2.2: Membrane Proteins and Transport across Membranes

Question 1

what are the two classes of membrane transport proteins

Answer 1

channel proteins, transporters (uniporters, symporters and antiporters, atp driven pumps)

Question 2

is the artificial bilayer impermeable or permeable to most water soluble molecules

Answer 2

impermeable

Question 3

can water soluble molecules conduct simple diffusion across the bilayer quickly

Answer 3

no, it takes a long time which is too slow for most functions

Question 4

what is the term of transport/regulation of membrane transport proteins transferring specific molecules

Answer 4

facilitated transport

Question 5

what type of molecules are permeable across the lipid bilayer (simple diffusion)

Answer 5

small nonpolar molecules (hydrophobic – o2, co2, n2, steroid, hormones), small uncharged polar molecules (h2o, ethanol, glycerol)

Question 6

are large uncharged polar molecules (aa, glucose, nucleosides) and ions permeable or impermeable across the lipid bilayer

Answer 6

impermeable

Question 7

are transport proteins selective

Answer 7

yes

Question 8

what determines the selectivity of the channels and transporters

Answer 8

channels: size and electric charge of solute
transporters: solute fits into the binding site

Question 9

organize the following into passive or active transport and determine whether it follows concentration gradient:
simple diffusion
channel mediated
transporter mediated

Answer 9

simple channel and channel mediated can be passive
transporter can be active

passive follows gradient and active doesn’t

Question 10

⭐️what composes the electrochemical gradient aka the voltage gradient

NET DRIVING FORCE

Answer 10

concentration gradient + membrane potential (electrical gradient)

side note: Eg glucose - uncharged molecules so it can also be determined by the concentration gradient

Molecules must have a charge in order to have an electrochemical gradient

Question 10

describe the additive nature of the electrochemical gradient and what is the opposite

Answer 10

the additive nature is when there is strong net driving force bc the transported molecules are moving to the side complementary to their charge

the opposite is when they work against each other and they are trying to transport molecules to the side with the like charge

Question 10

which side of the cell negatively and which is positively charged

Answer 10

inside is -, outside is +

Question 11

channel proteins are a ____________ pore across the membrane (hydrophilic or hydrophobic)

Answer 11

hydrophilic

Question 11

describe channel proteins in terms of selectivity, speed, and interactions with the channel

Answer 11

• they are selective, eg ion channels based on ion size and electric charge
• selectivity happens at the center, they have to fit (eg k ions have to shed h2o to fit)
• transient interactions with the channel wall
• faster transport through channels

Question 11

what type of ion channel is k+ leak channels and its significance to membrane potential

Answer 11

non gated ion channels

[c] of k+ in cytosol is always relatively higher so it’s always leaking out = sets up membrane potential

Question 11

describe non gated ion channels

Answer 11

• always open
• k+ leak channels: k+ moves out of cell, found in plasma membrane of animal cell,

Question 12

what type of ion channels are the following:
mechanically gated
ligand gated (intracellular and extracellular)
voltage gated

Answer 12

gated ion channels

Question 13

for voltage gated ion channels, can they flip back and forth

Answer 13

yes. can also randomly flip back and forth in the open or closed conformation so its really about what the steady state it’s in when there is or is not a signal

Question 13

what is the signal for the following:
mechanically gated
ligand gated (intracellular and extracellular)
voltage gated

Answer 13

mechanically gated - mechanical stress (plasma membrane stretched to open channel)

ligand gated extracellular - ligand eg nt (outside is the gate)

ligand gated intracellular
ligand eg ions, nucleotide (inside is the gate)

voltage gated - change in voltage across the membrane (membrane depolarization)

Question 14

describe the differences in rate of transported mediated diffusion and simple diffusion and channel mediated transport

Answer 14

simple diffusion and channel mediated transport is linear while transport mediated is similar to log as approaches max

Question 15

associate the following examples to what form of membrane transport they are
- k+ leak channels
- glut uniporter
- Na+ glucose symporter
- Na+ H+ exchanger
- Na+ K+, H+ Pump

Answer 15

passive transport by channel proteins: k+ leak channels

passive transport by transporter proteins: glut uniporter

active transport by gradient driven pumps (transporter proteins): Na+ glucose symporter

active transport by gradient driven pumps (transporter proteins): Na+ H+ exchanger (antiport)

active transport by ATP driven pumps (transporter proteins) Na+ K+, H+ Pump

Question 16

describe the uniport process of transport

Answer 16

one solute, passive transport down its electrochemical gradient with reversible direction of transport

Question 17

outline GLUT uniporter

Answer 17

transports glucose down the concentration gradient, can work in either direction of in or out of the cell

Question 18

is active transport with or against the electrochemical gradient

Answer 18

against

Question 19

for the gradient driven pumps, the first solute is ____ its gradient (for free energy) and the second is ______ its gradient

Answer 19

down, against

Question 20

differ between symport and antiport

Answer 20

symports are two solutes moved in the same direction and antiports are two solutes moved in opposite directions (but the [c] of both in and out are the same)

Question 21

outline the NaH exchanger (antiport) example

Answer 21

• cytosolic pH must be regulated for optimal enzyme function
• excess H+ in the cytosol from acid forming reactions and leaks from lysosome
• transporters maintain cytosolic pH through the NaH exchanger (plasma membrane) with a sensing domain to sense pH and has H+ transported out against its electrochemical gradient
• Na+ is transported down its electrochemical gradient and provides energy

