Week 8 Textbook

Question 1

what are membrane transport proteins

Answer 1

transmembrane proteins - path for the movement of select substances across a cell membrane
- import/export nutrients, sugars, AAs, remove toxins

Question 2

what direction does simple diffusion occur

Answer 2

from high conc to low con

Question 3

what is facilitated transport

Answer 3

membranes that have specialized membrane transport proteins

Question 4

smaller and hydrophobic/non polar molecules diffuse more rapidly across the bilayer

Answer 4

true

Question 5

explain which kinds of molecules can diffuse through the membrane easily

Answer 5

small, nonpolar = dissolve readily in lipid bilayers and diffuse rapidly

uncharged polar molecules = uneven distribution of charge - will only diffuse if small enough (water, ethanol)

larger uncharged polar molecules = glucose, hardly cross

ions = impermeable, no matter how small, their strong electrical attraction to water molecules inhibit the entry

Question 6

how do the transmembrane proteins allow hydrophilic molecules to pass thru the hydrophobic part of the bilayer

Answer 6

the transmembrane segments cluster together and allow the hydrophilic AA side chains to face the inside and interact with the hydrophilic molecules following in

Question 7

what are the 2 main classes of membrane transport proteins

Answer 7

1. transporters
   - transfers only ions that fit into specific binding sites on the protein - need specificity (specific binding site that gives transporters their selectivity)
2. channels
   - discriminate mainly on size and electric charge, when the channel is open, ions of good size and charge can pass thru

Question 8

T/F channels are slower than transporters since they need to select which molecules are charged and of size

Answer 8

false
the transports are slower
- they undergo conformational charges as they open and close on either side and have a specific binding site

Question 9

where can you usually find lots of Na+ and K+

Answer 9

Na+ outside the cell
K+ inside the cell
the charges inside and outside must be balanced
the high conc of Na+ outside the cell is balanced by Cl- charges outside the cell
the high conc of K+ inside the cell is balanced by lots of anions or other nucleic acids

Question 10

what is a membrane potential

Answer 10

when theres an electrical imbalance of charges across the membrane = potential

Question 11

what occurs when the the cell is in resting membrane potential

Answer 11

the voltage difference across the cell membrane
the charges are balanced - holding steady
animal cell resting potential = -20 to -200mV

Question 12

why is the resting potential represented in negative mV

Answer 12

bc the interior of the cell is more negatively charged than the exterior

Question 13

what is passive transport

Answer 13

when the substances flow downhill from high to low concentration
there is no other driving force
passive transport across through channel and transport membrane proteins
moves along its concentration gradient

Question 14

what is active transport

Answer 14

moving a solute against its conc gradient
driving from low to high
- needs ATP
- has transmembrane proteins (pumps) that use the hydrolysis of ATP

Question 15

what is the electrochemical gradient

Answer 15

the net driving force which determines the direction in which a charged solute will flow across the membrane via PASSIVE transport (channel, transporter)
one force = conc gradient
another force = membrane potential
Na+ (high outside the cell) tends to go into the cell
K+ movement is small, even when the K+ is open bc the electrochemical gradient across the plasma membrane of resting cells are small

Question 16

which type of membrane transport proteins can perform both active and passive transport?

Answer 16

transporters

Question 17

T/F there are many transport proteins for each type of selectivity

Answer 17

true
each cell membrane contains a characteristic set of different transporters appropriate to that particular membrane
- transporters that import nutrients, sugar, AAs, nucleotides, lysosomes, H+

Question 18

T/F glucose is negatively charged

Answer 18

false
neutral uncharged molecule

Question 19

what passive transport movement works with glucose

Answer 19

glucose transporter, reversible but highly selective to D-glucose
- has lots of conformations, one of the conformations exposed binding sites for glucose on the exterior of the cell
glucose = neutral - electrochemical gradient is zero
after eating a meal, lots of glucose, binds to external site on transporter, conformation changes, glucose is carried inside the cell where the conc of glucose is low, conformation switches again and is able to transport another glucose

Question 20

what are the three types of transmembrane pumps

Answer 20

gradient driven pumps
- moves one solute across a membrane depending on the concentration (high to low)
ATP-driven pumps
- uses energy released from the hydrolysis of ATP to drive uphill transport (low to high)
light-driven pumps
- found mainly in bacterial cells which uses energy derived from sunlight to drive uphill transport

Question 21

explain how the influx of Na+ through gradient driven pumps is linked with active transport pumps

Answer 21

the influx of Na+ going towards the concentration graident
provides the energy for active transport of many other substances into the cell against their gradient
Na+ pump has a central role in the active transport

Question 22

Na+-K+ Pump Process

Answer 22

transporter in most animal cells that actively pumps Na+ out of the cell and K+ in the cell
using energy derived from ATP hydrolysis
- if any step is prevented from occurring the entire cycle stops
- ouabain inhibits the Na+ pump by preventing the binding of extracellular K+
since this pump is used for coupling reactions, it only operates when it needs to to waste hydrolysis of ATP
Process:
3 Na+ binds from inside the cell to be transported out
phosphorylated and the conformation changes which allows Na+ to be ejected
K+ binds to the open outside conformation, this triggers the dephosphorylation of the pump, which returns it to its original conformation - K+ is ejected

Question 23

T/F Na+ pump provides the cell with a steep concentration gradient and = energy

Answer 23

true
Na+ outside the cell = large volume of energy

Question 24

explain the movement of Ca2+ and the Ca2+ pump

Answer 24

less available than Na+ Ca2+ like Na+ is kept at low concentration in the cytosol compared to its concentration in the extracellular fluid Ca2+ binds tightly to a variety of proteins in the cell has a large concentration difference using the ATP driven Ca2+ pumps

