselves. The cisternal model is favored

Front 46

what would deleting the first signal sequence do to membrane spanning domain?

Back 46

Deleting the first sequence would transform the next membrane spanning domain into a start transfer signal and would invert the orientation of the protein

Front 47

what would changing the hydrophobic aas to charged aas do?

Back 47

Changing from hydrophobic aas to charged aas would destroy the ability for the sequence to act as a signal sequence and the ability for it to become a membrane spanning sequence. So the adjacent membrane spanning doman will become the start sequence and be inverted. The mutated signal sequence will not be cleaved off because it will remain on the cytosolic side and signal peptidases are only found inside ER

Front 48

Active transport of Na-glucose

Back 48

Active transporters utilize the electrochemical gradient of Na to pump glucose against their concentration gradient. Transporters will undergo a conformational change whether both solutes are bound or none are bound. The binding of Na and glucose is cooperative. Na concentration is much higher in extracellular space and it binds to a transporter. This leads to the cooperative binding of glucose, a change in conformation, and the transport of both into the cell.

Front 49

Passive transort of Na-glucose

Back 49

Moves down glucose concentration gradient. The protein undergoes random conformational changes. In one state the binding site is available on extracellular space and in the other state binding site available from cytosolic side. If there are more glucose molecules on one side of membrane, more glucose molecules bind in one state than the ohter.

Front 50

Describe K channels

Back 50

They are made up of 4 identical transmembrane subunits that form a pore for passage of ions. Each subunit contains 6 transmembrane segments, the 1st 4 form the voltage sensing domain and the last 2 form the pore domain. Negative amino acids are on the cytosolic side of pore and attract cations and repel anions to make the pore cation specific. The polypeptide region between S5 and S6 form a short alpha helix, the pore helix, and a loop that forms a selectivity filter. The selectivity loops from the 4 subunits form a narrow pore that is lined by carbonyl oxygens which K is able to interact with but Na is not.

Front 51

How is strain 1 defective?

Back 51

it is defective in the signal peptidase that resides in mitochondira because COX4 was found in mitochondria but was moving much more slowly than normal COX4, suggesting strain one did not cleave signal sequence. Strain 2 is defective in both TOM and TIM and cant import proteins into mitochondria.
Strain 3 is defective in SAM which is important for instertion of proteins into mitochondrial outer membrane. COX4 does not require SAM because it is mitochondrial .

Front 52

Immune cells do not seem to recognize sythnesized glycoproteins. Which of the following is a likely explanation?

Back 52

Oligasaccaraide needs to be further modified before its mature.

Front 53

Caltriculen

Back 53

will be soluble because KDEL sequence is at the end so the protein will be found in cytosol of ER. Since KDEL is a ER retention signal, HMG reductase would probably be a transmembrane protein because it has stretches of mainly hydrophobic amino acids.