Transporters

Question 1

Rank in order of permeability:

Ions, small uncharged polar molecules, gases, non-polar molecules, large uncharged polar molecules

Answer 1

Highest permeability:
Gases (diffusion)
Non-polar/hydrophobic molecules (diffusion)
Small uncharged polar molecules (facilitated diffusion)
Large uncharged polar molecules (facilitated diffusion)
Ions (active transport or facilitated diffusion)

Question 2

What is the difference between active transport and passive transport?

Answer 2

Active: requires energy, positive free energy, goes against the concentration gradient
Passive: requires no energy, negative free energy, goes with the concentration gradient

Question 3

What are the various ways molecules can pass through the membrane?

Answer 3

Active transport: (carrier proteins)
Primary AT - uniporter, requires ATP
Secondary AT - cotransporter (supporter, antiporter), requires pre-existing electrochemical gradient

Passive transport:
Facilitated diffusion: carrier protein
Simple diffusion: channel proteins and directly through the membrane

Question 4

What is the equation for net rate of diffusion?

Answer 4

J = P*(C2-C1)

Net rate of diffusion = Permeability * (Concentration difference)

Question 5

What is the equation for free energy change?

Answer 5

G = RT*log(C2/C1)

Free energy change = gas constant * temperature * log(concentration outside / concentration inside)

Question 6

What does Na+K+ATPase do?

Answer 6

Primary transporter (uses ATP)
Transports 3 Na+ ions out of a cell while co-transporting 2 K+ ions into the cell (generating negative cell Potential)

Question 7

What are the components of the P-type ATPase (NaK-pump)?

Answer 7

Alpha subunit:
Contains phosphorylated Aspartate
Binding site for Na+, K+, ATP, Oubain (Toxin that inhibits)
Beta subunit:
Contains glycoproteins that direct the pump the the cell surface membrane

Question 8

What does Ca2+Mg2+ATPase do and describe its properties

Answer 8

Transports either Ca2+ or Mg2+ out of the cell
Primary (requires ATP)
Uniporter
High affinity, low capacity

Question 9

What does the Na+Ca2+Exchanger (NCX) do and describe its properties

Answer 9

Transports Ca2+ out of the cell for every 3 Na+ it transports into the cell
Secondary (requires on electrochemical gradient of Na+)
Antiporter
Low affinity, high capacity

Question 10

What does the Na+H+ Exchanger do?

Answer 10

Transports 1 H+ out of the cell for every Na+ it transports into the cell
Antiporter

Question 11

What does the Na+ Glucose Co-transporter do?

Answer 11

Transports 1 glucose into the cell for every 2 Na+ it also transports into the cell
Symporter

Question 12

What transporters are involved in cystic fibrosis?

Answer 12

CFTR (Cystic Fibrosis Transmembrane conductance Regulator): transports Cl- out of cells into the alveolar space
If this fails, less Cl- ions in the airway, less water, thicker mucous

Question 13

What transporters are involved in diarrhoea?

Answer 13

CFTR (Cystic Fibrosis Transmembrane conductance Regulator): transports Cl- out of cells into the lumen of the gut
Protein kinase A causes increased Cl- efflux, more water in the lumen, watery faeces

Question 14

How is Intracellular [Ca2+] controlled?

Answer 14

Primary active transport: PMCA and SERCA
Secondary active transport: NCX
Facilitated diffusion: Uniports into the mitochondria

Question 15

How do the PMCA and SERCA work?

Answer 15

Both are primary transporters (requiring ATP), taking Ca2+ from the cytosol (symporting with H+)
PMCA (Plasma Membrane Ca2+ ATPase): Ca2+ —> extracellularly
SERCA (Sarco/endoplasmic Reticulum Ca2+ ATPase): Ca2+ —> S/ER

Question 16

What happens when Intracellular [Ca2+] gets too high?

Answer 16

Uniporters transport Ca2+ into the mitochondria for storage

High [Ca2+] in the mitochondria can lead to apoptosis (initiate caspase cascade)

Question 17

What does electrogenic mean?

Which transporter show electrogenicity?

Answer 17

Current flows in the direction of the membrane potential

NCX

Question 18

How is cell pH controlled?

Answer 18

Acid efflux (NaH Exchanger, Na dependent HCO3 cotransporter)
Alkali influx (NA dependent HCO3 cotransporter, Na+HCO3- cotransporter)
Alkali efflux (AE)

Question 19

How do the acid effluxers work?

Answer 19

Na+H+ Exchanger; As 1 Na+ moves in, 1 H+ moves out
Na+ dependent HCO3- cotransporter: As 1 HCO3- moves in, 1 Cl- moves out this is dependent on 1 Na+ moves in, as 1 H+ moves out (acid effluxer and alkali influxer)

Question 20

How do the alkali influxers work?

Answer 20

Na+ dependent HCO3- cotransporter: As 1 HCO3- moves in, 1 Cl- moves out this is dependent on 1 Na+ moves in, as 1 H+ moves out (acid effluxer and alkali influxer)
Na+ HCO3- Exchanger: As 1 Na+ moves in so does 3 HCO3-

Question 21

How does the alkali effluxer work?

Answer 21

AE (Anion Exchanger): As 1 Cl- moves in, 1 HCO3- moves out

Band 3 is responsible for this exchange

Question 22

How is cell volume controlled?

Answer 22

Movement of osmotically active ions with H2O

Question 23

How is HCO3- reabsorbed in the proximal tubule of the kidney?

Answer 23

NaHCO3 dissociates in the lumen of the kidney —> Na+, HCO3-
Na+ moves into the kidney cell via Na+H+Exchanger bringing H+ into the lumen
H+ and HCO3- form carbonic acid (H2CO3)
Carbonic anhydrase breaks down H2CO3 —> H2O +CO2 which diffuse into the kidney cell
Carbonic acid is reformed from H2O +CO2 by carbonic anhydrase
H2CO3 dissociates into H+, HCO3-
HCO3- is transported into the capillary via Band 3 (AE) with Cl- entering kidney cell

Question 24

What is the Extracellular, Cytosolic, Reticulum concentration of Ca2+?

Answer 24

Extracellular: 10^-3
Cytosolic: 10^-7
Reticulum: 10^-5