Question 25

what is the main difference in the Ca2+ pumps

Answer 25

they return to their original conformation without a requirement for binding and transporting a second ion Na+ and Ca+ pumps have similar AA sequences and structures which show that they have common evolutionary origin

Question 26

Explain the significance of gradient-driven pumps

Answer 26

A gradient of any solute across a membrane, like the electrochemical Na+ gradient generated by the Na+ pump, can be used to drive the active transport of a second molecule. The downhill movement of the first solute down its gradient provides the energy to power the uphill transport of the second solute

Question 27

explain the symport/uniports/antiports gradient pumps

Answer 27

symport the pump moves a pair of solutes in the same direction across the membrane uniport gradient driven transporters that only move a single solute across the membrane at a time the passive diffusion of a solute down its concentration gradient - they arent pumps (passive glucose transporters) antiport moves 2 solutes from either sides of the membrane to the opposite sides

Question 28

explain the glucose-Na+ symport pump in terms of epithelial cells

Answer 28

symports of Na+ are impact driving force for transporting glucose in the gut they take up glucose from the gut lumen even when the conc of glucose is higher in the cells than in the gut bc the electrochemical gradient for Na+ is so steep when Na+ moves into the cell down its gradient, glucose binds to Na+ and are both driven - if one solute is missing, the other will not bind - needs both solutes to bind into the protein for transport to occur - the importance of this is ot make sure that the coupling Na+ doesn't leak into the cell without doing useful work

Question 29

what is the occluded occupied state of the glucose-Na+ symport

Answer 29

when the protein is closed from both sides with BOTH solutes inside it can also be in this state when BOTH solutes are not in the spots (empty)

Question 30

what would happen if the epithelial cells only had glucose-Na+ symport

Answer 30

they would always take up glucose and never release it for the other cells to use - the epithelial cells have 2 symports on either ends of the cell in the basal and lateral domains they have glucose uniports which passively release glucose down its conc gradient for other tissues to use

Question 31

in an epithelial cell, how are the two glucose transported segregated?

Answer 31

tight junctions around the apex of the cell = barrier preventing mixing of membrane components

Question 32

what is an example of an antiport used in the plasma membrane of many animal cells

Answer 32

Na+--H+ exchanger (antiport) uses the downhill influx of Na+ to pump H+ out of the cell - used to control pH in the cytosol to prevent the cell interior to become too acidic

Question 33

what do plants, fungi and bacteria use if they dont have Na+ gradients

Answer 33

rely on H+ to import solutes into the cell H+ pumps (creates the acid pH in the medium) the import of other solutes are mediated by the H+ symports (electrochemical gradient in the same way Na+ is used in animals) - uses energu of ATP hydrolysis to pump H+ out of the cell - in lysosomes/vacuoles of some plant, animal. fungal cells they have different H+ pumps in the membrane - which actively transport H+ out of the cytosol into the organelle

Question 34

what are bacteriorhodosin pumps

Answer 34

located in the plasma membrane of some bacteria active export of h+ light as energy source

Question 35

explain channel proteins

Answer 35

transmembrane pores that allow the passive movement of small water soluble molecules porins - that form pores in the outer membrane of mitochondria and some bacteria

Question 36

what are ion channels

Answer 36

- they are not always open, needed specific stimulus to trigger open - doesn't need to undergo conformational changes for an ion to pass thru when open - they show ion selectivity, allowing ion selectivity - depends on the size and shape of the ion channel and the distribution of the charged AA that line it - since every ion is surrounded by water molecules, the channels have selectivity filter which allows the narrowest part of the ion channel to force ions into contact with the interior of the channel wall - removing the water surrounding - since it select for the ion, it then moves down and then re binds with lots of water molecules on the other side of the membrane

Question 37

Explain K+ leak channels

Answer 37

allow K+ to move freely across the membrane when K+ moves out of the cell its down its conc gradient the loss of positive inside the cell creates a voltage difference (membrane potential) no further movement of K+, in equilibrium conditions K+ is kept inside the cell which is strong enough to counteract the tendency of K+ to move down its concentration gradient L+ = zero

Question 38

____ where the flow of positive and negative ions are precisely balanced so no further differences in charge accumulate across the membranr

Answer 38

resting membrane potential

Question 39

what is the nernst equation

Answer 39

expresses the equilibrium quantitatively and makes it possible to calculate the theoretical resting membrane potential Voltage = 62 Log (Co/Ci)

Question 40

what are the two components that drive an ion across a membrane

Answer 40

1. due to the electrial membrane potential 2. due to the concentration of the ion gradient

Question 41

what is a mechanically gated channel

Answer 41

an ion channel that allows the passage of select ions across a membrane in response to a physical perturbation - opened by a physical force

Question 42

what is a ligand-gated channel

Answer 42

the opening of the protein is based on the binding of a molecule which is either intracellular or extracellular ligand

Question 43

what is a voltage gated channel

Answer 43

it is the opening of the membrane protein which is controlled by the membrane potential

Question 44

how can a cell restrict movement of its membrane proteins

Answer 44

cells can create barriers that restrict particular membrane components to one membrane domain (functionally and structurally specialized region of the membrane which is in the presence of specific proteins)

Question 45

explain tight junctions

Answer 45

junctions in certain areas of the cell that allow junctional proteins to form a continuous belt around the cell where the cell contacts its neighbors - this creates a seal between adjacent plasma membranes - membrane proteins are unable to diffuse past the junction - thus restricting them to particular domains