Ion Transport

Question 1

Functions of Na+K+ ATPase pump

Answer 1

Forms Na and K gradients (need for electrical excitability)

VERY small (<5mV) resting membrane potential contribution

Drives secondary active transport

Question 2

What is secondary active transport important for?

Answer 2

Control pH
Regulate cell volume and calcium levels
Absorption of sodium in epithelia
Nutrient uptake (glucose)

Question 3

Why is intracellular calcium controlled?

Answer 3

Toxic to cells in high levels

Calcium and phosphate could crystallise

10,000-20,000 fold difference in levels across membrane (outside 50-100nM, inside 2mM)

Question 4

How is calcium level of cell controlled?

Answer 4

PMCA (plasma membrane calcium ATPase)
SERCA (sarco/endoplasmic reticulum)

NCX (Na+Ca2+ exchanger)

Mitochondrial uniports

Question 5

Ca2+ATPase function (PMCA)

Answer 5

PMCA expels Ca2+ out of cell and hydrogen into cell
High affinity for Ca2+
Low capacity (removes residual)

PRIMARY ACTIVE

Question 6

What does SERCA do? (Ca2+ATPase)

Answer 6

Drives Ca into endoplasmic/sarcoplasmic reticulum
H+ move out
High affinity, low capacity (removes residual)

PRIMARY ACTIVE

Question 7

What does Na+Ca2+ exchange do? (NCX)

Answer 7

3Na in, Ca out
Low affinity, high capacity (removes most Ca2+)

SECONDARY ACTIVE

Question 8

Facilitated transport to control calcium concentration

Answer 8

Mitochondrial uniporters
Operate at high calcium levels
Buffers to protect cell

Question 9

Primary active transport controlling calcium levels

Answer 9

PMCA
SERCA

(Hydrogen in, calcium out)

Question 10

Secondary active transport to control calcium levels

Answer 10

NCX

3 Sodium in, calcium out

Question 11

Sodium calcium exchanger NCX importance

Answer 11

Cell toxicity during ischaemia/repurfusion

Electrogenic - current flows in direction of Na+gradient

Question 12

Polarised cell NCX function

Answer 12

negative inside, positive outside

Calcium moves out
3 Sodiums move in

(High Ca in cell, low Na inside)

Question 13

Depolarised cell NCX activity

Answer 13

Negative outside, positive inside

3 Sodiums move out, calcium moves in
(High Na in, Low Ca in)

REVERSE - important for cardiac action potential

Question 14

Effects of Ischaemia on NCX

Answer 14

low O2 = low ATP
Na accumulates inside cell as not being pumped out
Cell depolarised

NCX reverses:
3 Na out, calcium in
Calcium TOXIC to cells in high levels

Question 15

How is cell pH controlled

Answer 15

Make less acidic:
NHE (Na+H+ exchanger)
NBC (sodium bicarbonate cotransporter)

Make less basic:
Anion exchanger

Question 16

NHE

Answer 16

Na+/H+ exchanger
Na in, H out
Electroneutral
Regulates pH and cell volume
Activated by growth factors

Question 17

What is NHE inhibited by?

Answer 17

Amiloride

Question 18

NBC

Answer 18

NBC - sodium bicarbonate chloride cotransporter
Hydrogen and chloride out
Sodium and bicarbonate in
= ALKALINE cell

Question 19

Anion exchange (AE)

Answer 19

Bicarbonate (HCO3) out
Chloride in
ACIDIFIES

Question 20

Two bicarbonate transporters

Answer 20

NBC Alkalinises
AE acidifies

Regulate pH and cell volume

Question 21

How is pH controlled and balanced

Answer 21

Alkali environment activates AE

Acidification activates NHE and NBC

Question 22

How is cell volume controlled

Answer 22

Swelling - extrude ions (water will move with)

Shrinking - influx ions (fill cell with ions and water)

Question 23

Mechanisms to resist cell swelling and shrinking

Answer 23

Swelling: REMOVE IONS
Conductive systems
Co transport

Shrinking: FILL WITH IONS
Cotransport and conductive systems