Question 22

outline the Na Glucose symporter example

Answer 22

• Na+ is transported down its electrochemical gradient and provides energy
• glucose moves against concentration gradient
• random oscillations between conformations, reversible
• conformational changes only occur after: both sites occupied with cooperative binding of Na and Glucose, both sites empty where the molecules both dissociated
• sodium will randomly bind but transporter won’t close until glucose

Question 23

how is the Na+ electrochemical gradient maintained in animal cells

Answer 23

through the NaK pump (plasma membrane atp driven pump)

Question 24

list the 3 types of atp driven pumps

Answer 24

1. proton type pump
2. abc transporter
3. v type proton pump

Question 25

state two examples of how transport proteins regulate critical cellular processes

Answer 25

• transcellular transport of glucose
• generation of membrane potentials

Question 26

what is the name of the basal + lateral domain

Answer 26

basolateral domain

Question 27

describe the process of how transporters work together to transfer glucose from the intestine to the blood stream

Answer 27

• glucose actively taken up from the gut from the gut lumen through the apical domain
• active transport of the na-glucose symporter
• passive transport of glucose down the [c] gradient whilst there is a NaK pump to set up the gradient – where glucose is passively reelased for use by other tissues

Question 28

in the parts of the intestinal lining do the following have low or high glucose concentrations:
- inside the lumen of the intestine
- cytosol of the epithelial cell
- extracellular fluid on the basolateral side of the epithelial cell

Answer 28

low
- inside the lumen of the intestine
- extracellular fluid on the basolateral side of the epithelial cell

high
- cytosol of the epithelial cell

Question 29

which transport process is in the apical membrane and which are in the basolateral plasma membrane

Answer 29

apical: Na glucose symporter
basolateral: glut2 uniporter, nak pump

Question 30

in the epithelial cells of the intestine, transport proteins are restricted by _________ _________ to seal it off so nothing can get by

Answer 30

tight junctions

Question 31

define membrane potential

Answer 31

difference in electrical charge on two sides of the membrane (about hte ions)

Question 32

why is the membrane potential important

Answer 32

used by gradient driven pumps to carry out active transport and electrical signalling (membrane depolarization)

Question 33

list the two methods of generating membrane potentials in animal cells

Answer 33

K leak channels and naK pumps

Question 34

is the flow of k leak channels in or out and what type of transport along what gradient

Answer 34

outward flow of K+ through passive transport down the electrochemical gradient

Question 35

describe the NaK pump in membrane potential

Answer 35

• ~10% of membrane potential
• maintains the na+ gradient w low cytosolic [na+] and k+ gradient w high cytosolic [k+]
• electrogenic: 3 na out 2 K in, so net 1+ ion pumped out

Question 36

what does it mean for membrane potential to be electrogenic

Answer 36

3 na out and 2 k in, Electrogenic bc extra ion pumped out = membrane potential w slight excess of positively charged ions on the outside

Question 37

can cells generally balance electrical charges inside and outside of cell

Answer 37

yes

Question 38

what is the net result of the generation of membrane potential in animal cells

Answer 38

a bit more + on outside (na, k), a but more - on inside (cl- and fixed anions (nucleic acids, proteins, cell metabolites))

Question 39

what is the equilibrium or resting membrane potential

Answer 39

vary from -20 mV to -200 mV

Question 40

why is resting membrane potential in - (minus)

Answer 40

bc it refers to whats going on in the cytosol

Question 41

describe the generation of membrane potential in plants cells

Answer 41

with the plasma membrane p type pump: h+ pump which generated an h+ electrochemical gradient of -230 to -160 mV
- used by gradient driven pumps to carry out active transport eg the h+ driven symport, electrical signalling, pH regulation

Question 42

describe p type pumps

Answer 42

uses atp, phosphorylated during pumping cycle, many p type pumps which transports ions, eg flippases which transports phospholipids (Maintains the asymmetry, uses AP hydrolysis in order to have moved the phospholipids to the cytosolic leaflet in golgi) and the NaK pump

Question 43

go through the pumping cycle of the Na-K pump

Answer 43

3 na binds, pump phosphorylates itself and hydrolizes atp, phosphorylation triggers conformation change and na is ejected, 2 k binds, pump dephosphorylates itself, pump returns to original conformation and 2k is ejected into the cytosol

Question 44

outline the function of the abc transporter

Answer 44

it is a type of atp driven pump. it uses 2 atp to pump small molecules across cell membranes.
Eg known for transporting toxins out of the cell - mech for cancer cells to gain resistance to chemo drugs is to have a lot of ABC transporters

Question 45

outline the v type proton pump

Answer 45

a type of atp driven pumps. it uses atp to pump h+ into organelles to acidify the lumen in lysosome, plant vacuole – DOES NOT BECOME PHOSPHORYLATED

Question 46

whats the difference between f and v types (proton pump and atp synthase)

Answer 46

f is structurally related to v type proton pump but opposite modes of action. it makes atp instead. uses h gradient to drive the synthesis of atp. present in mitochondria, chloroplasts, bacteria

Question 47

is f type atp synthase reversible

Answer 47

yes, it depends on atp concentration and h electrochemical gradient