Ion Gradients, Ion Channels and Membrane Transport Systems

Question 1

Cholesterol affect on cell membranes

Answer 1

• Abolishes endothermic phase transition of bilayers
• Reduces phospholipid packing - inc. fluidity
• Reduces phospholipid chain motion - decreases fluidity

Question 2

Co-transport systems

Answer 2

• Na+ - glucose - small intestine and kidney, entry of Na+ gives energy for glucose entry (symport)
• Na+ - Ca2+ exchanger - inward flow of Na+ down its concentration gradient drives outward flow of Ca2+ up its conc. gradient (antiport)
• Na+ - H+ exchanger - inward flow of Na+ down its conc. gradient drives outward flow of H+ leading to cell alkalisation (antiport)

Question 3

Symport

Answer 3

The transfer of two molecules in the same direction

Question 4

Antiport

Answer 4

The transfer of two molecules in the opposite direction

Question 5

Uniport

Answer 5

When one solute molecule species is transported from one side to the other

Question 6

Na+ - K+ - ATPase

Answer 6

Forms Na+ and K+ gradients 
Drives 2ndary active transport processes...
- Ion homeostasis 
- Intracellular [Ca2+]
- Intracellular pH 
- Cell volume
- Ion gradients underpinning resting membrane potential 
- Nutrient uptake

Question 7

Control of resting intracellular [Ca2+]

Answer 7

High intracellular [Ca2+] is toxic to cells
2mM/50-100nM (extracellular/intracellular)
- PMCA expels Ca2+ out of cell
- SERCA accumulates Ca2+ in SR/ER
- Na+ - Ca2+ exchanger removes most Ca2+
- Mitochondrial Ca2+ uniports

Question 8

Na+ - Ca2+ exchanger (NCX)

Answer 8

Exchanges 3 Na+ for 1 Ca2+
In depolarised cells it is reversed and brings Ca2+ into cells
Contributes to Ca2+ influx during cardiac AP and toxicity during periods of ischaemia

Question 9

pH regulation in cells

Answer 9

• Sodium-potassium pump - creates Na+ gradient to provide energy for other transporters
• Na+ - H+ exchange - acid extrusion
• Na+ - Cl- - HCO3 - - H+ - co-transporter - acid extrusion and alkali influx
• Na+ - HCO3- -co-transporter - alkali influx
• Anion exchange - Cl- enters, HCO3- leaves - alkali extrusion

Question 10

Cell volume regulation - resisting cell swelling

Answer 10

Efflux of osmotically active ions Na+, K+ and Cl-

• Conductive systems - K+/Cl- channels expel ions, water follows
• Co-transport systems - expel ions and water follows

Question 11

Cell volume regulation - resisting cell shrinking

Answer 11

Influx of osmotically active ions Na+, K+ and Cl-

• Conductive systems - Na+/Ca2+ into cell, water follows
• Co-transport systems - Na+ - Cl- / Na+ - K+ - 2Cl-